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MatterBeam

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  1. Yes, HE alone is worse than rocket fuel mixes. However, with the addition of fusion energy, they come out on top again. All my calculations in the blog post were for single stages.
  2. The power source is high explosives. Chemical reactants, 5 MJ/kg delivering heat and pressure at a rate of several gigawatts. Yes, it is possible. It will be very difficult though.
  3. This is from my latest blog post: http://toughsf.blogspot.com/2022/03/fusion-without-fissiles-superbombs-and.html Fusion technology today relies on expensive, building-sized equipment for ignition, or the help of an already powerful fission detonation. What if we could do away with both? Fusion power without the need for fissiles, but also small enough to be launched into space. It is possible, and eventually it will be practical. Let’s look at how that would work and its implications. The lead image is artwork commissioned from the talented Daemoria on the ToughSF Discord. It features a spacecraft powered by an Orion-type nuclear pulse propulsion system refueling using the ices of an asteroid deep in the Outer Solar System. Click to zoom in! Too big to launch The point of convergence of all the National Ignition Facility's 192 lasers. Fusion research today focuses on igniting small quantities of deuterium and tritium using the concentrated energy of lasers, magnetic fields, plasma jets or particle beams. This puts the fuel in conditions far more intense than the core of our Sun, which is enough to ignite the nuclear reaction. However, the total amount of energy being handled is not all that great. The latest record-breaking fusion attempt at the National Ignition Facility added 1.8 MegaJoules of energy in the form of a laser pulse to a tiny gold Hohlraum containing a few milligrams of frozen fuel. Only 150 kiloJoules was actually absorbed by the fuel. From this, the fusion fuel yielded 1.3 MJ, or 8.6 times the input. The energies involved here are equivalent to the kinetic energy of a small truck at highway speeds or the heat released by burning about 50 milliliters of gasoline. Even if we include the total electrical input of the NIF facility during the attempt, 422 MJ (mainly due to the ridiculously low 0.8% efficiency of the lasers), then we are talking about equivalent to the kinetic energy of a medium-sized passenger jet on takeoff or the explosives in a Mark 82 bomb. It is more than we usually encounter in everyday life, but within reach with a little effort. The full NIF facility houses 7680 xenon flash lamps and 3072 glass slab lasers. The NIF cost $3.5 billion to build and spans at least 300 meters. It probably weighs thousands of tons. All just to deliver 150 kJ to a tiny ball of DT. Sure, a more efficient laser and a more compact arrangement of the components could be used, but it is clear that existing fusion technology cannot fit inside the size and mass constraints of modern space launch capabilities. Even the upcoming SpaceX Starship, a superheavy lift vehicle, can only accommodate 100 ton payloads that are less than 8 meters wide. There is a gap of several orders of magnitude between the two. So how do we move fusion technology into space? Stars in small boxes There is an easy path and a hard path to placing fusion technology in space. We are on the hard path. It involves progressing our current technological development of ignition methods to the point where the equipment needed for fusion ignition becomes lightweight and manages an input-to-output energy ratio (the fusion gain factor) by two orders of magnitude. For example, we could look at the Gradient Field Imploding Liner concept. This design pushes 50 tons of payload to Mars using a 1.2 GW fusion drive. It uses a novel method for ignition (an imploding lithium liner shot through a magnetic coil of over 20 Tesla) that produces a fusion gain factor of 982. After adding up the mass of the equipment needed to generate electricity from the fusion reaction (to power the ignition process) and radiators to remove waste heat, it ends up with a fantastic power density of over 10 kW per kg. A single Starship launch of 100 tons would be able to deliver a reactor with an output of 1 GigaWatt if fusion technology achieved that performance. That’s enough to tend to the needs of over a million people. However, these advances are a long way away. It will require immense effort and research investment over the course of several decades to even come close to these figures. What about the easy path? The 15 Megaton yield Castle Bravo test. Fusion reactions have been produced easily and in small packages since the 1950s in thermonuclear bombs. The shortcut here is to create the necessary conditions for igniting fusion fuel using the awesome power of another nuclear reaction: fission. It is much easier to extract energy from unstable uranium or plutonium isotopes. It can be as simple as bringing enough of these substances together in one place. The only challenge that remains is to channel that energy into the fusion fuel - an idea first proposed by Enrico Fermi that resulted in the Teller-Ulam design that used the radiation from a fission stage (the primary) to implode a fusion stage (the secondary). From a physics perspective, it is very elegant: it turns a hard problem (igniting fusion) into two easy problems (igniting fission, then transferring the energy). From a practical perspective, it is terrifying. Any plane or rocket that could lift a few hundred kilograms had its destructive capability upgraded to levelling an entire city. The W56 warhead weighs only 272 kg but manages a yield of 1.2 million tons (megatons) of TNT. The incredible yield-to-weight ratios of nuclear warheads. ICBMs have carried these thermonuclear warheads into space, but not into orbit. These missiles cannot achieve orbital velocity, but only because it is not necessary and not because it is impossible. Their deltaV capability is about 6 to 7 km/s and they would need an additional stage to achieve the necessary 9 km/s for Low Earth Orbit. Incidentally, this is how we got the Soyuz rocket; by adding an extra stage to the R-7 ICBM. Thermonuclear weapons have been tested in space. The most famous example is the Starfish Prime shot. A W49 warhead with a yield of 1.4 megatons was detonated at an altitude of 400 km. The Starfish Prime test of 1962. A naïve calculation would find that a SpaceX Starship could be filled with W56 warheads and hold a combined yield equivalent to 441 megatons of TNT. The previous 1 GW reactor would have to work for 58 years to match the energy these warheads could release in microseconds. It is not so straightforward though. Thermonuclear warheads have many downsides that prevent them from being an acceptable fusion technology in space. The first is their minimum size. The fusion reaction must be initiated by a fission reaction, which requires a critical mass of fissile material. In the smallest warheads, this is brought down to a few kilograms, resulting in a minimum yield of roughly 42 GJ or 10 tons of TNT. A warhead at this scale is extremely wasteful in its use of fissile material. The smallest design that actually liberates a good fraction of its potential energy would release 4,200 GJ or 1000 tons of TNT. Funnily enough, it obtains this from the same amount of fissile material but with a much larger and more complex compression scheme. A fusion stage on top would need to release a multiple of this yield (10 to 20 times more) to be worth its inclusion. A propulsion system that uses thermonuclear bombs would have trouble if it were hammered by pulses with a yield equivalent to tens of thousands of tons of TNT. A nozzle or pusher plate that receives this blast would be immense, and the suspension system needed to translate the pulses into a continuous acceleration would bring us back to the building-sized equipment we are trying to avoid in the first place. The second is their need for fissile material. It is in fact the biggest problem with producing thermonuclear warheads. Today, it means that they need a highly controlled substance, which is enriched uranium or plutonium. It is expensive, difficult to manufacture, easily weaponizable and dangerous if accidentally dispersed. Political considerations and social fears have already prevented the launch of much milder nuclear propulsion system, in the form of Nuclear Thermal Rockets, and ruled out designs like the Orion nuclear pulse propulsion rocket by international law. Even in a fictional setting or alternate-future where these concerns are minimized, there is still the logistical problem of sustaining the use of these materials. The Midnite mine. Uranium is only found in high concentrations on Earth thanks to the action of the terrestrial water cycle. Dry surfaces like the Moon or small bodies like asteroids have their uranium dispersed within them at concentrations similar to the primordial composition of our Solar System. Instead of mining rich veins for uranium at 200,000 parts per million, settlers on Venus or Ceres would be sifting through vast quantities of rock to extract less than 2 parts per million. Map of uranium on the Moon. That’s 5 grams per cubic meter of rock. Worse, only 0.7% of this uranium is of the desired U235 isotope, so only 35 milligrams of enriched material would go towards the thermonuclear warhead. The rest would have to go through a laborious burnup and transmutation process inside breeder reactors. If the minimum critical mass is about 2 kilograms, then over 57,000m^3 of rock would need to be processed for each thermonuclear pulse. A rocket that uses these pulses for propulsion may need thousands of pulse units to complete a trip… it is clearly unsustainable! Deuterium/Hydrogen ratios in the Solar System The fusion fuel is a minor concern in comparison. Deuterium is abundant in all waters of the solar system at 312 parts per million (0.312 grams per kg), and can be higher in the outer solar system. Deuterium concentration was 3 times higher in the samples returned from the comet 67P/Churyumov-Gerasimenko than on Earth. It can be melted out of the ices of a comet and separated by electrolysis. Tritium is trickier to obtain, but it can be manufactured out of lithium, which is a rather common element. It decays with a half-life of 12 years but with the speed of fusion propulsion, most trips will be completed well before then. Helium 3 is very rare in comparison, but obtaining it is still possible from the lunar surface or by scooping up the atmospheres of Venus or the gas giants. Filtering gases is a much easier task than digging through kilometers of rock after all. Going by the abundance of their fuels, we would want to use Deuterium-Deuterium fusion, then Deuterium-Tritium, then Deuterium-Helium 3. Pure Fusion A hemispherical implosion test device. The solution is to find a way to use a simple non-nuclear energy source, and concentrate it in a way that can ignite a fusion reaction but without the need for complex or heavy machinery to serve as an intermediary. Fusion, without the ‘dirty’ fissile aspect. This is the ‘pure fusion’ concept that has long been on the minds of scientists since the first fusion bomb was tested. It found renewed interest ahead of and following the Comprehensive Test Ban Treaty in 1996. Some of the methods for achieving pure fusion ignition, especially by Soviet and then Russian scientists, were tested in the 1990s and 2000s in collaboration with LANL. It might be because they feared that they might not have access to the multiple billion dollar investment needed to pursue conventional ignition research. More recent concepts have appeared too. Interest in them has waned since fusion research has become a well funded international effort, like JET and NIF. 'The Gadget' from the Manhattan project. This is a prickly topic to discuss with any nuclear scientist today. The design of a pure fusion device overlaps significantly with that of a regular nuclear warhead. Discussing this topic in detail with the general public generally goes against the rules they have to follow to retain their security clearances. They might inadvertently reveal facts or figures they are not allowed to share, even for far off speculation like this. It is wise to not test their patience. Nuclear weapons after all threaten human civilization on one hand, and offer absolute protection against invasion or loss of sovereignty on the other. Aggressive posturing by small and otherwise weak states like North Korea is only possible because they have incredible destructive power at their disposal. The proliferation of nuclear weapons weakens the protection they offer to existing holders while increasing the risk that they are deployed by someone who doesn’t have much to lose. Anything that threatens to share nuclear power to a wider group is therefore taken very seriously. Pure fusion technology could be considered to be one such proliferation concern. The creation of nuclear weapons that circumvent the most effective anti-proliferation control, which is access to fissile material, could destabilize the relations between nuclear states. Global annihilation would come closer. More specifically, it is a restriction on the enrichment of uranium from 99.3% U238 into >90% U235 (or into Pu239). Uranium gas centrifuges for U235 enrichment. Natural uranium cannot be made into a bomb, and it is regularly shipped around the world by the hundreds of tons to feed nuclear reactors. It would be practically impossible to restrict access to it. ‘Reactor grade’ uranium, which is enriched to less than 5% U235, won’t work either. Climbing up to ‘weapons grade’ is a long and arduous process that requires gas centrifuges that take up several football fields and many megawatts of electricity. The machinery is delicate and needs trained personnel to run… even moderate damage or a cyber attack can take them down. India's Bhabha Atomic Research Centre reactor. The other route, which is to operate a reactor specifically designed to produce Plutonium 239, is also difficult to hide, but it has been successful in the past. Pure fusion ignition does not need enriched uranium. There is discussion around how the technology could destabilize the current nuclear arms balance, especially since the Comprehensive Test Ban Treaty left open the door to conventional ignition research and therefore there is a legal ground for the development of alternate ignition schemes. However, as we will calculate later, pure fusion devices cannot result in weapons with the same destructive potential as actual nuclear warheads. They might have an effect on warfare at the tactical scale but not really at the strategic level. Still, there is a real possibility that these designs will be developed seriously in the future, for military purposes or not. They have advantages that are not useful today but might be critical for a space settlement at the edge of the Solar System. Looking into these pure fusion concepts can help inform us about their future potential in propulsion, energy generation and elsewhere. We will look at two plausible concepts for igniting a pure fusion device. The first is Magnetized Target Fusion using explosive-driven flux generators. The second is Multi-Stage High Explosive-driven Implosion Fusion. To these documented concepts we will add invented variants based on other speculative technologies that have been demonstrated in some way or another. Magnetized Target Fusion using Explosive-driven Flux Generators A helical explosive-driven flux generator design for the MAGO experiments. Explosive-driven Flux Generators are able to convert the chemical potential of a high explosive (HE) into a powerful magnetic pulse. This is done by first creating a strong magnetic field by running an electrical current from a small capacitor through a number of conducting disks (Disk Explosive Magnetic Generator or DEMG) or coils (Helical Explosive Magnetic Generator or HEMG). The detonation of a high explosive compresses these conducting structures into a smaller and smaller volume, which magnifies the electrical current and multiplies the initial magnetic field to several hundred tesla. These steps can be staged, with the magnetic field produced by the first compression being multiplied again by a second compression. The Tsar Bomba was developed at the Russian VNIIEF. Experiments at the Russian VNIIEF (All-Russian Scientific Research Institute of Experimental Physics) demonstrated a 20 to 25% conversion of high explosive energy into magnetic energy, with electrical currents on the order of 100 MegaAmperes producing magnetic fields of 200 Tesla strength. It should be noted that actual efficiency is likely much higher (1.5x times higher, so in the 30-40% range) but only a fraction of the total output is delivered at a useful rate, as explained in the Efficiencies section in this document. There is also an explanation that these results are from designs that did not really require high explosive-to-magnetic efficiency, and that instead of 70% is possible with end-initiated coaxial generators. A DEMG with 3 modules, containing disks a meter wide, was shown to deliver 100 MJ of energy and an electrical current of 256 MA, and it is possible to stack 25 of these modules and maybe more. DEMGs tested at the VNIIEF. These powerful magnetic pulses can be used to drive Magnetized Target Fusion (MTF). In this ignition scheme, fusion fuel is first heated into a ‘warm’ plasma, and then it is rapidly compressed by imploding a spherical metal shell (the liner). The shell implodes because of the powerful magnetic pulse we have created using a flux generator. It achieves a substantial velocity of several tens of kilometers per second, enough to raise the pressure and temperature of the plasma trapped inside to fusion ignition conditions. Almost all the fusion energy that is then released is absorbed by the metal shell, causing it to vaporize and expand as a plasma explosion, which can be redirected for thrust or absorbed to generate electricity. MTF has been demonstrated successfully several times with actual fusion neutrons being detected. The biggest current project aiming to use MTF is General Fusion. General Fusion's piston-compressed MTF scheme. It has many advantages over achieving fusion using conventional means. The pressure it can achieve far exceeds anything a tokamak can manage by using static (non-pulsed) magnetic fields, which really helps push fusion fuel particles together. The implosion velocity is much lower than the several hundreds of km/s that need to be achieved at the NIF or most other inertial confinement fusion schemes and it receives that energy far more efficiently than could be managed by a laser or particle beam blasting away at a pellet of frozen fusion fuel. However, it has its own set of challenges and far less investment in its development than the other ignition methods. For our purposes, we are looking at the following chain of events: HE -> Flux Generator -> Metal Liner -> Fusion Ignition -> Fusion Output Each arrow has a certain efficiency figure associated with it. The only source of energy input is the high explosive, and the only source of energy output is from the fusion reaction. There are some small steps we are omitting here, like losses to electrical switching or the initial heating of the fusion fuel, but they are far smaller (kJ scale) than the energies involved in the main steps (MJ scale). The objective is to have a far greater fusion output than the HE energy input. The MAGO plasma chamber. The VNIIEF’s MAGO project (MAGnitnoye Obzhatiye or magnetic compression) found that if the metal liner had a kinetic energy of 65 MJ and imploded at 20 km/s, it could get 8.9 milligrams of deuterium-tritium plasma pre-heated to 1 million Kelvin to undergo fusion and release 1 GJ of energy. Deuterium-Tritium reactions have an output of 340 TeraJoules per kilogram. The full potential of the 8.9 milligrams of fuel is 3.03 GJ. This means that the implosion got 33% of the fuel to undergo fusion (also called the burnup ratio). The result is a ‘fusion gain’ of 16x. They based these results on experiments with 200 MJ flux generators creating >1000 Tesla fields adding up to 25 MJ into the metal liners. If we assume that 25% of the high explosive’s energy can be converted into magnetic energy, and that 60% of the magnetic HE is around 5 MJ/kg for denser compositions like ‘PBX 9501’, so working backwards, it would take 86.6 kg of HE to deliver 433 MJ as energy input, that gets converted into 108.25 MJ of magnetic energy, which results in 65 MJ of metal liner kinetic energy. The final output is 1000 MJ, giving a return on energy investment of 2.3 times. Component weights for a DEMG-powered pure fusion device. Other estimates in this document’s appendix B suggest that a multi-stage device with a plasma chamber would fit 320 kg of HE inside 3400 kg of equipment to be able to deliver 100 MJ to a metal liner that compresses up to 30 milligrams of DT fuel. The fusion output is 10 GJ, which is a 33% burnup ratio. The performance of the flux generators is pessimistic, with only 6% of the 1600 MJ chemical potential in the HE actually being delivered to the plasma chamber. That means a return on energy investment of 6.25 times. The majority of the mass is dedicated to a 2000 kg DEMG device. In the footnotes, it is explained as a necessarily conservative estimate, far greater than the minimum amount of copper wires needed for simply conducting the electrical current. In fact, it seems like the masses of all the explosive flux generators have been estimated by multiplying the mass of the explosive they contain by a factor 10. There are few other figures to rely upon for further speculation. Nonetheless, we can put together the data we have to obtain a ‘reasonable’ MTF design that is powered by high explosives. We’ll call this the Early EMG-MTF device. Early EMG-MTF Total mass: 1600 kg HE mass: 100 kg HE energy: 500 MJ HE-to-magnetic efficiency: 25% Magnetic energy: 125 MJ Magnetic-to-kinetic efficiency: 60% Liner kinetic energy: 75 MJ DT fuel: 22.5 milligrams DT burnup: 33% Fusion output: 2.52 GJ Average energy density: 1.57 MJ/kg This design is admittedly not very powerful. 2.52 GJ of fusion output might sound like a lot, but it is only a 5 times return on energy invested. It is also important to look at the average energy density of the device. It is much less powerful than the same mass of simple HE, so it would be a terrible weapon and even worse propulsion system - for comparison, a mixture of hydrogen and oxygen in a rocket engine has an average energy density of 15 MJ/kg. It actually compares poorly to lithium-ion batteries, which is laughable for a thermonuclear reaction. Comparison of the huge structures need to provide an electrical pulse with capacitors or high explosives. Technology is expected to improve. If we conceived of this technology today instead of in 1998, we should hope to get better results. This can include the use of stronger materials, aluminium conductors instead of copper wires or even high temperature superconductors, better HE compositions and perhaps a different explosive flux generator design that comes closer to the 70% HE-to-magnetic efficiency mentioned previously. These would all lead to a lighter device. It is unlikely to fall below 2x the weight of the explosives, because the HE needs to push against something to transfer its momentum efficiently, but a reduction from 10x to 5x the weight is plausible. More explosive flux generator configurations. Today’s MTF schemes also aim for much higher fusion gain ratios. Tricks to improve the efficiency of the reaction, such as turning the initial warm fuel plasma into a field reversed configuration that is self-containing and prevents heat losses by touching the imploding metal liner too early, can be used. General Fusion’s initial Acoustic MTF concept had pistons compressing a plasma, with 14 MJ being delivered to the plasma in the final step. This was enough to release 704 MJ of fusion energy, which is a fusion gain of 50 times. We can work out that they use 10 milligrams of fusion fuel with each shot, and that the burnup ratio they assume is 20%. The Fusion Driven Rocket's magneto-inertial ignition concept. John Slough’s Fusion-Driven Rocket uses a type of Magnetized Target Fusion where the metal liner is made of lithium and receives a kinetic energy of 2.8 MJ. In return, it provides a fusion gain of 200. This is far above the fusion gains mentioned previously. There are hotspot ignition schemes that can attain fusion gain ratios in the thousands by starting a burn wave in a much larger quantity of fuel, but let’s not be excessively optimistic. If we assume that these promises will be fulfilled, then we can guess at the performance of an EMG-MTF built to an advanced technology standard. Advanced EMG-MTF Total mass: 500 kg HE mass: 100 kg HE energy: 500 MJ HE-to-magnetic efficiency: 70% Magnetic energy: 350 MJ Magnetic-to-kinetic efficiency: 60% Liner kinetic energy: 210 MJ DT fuel: 150 milligrams DT burnup: 33% Fusion output: 16.8 GJ Average energy density: 33.66 MJ/kg We get a much more interesting device. It is 6.7 times more powerful than HE on its own and exceeds the performance of any chemical reaction. But even these improved figures are nowhere near the power of a conventional nuclear warhead which manages energy densities on the order of 10,000,000 MJ/kg. Multi-Stage High Explosive-driven Implosion Fusion This approach attempts to ignite a fusion reaction by imploding the fuel without using a flux generator as an intermediary. High explosives press directly against a metal sphere to cause it to implode into fusion ignition conditions. Normally, this is impossible. HE is powerful and their detonation velocity ranges from 7 km/s to over 10 km/s. The Gurney Equations state that they can push a plate of metal (called a flyer in this situation) up to a third of their detonation velocity, so 2.3 to 3.3 km/s. The UTIAS explosive-drive implosion of a hemispherical chamber. However, some ignition schemes get around this by concentrating the energy of the high explosive shockwaves in some manner. This was demonstrated by using a Voitenko compressor to send a shockwave into a hemispherical chamber filled with deuterium gas. Fusion neutron were successfully produced and detected. The theoretically simple collapsing spherical chamber. Even more effective (in theory) is use explosives to surround a 1m wide sphere of metal and get it to implode into a tiny 0.1 cm-sized volume. This 1000x decrease in volume would bring the initial inward velocity to several thousand km/s and multiply the internal pressure by tens of millions of times, enough to ignite a fusion reaction. Tests have successfully demonstrated 1 MJ-scale detonations imploding metal spheres and hemispheres and causing some fusion reactions to occur. However, they used 20 cm wide spheres and tried to explain how scaling up their designs will not provide much improvement. Rayleigh-Taylor instabilities forming. The tiniest imperfections in the sphere or the explosive would be magnified as the sphere’s size decreases and would cause the compression to fail. Rayleigh–Taylor instabilities would also cause the smooth surface of the metal sphere to bubble over into a turbulent storm that isn’t very effective at compression fusion fuel. Mitigating these imperfections involves scaling up the sphere to tens of meters in width, and therefore surrounding it with thousands of tons of HE. Not a great solution either. Instead, what we could do is perform a more moderate implosion, and then convert the energy into another form that can do more work on compressing the fusion fuel. Two methods are documented. Winterberg's magnetic booster concept. The most complicated method involves the use of a ‘magnetic booster’. The metal sphere that the HE will implode is given an electrical current, which produces a magnetic field. The sphere is also filled with low density fusion fuel in the form of a gas and at its center is a special target. The initial implosion takes place at a velocity of 5 to 8 km/s, depending on the initial size of the metal sphere. Near the end, the walls are closing in at over 20 km/s. This is enough to raise the temperature within the fuel gas to millions of Kelvin. Not enough for ignition, but enough to get the special target to work. The implosion also multiplies the initial magnetic field into something of massive strength. A diagram of this mag-booster concept. The special target is the magnetic booster and a fuel pellet surrounded by ablative material in a small closed chamber next to it. The magnetic booster is a Z-pinch device, basically a number of coils connected to a capacitor and surrounding a conductive tube. The circuit is open, so there is no electrical current. At the final stage of the metal sphere’s implosion, the circuit is closed. Current runs through the coils and creates a small magnetic field. This does nothing on its own, but it does react to the massively strong magnetic field that surrounds it. The interaction of the fields causes a similarly massive electrical current to start running through the conductive tube. This causes the Z-pinch effect, which exerts enormous pressure on the tube and causes it to collapse. This collapse causes the remains of the tube to radiate heat. This comes in the form of energetic UV and X-rays. Penetrating radiation digs into the adjacent chamber that has held the fuel pellet safe so far. The ablative layer surrounding the fuel pellet vaporizes. The reaction force of the vaporized gases forces the fuel pellet inwards, in turn bringing it to fusion ignition conditions. You may have noticed the similarities between this ‘magnetic booster’ and the steps taken by the Teller-Ulam design of a thermonuclear warhead to turn the energy released by a fission primary into X-rays that then cause a fusion secondary to implode and ignite. The ignition of the tiny fuel pellet raises the temperature of all the gases compressed within the metal sphere. It creates a much larger fusion reaction, which could then be used to ignite even larger quantities of fusion fuel… if we were not tired yet of the great complexity and number of steps involved so far. The complete propulsion system. Winterburg gives us some estimates for the performance of this pure fusion device. It would be a 20 cm wide metal sphere, about a millimeter thick and weighing 40 kg, surrounded by a 10 cm thick layer of HE. The explosive is assumed to be Octol, which has a density of 1700 kg/m^3 and an energy density of 5.3 MJ/kg. This layer is itself contained inside a 10 cm thick iron sphere (the tamper) that weighs 800 kg. The iron is the single biggest contributor to the device’s mass. Its job is to contain the 70 MJ high explosive detonation for a maximally efficient implosion. The total mass of the device is 853 kg, rounded up to 1000 kg by Winterberg. The fusion reaction within it releases 400 GJ of energy. Most of it is in the form of neutrons, but the iron sphere does an excellent job at absorbing them all. We can call it the Magnetic Booster Implosion Fusion device or MBIF. Here is the summary: Winterberg MBIF Total mass: 1000 kg Tamper mass: 800 kg HE mass: 53 kg HE energy: 70 MJ DT fuel: 2.53 grams DT burnup: 50% Fusion output: 400 GJ Average energy density: 400 MJ/kg This is an incredible performance, blowing away even the best assumptions for the Advanced EMG-MTF. We can attribute this to the much larger quantity of fuel that gets heated to ignition conditions and the elimination of the heavy flux-generator equipment. Still, this is nowhere near the power of a conventional nuclear warhead. A Winterberg pure fusion design, this time relying on compressed 'super-explosives'. Winterberg’s original conception of a ‘mini-nuke’ had a metal sphere collapsing to the point where it radiates in the X-ray wavelengths and causes another ablative stage to compress fusion fuel to the point of ignition, without the need for a complex ‘magnetic booster’. It might reduce the number of steps needed to achieve fusion, at the cost of tightened tolerances on how smooth the metal sphere is and how evenly the HE detonates. These advantages would be seen during the manufacturing stage and not in the actual performance. Another method attempts to improve on the design offered by Winterberg but combining it with more recent techniques. Finn van Donkelaar suggests that a staged HE accelerator using overdriven detonations can do away with the imploding spheres and heavy iron tamper. It is a less rigorous treatment of the topic, but it does have some interesting figures to offer. There are four steps: acceleration of metal plates (flyers), piston-compression of deuterium-tritium gas followed by a spherical implosion, and finally a fuel pellet surrounded by ablative material that undergoes the final compression. The same principles as those for creating EFPs are used here. The HE is separated into disks lined up behind metal plates (called flyers). The first HE stage is ignited and it pushes a flyer to 3 km/s. This flyer hits the back of the second stage, creating a shockwave. This second stage adds its own velocity to its own flyer, allowing for flyer velocities greater than what is possible with a single stage - a solution very similar to one adopted by rockets to overcome the deltaV limitations of a single stage. Explosives act differently when compressed due to a shockwave. The shockwave has an additional effect. It causes a sudden compression of the material it passes through. Compressed matter has a higher density and therefore a greater speed of sound. The compression also causes the chemical composition to ignite. Theoretically, the travelling wave will pick up more energy from this combustion, causing it to compress more HE even harder, which again increases the speed of sound and allows it to reach higher velocities. The result is an 'overdriven' detonation velocity superior to the ordinary uncompressed detonation velocity. The combined effects of staging and overdriven explosion velocity would allow flyer plates to achieve 8-12 km/s. The final flyer hits a converging section that focuses its energy on a ‘cup’. That cup acts like a piston travelling down a tube that contains DT gas before meeting a ‘bowl’. The temperature at this point has increased to 9500 K. The cup and bowl then meet to form a sphere that undergoes its own implosion that forces the fusion fuel into a volume a thousand times smaller. Temperatures reach millions of Kelvin, providing the X-ray radiation needed to make the surface of the fuel pellet surrounded by ablative material explode and finally achieve ignition. The fusion reaction in the fuel pellet provides the spark that gets the rest of the fuel gas to react. We have some performance figures, but with few details. A scaled up device would mass 1600 kg in total, have a length of 2.5m and a width of 0.4m, and yield an output of 8,368,000 MJ. Energy density is 5,230 MJ/kg. The amount of fusion fuel consumed is between 50 and 100 grams, depending on assumptions about burnup ratio. We can call it the Staged Overdriven Accelerator Fusion device. SOAF device Total mass: 1600 kg DT fuel: 50 grams DT burnup: 50% Fusion output: 8.37 TJ Average energy density: 5.23 GJ/kg This performance figure is ridiculously high, and it speaks to the true potential of fusion technology. And yet, it is about 1900 times weaker than a thermonuclear warhead. Other ways to spark the fire There are even more ways to get fusion reactions without needing any fissile material or heavy equipment. They are, however, even more speculative. A SMES device using niobium-tin coils. One example is to use Superconducting Magnetic Energy Storage (SMES) devices. SMESs pushed to the limits of the tensile strength of the materials holding them together can manage impressive energy densities. The quenching process allows them to release their stored energy nearly instantaneously too. Using the maximum strength-to-weight ratio of modern mass-produced materials, such as the 7 GPa strength at 1790 kg/m^3 density of Toray T1100G carbon fibers, would be able to store 3.9 MJ/kg. This is less energy than the 5 MJ/kg of dense explosives like RDX. However, SMES output their energy in the form of electricity, allowing it to be converted into magnetic energy with near-perfect efficiency, and at extremely rapid rates. They also greatly reduce the mass of copper conductors and various magnetic coils needed as they can pass huge currents through small wires (assuming the wires are also superconductors). In effect, 1 kg of Toray 1100G-backed SMES is worth 1.4 to 3.1 kg of HE due to increased efficiency. It would be even better in practice as SMES do not need to explode or push against something to operate (so no need for a heavy tamper), so they can allow for even greater mass savings. At their best, SMES backed by more advanced materials, such as carbon nanomaterials, could exceed 50 MJ/kg while retaining the efficiency benefits over HE. Superconducting materials applied to other parts of an explosive flux generator could result in the following device: SMES-EMG-MTF Total mass: 200 kg SMES mass: 100 kg SMES energy: 5000 MJ SMES-to-magnetic efficiency: 99% Magnetic energy: 4950 MJ Magnetic-to-kinetic efficiency: 80% Liner kinetic energy: 3960 MJ DT fuel: 2.83 grams DT burnup: 33% Fusion output: 320 GJ Average energy density: 1.6 GJ/kg This would bring it more in line with the performance of the staged HE accelerator. Of course, applying SMES technology to the SOAF device itself would bring performance to an even greater level. Simulation of a shear-flow-stabilized Z-pinch, one of the most promising approaches. There are even more ways to use the energy of a large explosion. The flux generators could exploit their ability to produce electrical currents in the hundreds of mega-amperes to drive a large Z-pinch. This could be used to directly compress a metal liner around a fuel pellet, as in the HOPE Fusion propulsion approach (an MTF version was also designed). In that design, 333 MJ is delivered to the specially shaped fuel target, and in return, 1 GJ of fusion energy is released. This energy gain ratio of just 3x is too slim to work with HE, but an improved concept could allow it. An explosive-driven railgun. Or, the electrical current could be used to power a short but extremely high acceleration electromagnetic gun. It would be connected by long wires to the EMG so the debris from its remains do not damage the accelerator. Whether it is a coilgun or a railgun, a projectile velocity of 20 km/s could be achieved before the current falls off. This is enough to start the multi-staged compression cycle proposed here for low velocity fusion ignition. It would be even easier to use the electrical discharge from SMES, although that raises the difficult question between throwing away empty SMES or installing the equipment to recharge them. The Wilderness Orion The application that stands out the most for these pure fusion devices is in the domain of space propulsion. A pure fusion device could be used to create a large plasma explosion. A magnetic nozzle or pusher plate could be used to turn that fusion energy into thrust, similarly to the various nuclear pulse propulsion designs. To estimate the performance of these devices as rockets, we use the method described in a previous blog post. This equation is most useful: Plasma RMS velocity = (2 * Energy Density)^0.5 Plasma RMS (Root Mean Square) velocity is in m/s. Energy density is in J/kg We can turn this into an exhaust velocity by including an efficiency figure for how good a nozzle is at turning an expanding plasma into an exhaust stream. Exhaust velocity = Nozzle efficiency * (2 * Energy Density)^0.5 Exhaust velocity is in m/s. Nozzle efficiency is a ratio. We’ll use 90% (0.9) for the following calculations. Energy density is in J/kg The energy density we use here is that of the entire device. This is because we must assume that the fusion reaction and its X-rays, charged particles, neutrons and other products are all fully absorbed into the device’s mass and converted into heat. For the Early EMG-MTF design, we get Energy Density = 1,570,000 J/kg. With a nozzle efficiency of 90%, we calculate an exhaust velocity of 1594 m/s. That’s a specific impulse (Isp, or exhaust velocity divided by 9.81) of 162 seconds, which is worse than most cold gas thrusters. No spaceship is going to bother with that. The Advanced EMG-MTF and its 33.66 MJ/kg is much more interesting. We calculate an exhaust velocity of 7384 m/s. That’s an Isp of 752s. This is better than any chemical thruster and comparable to a low performance solid-core nuclear thermal rocket or a solar thermal thruster restricted by poor materials. The Winterberg MBIF manages 400 MJ/kg. That results in an exhaust velocity of 25,455 m/s. An Isp of nearly 2600s is better than most high-thrust electric thrusters and is only matched by advanced gas-core nuclear rockets. Performance reaches another level once energy density is measured in GJ/kg. The SMES-EMG-MTF would get us 5,200s Isp and the SOAF design manages an even higher 9.400s. Even the most advanced electric thruster would struggle to meet this performance level. For the higher specific impulses, you would want a magnetic nozzle to handle the plasma, as shown in this beautiful piece by Seth Pritchard. This is not to say that high specific impulse is the only thing to aim for. Like other forms of nuclear pulse propulsion, a rocket that drops pure fusion devices into its nozzle also gets very high thrust. More thrust can be delivered by simply sending out these devices to explode more frequently behind the spaceship. All the ignition energy is contained inside the devices, so there is no major rate limit to how often they can be used. Drop a single 1 GJ device per second, and the drive power is 1 GW. Drop ten of them, and it becomes 10 GW. This is most similar to the original Orion design and its Outer Space Treaty-violating nuclear pulse units. The Advanced EMG-MTF dropped at a rate of 1 per second would get you a drive power of 16.8 GW and a thrust (with 90% nozzle efficiency) of 4.1 MegaNewtons. The main interest in these devices is how they free space propulsion from the need to obtain fissile material from Earth, while also providing a level of performance unmatched by chemical or solar energy. Fusion fuels can be found in any patch of ice in the solar system. High explosives are composed of nitrogen, oxygen, carbon and hydrogen. The red-coloured ices on some comets and icy moons is due to organic compounds, as we can see in this Viktus Justinas piece. Various volatiles like ammonia and carbon dioxide can be found on the surfaces of comets or icy moons. It is not a good idea to research exactly how they are made, but turning those raw materials into the H2N2O2 nitroamide building blocks for C3H6N6O6 cannot be more complex than the processes needed to resupply life support systems. A potential obstacle is the need for metals like copper to create conductors and coils. It is the 25th most abundant element in the Solar System, which might not sound like a lot, but you might expect to find 1 kg of copper for every 1724 kg of iron. A metal-rich asteroid like 16 Psyche or 21 Lutetia would contain 10^18 to 10^19 kg of iron. Roughly, we would expect a near-limitless supply of 10^14 to 10^15 kg of copper. Similar ratios would exist on the surfaces of Mars and the Moon. 3D printing and ISRU are key to NASA's future plans. 3D printing of metals and laser cutting of the HE can create the structures needed to implode the fusion fuel. It should be of similar difficulty as printing solar panels, and NASA already considers that a press-to-print process in the near future. This is the origin of the Wilderness adjective: pure fusion devices allow for ‘wilderness refuelling’ or In-Situ Resource Utilization, the same way chemical rockets can manufacture new fuel out of any mass of water they encounter. How would these devices look like on a spaceship? Let’s draft two designs for pulse propulsion spacecraft. The first one, the ‘Mars Circuit’ spaceship, aims to travel from Earth to Mars and back, and the second one, the ‘Saturn Circuit’ spaceship, will jet around the outer Solar System. The Mars Circuit spaceship uses the Advanced EMG-MTF devices. It is a 100 ton spaceship carrying onboard power generation, radiators, life support system, habitation spaces and everything else needed for drifting through interplanetary space. It also has a payload bay that can fit 100 tons. Behind it is a magazine stack of fusion devices. The stack is 35 tons while empty. For a Mars mission, it is filled with 5064 units of half-size (250 kg) versions of the Advanced EMG-MTF devices, totalling 1266 tons. These provide a specific impulse of 753s. Utilizing them is a propulsion system of 108 tons. A USAF Orion with its pulse unit magazines highlighted. This system includes a pusher plate, suspension arms and structural support that can handle 2 MN thrust per pulse. It is directly modelled on the propulsion section of the 10m USAF Orion design (although it would be overbuilt by modern standards). It can drop one pulse unit every 0.8s. Average thrust would be 2.5 MN. Here is the summary for this spaceship: Mars Circuit spaceship Payload: 100 tons Dry mass: 243 tons Propellant mass: 1266 tons Total mass: 1609 tons DeltaV: 11.4 km/s Acceleration: 0.16g (full) to 0.74g (empty) This is not a zippy ship that can just take straight lines to its destination. It does however have enough deltaV to complete fast 120 day trips to Mars. It curves out of Low Earth Orbit and gently slows down into an orbit around Mars, without aerobraking. All neutrons are absorbed within the EMG-MTF units so this is not a radioactive hazard to its surroundings and won’t be ‘hot’ after use. It can directly approach space stations or other spacecraft, like the vehicles that will take the payload down to the Martian surface. Fresh pulse units can be manufactured entirely out of the resources available from the moons Phobos and Deimos. Within the 1266 tons of propellant, there would only be 37.8 grams of fusion fuel. The Saturn Circuit spaceship is much larger and goes much faster by exploiting the power of SMES-EMG-MTF devices. It has 500 tons of onboard equipment, which include comfortable living spaces and a fully self-contained manufacturing facility. Payload capacity is 100 tons. Its magazine stack is filled with 100 kg pure fusion devices that contain 0.566 grams of fusion fuel and output 63.5 GJ thanks to SMES technology that stores 10 MJ/kg. Each unit provides a specific impulse of 3632s and a thrust of 3.56 MN. The average temperature of the plasma created by the use of each fusion device is 600,000 K. The Mini-Mag Orion. This allows it to be harnessed by a magnetic nozzle at the rear of the spaceship. A 40 ton propulsion system (based on that of the Mini-Mag Orion) drops a total of 20,000 of these units at a rate of 1 per second. Here is the summary for this spaceship: Saturn Circuit spaceship Payload: 100 tons Dry mass: 560 tons Propellant mass: 2000 tons Total mass: 2660 tons DeltaV: 49.6 km/s Acceleration: 0.14g (full) to 0.55g (empty) This spaceship can really build up speed. Starting in Low Earth Orbit, it stops at Mars in 38 days, orbits Jupiter after 6 months or gets to Saturn in 1 year. It is not the fastest craft conceivable at that technology level, but it can be relied upon to connect the furthest planets without any initial infrastructure or external support. Even its longest trips are short enough that the 12 year half-life of tritium is not really a concern. It does all this using just 11.3 kg of fusion fuel so carrying an excess isn’t difficult. At 3632s Isp and technically unlimited thrust, made possible by detonating pulse units more frequently or just using larger plasma explosions, there is a clear opening for high performance spacecraft with military potential. The Orion Battleship, a 4000 ton design equipped with 20 Megaton nuclear missiles and naval guns. The combination of wilderness refueling and high performance makes wandering fleets, or more likely pirates, a realistic possibility. Stealth also becomes more effective if you do not need to heat up a nuclear reactor or ignite a fusion core to start maneuvering. Superbombs It is obvious that pure fusion devices have a real potential as weapons. But by now, we hope that the numbers we have arrived at make it clear that they have nowhere near the destructive potential of existing nuclear warheads. They are thousands to hundreds of thousands of times weaker than a thermonuclear device initiated by a fission primary. An F-35A testing the deployment of a B61 thermonuclear bomb. A B61 nuclear bomb with a yield of 300 kilotons of TNT can easily be carried by any aircraft with a hardpoint capable of more than 324 kg. Matching its performance with the wildest SOAF design would mean a warhead with a mass of 235 tons. It would barely fit inside the payload limits of the An-225, the largest cargo plane in the world. Using the Early EMG-MTF design would require 800,000 tons to reach that yield. That’s closer to the weight of all the US Navy’s nuclear aircraft carriers… combined! The destructive radius of a 2000 lb bombs. It does not mean that there would be no consequences to the development of pure fusion devices. A plausible design with an energy density of 30 MJ/kg would be six times more powerful than simple HE. Real weapons are about 40% to 60% filled with HE, so it is practically a 12x increase in destructive potential. It would be a ‘superbomb’. By another comparison, the effect of a 907 kg (2000 lb) bomb could be matched by that of a 75 kg (165 lb) pure fusion device. Warfare at the tactical scale has already known a significant shift in the effectiveness of bombs with the introduction of precision guidance systems. It allows large and bulky loads, like a Vietnam-era B-52D Stratofortress bay filled with 66 of the US Air Force’s 340 kg (750 lbs) bombs, to replaced by a precision strike by a JDAM-equipped GBU-12 at 227 kg (1000 lbs), of which fighter jets can carry several. A Super Hornet with a full bomb loadout. Superbombs would cause another change in loadouts. The F/A-18 Super Hornet could be carrying 3600 kg of bombs and 1800 kg fuel for a long range strike mission. It would rely on other aircraft to protect it with their air-to-air missiles, and yet more to guide its munitions using equipment like Litening pods. With 30 MJ/kg Superbombs, its loadout could instead be 360 kg of bombs, 1800 kg of fuel and 3240 kg distributed between missiles, electronic warfare equipment, targeting pods or even more fuel. A single fighter could replace an entire squadron. It might even be able to hide its bombs inside internal bays to be able to maintain a stealthy outline, like an F-35B, while delivering the same power as an F/A-18 bristling with weapons. An MQ-9 Reaper drone equipped with precision-guided Mk 82 bombs. Or, the expensive jets could be replaced by small drones, each only having to hold a few hundred kg of munitions. Pure fusion devices would make delivering destruction to far away targets even cheaper and easier. A side-effect of the development of pure fusion devices is the access to ‘neutron bombs’. These are weapons that intentionally leak the radiation produced by the fusion reaction instead of trying to absorb it to maximize the amount of energy that becomes heat. The intention is to deal a lethal effect via penetrating radiation out to a further radius than the blast effect can manage. The Early EMG-MTF device with its 2.52 GJ output would have a blast radius of 36 meters. An Advanced EMG-MTF yielding 16.8 GJ increases this radius to 68m. If these were converted into neutron bombs, they would deliver a lethal dose of radiation out to 272 meters and 512 meters respectively. It is enough to depopulate multiple entire city blocks. These radii are only reduced by about 50% when concrete walls stand in the way. Another consequence is that tank armor becomes much less useful. Today, a nuclear warhead that can kill a tank crew by radiation has to be close enough to destroy the tank itself by blast effect anyway. In this case, a near miss with a small superbomb is enough to deliver a lethal dose. It is unlikely that the neutron effect can be scaled up to many kilometers (which would empty an entire city center with one hit) as air absorbs and scatters the neutrons after some distance, but it is still enough to create a frightening change of priorities during battle. An invading force could hit populated areas with neutron bombs and rid them of any inhabitants, whether they are innocent civilians or potential defenders. They could then move in and easily hold it. No siege involved, no prolonged cries of the oppressed on social media and news channels. Just a single action that hands an entire city and its economic value, infrastructure and factories, mostly undamaged. Offensive actions would be immensely profitable. Defenders would have to pay an even higher price for letting any missile through their defenses. The general result would be a gradual evolution of the state of warfare. Nothing as drastic as the invention of the nuclear weapon, far from disrupting the balance between nuclear-armed states, and not worthy of proliferation fears. Significant enough however to change what military planners worry about or aim for. Conclusion Pure fusion devices are still a thing of the future. But, we must start considering the potential consequences of their development today. If their arrival is expected and regulated, we could open up human exploration of the Solar System like never before with spaceships untied from the rest of civilization for years. But if we are unprepared, or we dismiss their potential effectiveness, then we could end up with yet another shift of warfare towards greater destruction at lower cost.
  4. That's awesome. Contact me for any questions.
  5. Plasma weapons as depicted in scifi don't and cannot exist. Plasma won't hold itself together and will just puff out like hot gas. The closest real world equivalent is particle beams. They act like lasers in most cases. The best advice so far! The missiles are coming towards you. They have a very high closing velocity, and necessarily come from a narrow range of angles. The sand just has to be between you and the missiles in the last moments before impact. If the missiles dodge the sand, they won't be able to hit without spending many more hours turning back around. If they go through the sand, they'll have sensors, mirrors, antennae and anything else exposed scraped off, turning them into blind sticks that you can dodge with a short RCS burn. 15g x 9.81 m/s^2 x 3600s x 2 = 1,059,480 m/s Which is 0.35% of the speed of light. The Expanse spacecraft are designed with 'no armor is best armor' in mind. They know that they can never withstand a direct hit from a railgun or a torpedo, so they sacrifice that dead weight for extra maneuverability and point defences. The PDCs in the Expanse have never overheated in the 9 books or 6 seasons of the TV show. They have jammed though. ------------------------------------------- Regarding the main question: the best way to defeat these hyper-missiles with 15g accelerate is to use an interceptor drone. This is a small and cheap drone with a ring of small RCS thrusters around a section of steel plate. It just drops off your spaceship's hull and maneuvers itself between the incoming missile and yourself. The incoming missiles have such a high closing velocity that just scraping the tip of this steel plate is enough to obliterate them. One drone per missile. It doesn't need a big gun to shoot the drone off. You don't need to supply any energy. Their 'firing rate' is just how many you choose to drop at once. They continuously guide themselves into a collision course, so accuracy is perfect. Each drone is massively cheaper and lighter than the missile it destroys, so you can easily carry several interceptor drones for each missile that could be launched at you.
  6. @ItsJustLuciCould you please provide an estimate for how much RAM is needed to play with the 32k texture pack in RC1?
  7. I am very impressed than an 8 year old mod is still being maintained. Kudos to @NathanKell
  8. This is from the latest ToughSF blog post. Read here: http://toughsf.blogspot.com/2021/03/fusion-highways-in-space.html Fusion Highways in Space A transport system that can get spacecraft to Jupiter in 10 days, but without a massive onboard reactor, using antimatter fuel or riding a gigantic laser beam? What we need instead is a Fusion Highway to connect the Solar System in unprecedented ways. The art above is by GrahamTG. It depicts a Bussard Ramjet, which is relevant as all the same components (collection scoop, reaction chamber, magnetic nozzle) are necessary for the Fusion Highway to work, but are used in slightly different ways. The ideal rocket In Star Trek, propulsion is never a problem unless the plot demands it. If you had to imagine the perfect rocket, what features would it have? Solving the troubles we have with our existing chemical-fuelled engines can serve as a starting point. Limited specific impulse, limited thrust, great complexity and high cost are standard features of today’s rockets. Logically, a perfect rocket would have maximal propellant efficiency, incredible thrust, minimal complexity and cost… or how about no propellant at all? The perfect rocket takes us up to relativistic speeds, but is also lightweight and accelerates quickly. It is instantly available and safe to use. Only a few propulsion systems have approached this ‘ideal’ status. A Bussard Ramjet, as initially conceived, would need no propellant except what it could gather from the interstellar medium, and it could accelerate all the way up to the speed of light and back. Relativistic ramjet. As we know today, it didn’t really work as advertised. An antimatter beam rocket promises amazing performance with great thrust and efficiency, but fails with regards to cost and safety. Fission fragment propulsion attempts to provide similar efficiency and uses a much safer fuel, but it lacks thrust and no-one would call it a perfect rocket. There is another type of candidate for ‘perfect rocket’ status. Externally propelled ‘beamrider’ rockets leave the power and propellant at home and receive instead a beam that they only have to convert into thrust. Laser-driven sails are the most famous example of this approach. Powerful generators produce a laser beam that gets focused by a huge mirror so that it can concentrate its output onto very distant targets. That target, a spaceship, only has to reflect the laser beam to accelerate towards its desired direction of travel. A kinetic mass-beam rider and its magnetic nozzle. Kinetic mass-beam propulsion creates a stream of high velocity projectiles that the target can deflect magnetically. However, you would need a very expensive beaming installation or very long accelerator to make these beamrider concepts practical. We will be focusing on another external propulsion system that has many advantages over laser sails and kinetic streams. The ‘beam’ is a trail of fusion fuel pellets that is simply pre-positioned ahead of a spaceship so that it can ‘ride’ it with no additional power input of its own, up to relativistic speeds. Fusion Highways There are three elements to a Fusion Highway: -A ‘road-laying’ system that moves pellets into position -A series of fusion fuel pellets that align into a ‘road’ -A spaceship that ‘rides’ the ‘road’ by igniting the pellets as they pass into a reaction chamber. There are many ways to position pellets in space. There will be very many of them in number, so a positioning method that is very inexpensive would be preferred. The ‘pellets’ are not necessarily dumb masses of frozen fusion fuel. At the very least, they are coated in insulation and devices that report its position (like a corner reflector or low power transmitter). If they are not placed immediately ahead of the spaceship, they would need a method for correcting their position in the long term. The spaceship itself is very simple. It has an opening that guides the pellets into its reaction chamber, using magnets or laser pulses to make last-second adjustments. Specially shaped targets. The reaction chamber holds a specially shaped target mass. Ignition itself is the result of the high velocity impact between the fusion fuel pellet and the target mass. ‘Impact fusion’ can take place at velocities as low as 100 km/s, if we are able to convert the linear force from the impact into a more efficient 2D or even 3D compression. The result is an expanding volume of energetic plasma. It bounces off the fields generated inside a magnetic nozzle so that energy is converted into thrust, and so the spaceship accelerates. When the spaceship reaches the next pellet, the cycle starts again. The main advantages of this method is that the spaceship does not need to have a heavy reactor or a complex fusion ignition system. It just drops masses in front of the pellets and harnesses the plasma with a relatively lightweight magnetic nozzle. Unlike a remote laser beam, the energy that propels the spaceship is not the result of a massive beaming installation, but derived from the fusion fuel on-the-go. None of that energy needs to be transmitted by immense focusing optics either, and it does not get harder to operate as the spaceship gets farther from its starting point. The pellets themselves do not need to have a huge velocity, another major advantage over a concept like kinetic stream propulsion. This means you don’t need massive accelerators to bring the projectiles up to incredible velocities, with the expectation that the spaceship can achieve at least a fraction of that velocity. On the Fusion Highway, the spaceship’s velocity is mostly independent of the fuel pellets’ velocity. These factors mean that a Fusion Highway can be affordable and have open-ended performance. The actual performance of this propulsion system depends on several factors. They are: The mass ratio between pellet and target The impact velocity Fusion fuel energy content Average molar mass of the pellet/target mix Fusion burnup and use efficiency Nozzle thrust efficiency Let’s go through two worked examples to demonstrate how those factors are used. Imagine a spaceship of 1,500 tons travelling at a velocity of 300 km/s relative to a fuel pellet track. The track is composed of 1 kg pellets, composed of 500 grams of Deuterium and Tritium fusion fuel, surrounded by 500 grams of frozen hydrogen ice. It has the potential to release 170 TJ of energy. The mass ratio between pellet and target is 0.001; this means the spaceship is dropping a 1 gram target for the 1 kg pellets to hit. The impact velocity is 300 km/s. At this velocity, the impact of 1 gram releases 45 MJ of energy, enough to ignite the fusion fuel if the appropriate techniques are used. The frozen hydrogen ice can be shaped to help direct the kinetic energy of the impact into a compressive force that ignites the fuel. We know that the maximum potential for the fusion fuel is 170 TJ, but not all of this energy will be transferred to the spaceship. Firstly, not all the fuel will undergo fusion. The burnup percentage might be just 10%, so only 17 TJ is released. Of that energy, 20% will be in the form of X-rays and charged particles, which will be easily converted into heat by the frozen hydrogen layer. 80% will be in the form of neutrons, which escape more easily. However, hydrogen ice is an excellent neutron absorbing material, and it should be thick enough for half the neutrons to be captured and turned into heat, so the final amount of ‘usable’ fusion energy is closer to 10.2 TJ. The kinetic energy from impact adds a negligible amount. All this energy converts the target+fuel mix into a very high temperature plasma that expands (if timed right) inside the spaceship’s magnetic nozzle. The temperature is high enough that all particles involved become fully ionized, which simplifies our calculations as we can use perfect gas laws with reasonable accuracy. We also assume that all heating is done while the target+fuel mix is solid (so at constant volume) and that the contribution of phase changes and ionization is negligible. The heat capacity of a perfect monoatomic gas at constant volume is 12470/Molar Mass, in J/kg/K. The temperature of a gas is its energy density (Joules per kilogram) divided by its heat capacity. The rate at which the gas expands is the Root Mean Square gas velocity, which is (24942 * Temperature / Molar Mass)^0.5. If we put these equations together, we find that the molar mass cancels out and therefore: Plasma RMS velocity = (2 * Energy Density)^0.5 In this example, 10.2 TJ of energy is distributed in 1.001 kg of matter. This gives a value for the plasma expansion velocity of 4,511 km/s. A noteworthy consequence of molar mass and heat capacity cancelling out is that the nature of the gases expanding does not matter. In theory, we are free to use abundant propellants like water or silicate rocks instead of being restricted to bulky hydrogen, although lighter molecules absorb neutrons better and lead to greater overall efficiencies. We must make an adjustment to this expansion velocity. The fuel pellet is initially retreating from the spaceship at 300 km/s. After impact, it loses 0.1% of its relative velocity, becoming 299.7 km/s. This must be subtracted from the expansion velocity to find the actual velocity of the plasma relative to the magnetic nozzle. That value becomes 3,198 km/s. Fusion plasma within a magnetic nozzle. The performance of this propulsion system is quite spectacular. Nozzle thrust efficiency is realistically 80%, so the spaceship inputs 1 gram, and it gets 1.001 kg exiting the nozzle at 2,558 km/s. The effective exhaust velocity is multiplied by a thousand to 8.52 times the speed of light. If there is a 300 km gap between the fuel pellets, the net propulsive power the spaceship outputs is 3.27 TW and its average acceleration is 0.17g. Now let’s repeat these calculations for a much higher relative velocity. The same 1,500 ton spaceship rides a track of the same 1 kg pellets, but at 90,000 km/s. The mass ratio between pellet and target is increased to 2. The spaceship drops a 2 kg frozen hydrogen target to impact the same 1 kg fuel pellet. The total mass of the mix after impact is 3 kg, so the ‘retreating velocity’ is reduced to 30,000 km/s. This also allows us to extract two thirds of the potential kinetic energy from the impact; 2700 TJ. The fusion fuel is compressed by a large amount of target material at much higher velocities, so excellent burnup percentages are to be expected, up to 25%. We can hope for 42.5 TJ to be released, and all of it to be absorbed by the extra target mass we’re putting in. Total energy adds up to 2742.5 TJ. Energy density is 2742.5 TJ over 3 kg or 914.17 TJ/kg. We can expect a plasma expansion velocity of 42,759 km/s. You will notice that the margin between plasma expansion velocity and retreating velocity at 12,759 km/s is much slimmer than in the previous calculation. The spaceship puts in 2 kg of propellant and gets 3 kg of plasma, so its effective exhaust velocity is a bit higher at 19,138 km/s, or 15,310 km/s if we consider nozzle efficiency. That same 300 km gap between fuel pellets means that the spaceship encounters 300 pellets per second. Net propulsive output is 105,478 TW (if the spaceship’s nozzle can survive it!) and average acceleration is 3.12g. Velocity Bands The performance of the Fusion Highway depends on the velocity of the spaceship relative to the fuel pellets. There are four distinct ‘velocity bands’ that significantly affect performance: Logarithmic scale on y-axis, all units in C. Green is Fusion band, Yellow is Kinetic band, Red if Relativistic Band. -Sub-ignition band The sub-ignition band of velocities is where the relative velocity of the fuel pellets and the spacecraft is insufficient to ignite fusion reactions. With dumb pellets of fusion fuel and a simple target, this can be as high as 1000 km/s. With specially shaped sphere-section imploding targets and other features that improve compression upon impact, this can be brought down to below 100 km/s. Further into the future, a few tens of km/s might be all that is needed for impact fusion thanks to hotspot ignition or the assistance of external magnetic fields. A Fusion Highway would have multiple entry and exit ramps. A spaceship would have to reach this minimum velocity by some other means before it can start using the Fusion Highway. Think of it as a car accelerating along the entrance ramp to a highway. This could be accomplished by consuming the first few fuel pellets using an onboard ignition system. The frozen fusion fuel could be compressed by magnetic fields, blasted by plasma jets or compressed by ablative laser beams… ignition of the fusion reaction would produce energy that is converted into thrust, allowing acceleration up to the impact fusion threshold velocity. It would not be an ideal solution, as the heavy fusion ignition system would not be of much use for most of the spaceship’s journey, but it would allow for free entry and exit from the Fusion Highway at any point. A better solution could be the use of ‘boost tracks’ that have a high relative velocity to the spaceship, somewhat like a conveyor belt that the spaceship can ride until it reaches the Fusion Highway at the necessary velocity. The boost track is a series of fusion fuel pellets that are shot at the spaceship’s position at above the threshold velocity for impact fusion ignition, doing away with the need for heavy onboard propulsion or ignition systems. The spaceship can then ride this short boost track and then divert to the main Fusion Highway once it has built up enough speed. If the threshold velocity is very low, then some alternative options become available. For example, the boost track is composed of pellets put on a retrograde orbit that the spaceship only needs to intercept at the right time. A spaceship in Low Earth Orbit would be travelling at about 7.7 km/s relative to the surface. Pellets in a retrograde orbit would be travelling at 7.7 km/s in the opposite direction, adding up to a relative velocity upon impact of 15.4 km/s. Pellets on a retrograde near-escape trajectory, perhaps falling from the Moon, could reach a peak velocity of over 11 km/s and achieve 18.7 km/s upon impact. If these orbital velocities are too low, then interplanetary relative velocities can be used. An Earth-orbiting spaceship facing retrograde fuel pellets along the same orbital path would achieve a relative velocity of up to 7.7+29.8+29.8: 67.3 km/s. -Fusion band A fusion rocket at full blast, featuring liquid droplet radiators. Imagine it has a collection scoop for fuel pellets in front. The fusion band of velocities is where the spaceship’s velocity relative to the Fusion Highway is enough to ignite the fuel pellets by impact. There is a minimum and maximum velocity here. The minimum velocity, as described above, is the threshold for igniting fusion reactions upon impact. The maximum velocity is more complicated. In this band of velocities, the energy gained from each impact is dominated by the output of the fusion reaction. In the 300 km/s example that was calculated in the previous section, 99.9996% of the energy was derived from the fusion reaction. Because the same amount of energy comes from igniting the same amount of fuel, the expansion velocity of the resultant plasma is nearly constant. However, as the spaceship’s velocity on the highway increases, the retreating velocity increases. At very low relative velocities, the difference between expansion velocity and retreating velocity is huge. Effective exhaust velocity is at its highest. At increasing relative velocities, the difference between expansion velocity and retreating velocity becomes smaller and effective exhaust velocity falls quickly. At some point, the relative velocity is nearly equal to the expansion velocity and no thrust is generated; effective exhaust velocity becomes zero. This is the limit of the fusion band. The maximum velocity is therefore close to the expansion velocity of the ‘pure fusion’ plasma. This depends, as shown in the previous calculations, on how much energy can be extracted from the fusion fuel divided by the mass of the fuel pellet. For example, a fuel pellet that is 50% Deuterium-Tritium fuel, has a 10% burnup ratio and is able to convert 60% of the fusion energy into heat would manage an energy density of 10.2 TJ/kg, and create a plasma that expands at 4516 km/s. The maximum velocity in the fusion band using this pellet will be around 4516 km/s. A better pellet helps extend the fusion band of velocities. Deuterium and Helium 3 release nearly 95% of their output in a form that can be converted into heat. Advanced compression and confinement techniques can improve burnup to perhaps 25%. If the fuel pellets can be made entirely of DHe3 fuel, we could manage an energy density of 83 TJ/kg and therefore have a plasma that expands at 12,950 km/s. It is important to extend the fusion band of velocities to be as wide as possible as this is where the outrageous effective exhaust velocities are possible, multiple times the speed of light in many cases. The spaceship only needs to drop the smallest target masses to ignite the fusion reaction, and can then ramp its speed up and down easily. -Kinetic band A RAIR spaceship. After the fusion band’s maximum velocity is crossed, there comes a point where tiny target masses are no longer possible. The target/fuel mix must have a retreating velocity lower than the plasma expansion velocity. Calculations show that this requires a target to fuel pellet mass ratio of over 2, i.e. 2 kg of target mass to catch 1 kg fuel pellets. The kinetic energy added upon impact quickly becomes dominant. In the 90,000 km/s example above, the kinetic energy from the impact represents 98.45% of all the energy that the plasma gains. The fusion fuel in the pellets can actually be replaced with inert material and we won’t see a significant drop in performance (and this will really help keep the overall costs low!). There is an optimal mass ratio between the target and the fuel pellets that provides the best effective exhaust velocity at any impact velocity. Since the fusion output provides only a small fraction of the energy gained from impact, this optimal mass ratio depends mostly on the performance of the magnetic nozzle and less on the composition of the fuel pellets. Furthermore, as the impact velocities increase, retreating velocity increases linearly (it is a momentum transfer) but the kinetic energy added to the expanding plasma increases quadratically. Calculations show that effective exhaust velocity improves gradually at higher velocities. A spaceship can ride the Fusion Highway more efficiently the faster it goes. However, the great reduction in effective exhaust velocity and the extreme velocities involved make this unsuited for interplanetary travel. Also, in this band of velocities, a spaceship travelling along Fusion Highway acts very much like a Ram-Augmented Bussard Ramjet. -Relativistic band After a while, relativistic effects come into play. The equations we’ve used to estimate the performance of this propulsion system tell us that a spaceship can ride a Fusion Highway up to large fractions of the speed of light with only moderate amounts of target masses. However, some assumptions start to break down. For example, we assume that the collision between the target mass and the fuel pellet is elastic, that the kinetic energy is fully absorbed and converted into heat, and that the fusion reaction has time to ignite and spread its energy throughout the mix before it all expands outwards. Some of these things won’t hold up at relativistic velocities. The fuel pellet will start to act instead as penetrating radiation that digs through the target masses. The plasma might expand too quickly for the fusion reaction to transfer its energy efficiently, or it might reach temperatures so great that there is significant energy loss through blackbody radiation before it fully expands. When do these relativistic effects come into play? It is hard to say. 30% to 50% of the speed of light seems like a plausible limit. At 0.5C, the Lorentz factor is only 1.15, but hydrogen acts as 145 MeV radiation and the plasma temperature is supposedly in the hundreds of billions of Kelvin. This is not to say that a Fusion Highway can’t be used beyond 0.5C, but that a much more complicated analysis is required to determine how its performance is affected. What we can conclude for now is that attempting to extend a Fusion Highway beyond the Solar System, to enable interstellar voyages, is a topic that needs its own separate treatment. Interplanetary Design Let’s go through two complete Fusion Highway designs for use in interplanetary travel. One is modest and uses conservative assumptions, the other is more futuristic and fully illustrates the awesome potential of this propulsion method. You will note that we do not go beyond velocities within the fusion band. Entering the -Modest example For the modest example, we will use 0.5 kg fuel pellets that are 10% Deuterium, surrounded by 90% water ice. Deuterium is abundant throughout the Solar System and provides about 80 TJ/kg of fusion energy. Fusion burnup will be about 10% and the usable fraction of that energy is 70%; the expected energy density is about 560 GJ/kg. An interplanetary transport system will consume a lot of fusion fuel and propellant, so it would appreciate getting to use cheaper options. Deuterium is a relatively abundant fusion fuel and it can be extracted from water anywhere in the Solar System. This could be a solar sail carrying deuterium off a comet resupply station. Each fuel pellet is covered in multiple layers of very thin reflective aluminium sheets, which serve as thermal insulation from sunlight, as well as a ‘harness’ made of plastic wires. That harness allows for clusters of pellets held inside a payload bay, and attached to large solar sails. These sails depart from Earth and dive down towards the Sun. A close pass allows for great acceleration and a trajectory that shoots back up to Earth with a relative velocity of about 100 km/s. They then drop the fuel pellets in a line, forming a boost track. Each solar sail can position these boost tracks with only a few months’ notice. It is more practical to send off multitudes of these sails, to create regular opportunities for travel, perhaps every week. After dropping off their payload, the solar sails can adjust their outwards trajectory to encounter a gas giant planet for a gravity assist back into the Solar System, and as they are dozens of times lighter than before, they can very slowly cancel out their velocity and return to Earth. The Fusion Highway itself is a 35 million km long track of fuel extending away from Earth, consisting of around 12 thousand fuel pellets. They are positioned in sections of perhaps 100 pellets by solar statites, which are solar sails large enough and lightweight enough to counter the Sun’s gravity and hold a position in interplanetary space indefinitely. Another 35 million km long segment leads up to the destination. The spaceship is a 711 ton vessel. It carries a 100 ton payload and 11 tons of target masses. 500 tons are dedicated to the propulsion system, including a magnetic nozzle that is only 50% efficient at converting the expanding plasma into thrust. Small spacecraft performing rapid trips to the Outer Planets and back. We set the power density of the propulsion system to 2 MW/kg (totalling 1 TW), which might seem excessive, but note that this is only a magnetic nozzle and very high temperature radiators, nothing else. It must not be compared directly with typical fusion rockets, who have to use heavy ignition equipment, power recovery cycles and lower temperature radiators. The remaining 100 tons consists of shielding, electrical equipment and comfortable living spaces. As mentioned before, the performance of the Fusion Highway depends on the velocity you ride it at. In the table below, we can see that the deuterium releases 280 GJ of useful energy, allowing for a plasma expansion velocity that is a rather constant 1058 km/s. The initial effective exhaust velocity is an impressive 479,420 km/s, dropping to 79,800 km/s at a relative velocity of 900 km/s. Here’s the performance table: Repeating the calculations for every 50 km/s increase in relative velocity allows us to calculate the necessary mass ratio required to accelerate across each 50 km/s step. To accelerate from 100 km/s to 150 km/s, the spaceship needs to expend about 147 kg of target masses. For the final 850 km/s to 900 km/s, it expends about 890 kg. The cumulative mass ratio for accelerating all the way from 100 km/s to 900 km/s involves multiplying the mass ratios of each step, for a final value of 1.008, or about 5.6 tons on top of the spaceship’s 700 ton dry mass. Here’s a table of mass flow, acceleration and displacement parameters for a spaceship limited to 1 TW riding this modest Fusion Highway: Acceleration increases over time because the exhaust velocity of the plasma decreases the faster the spaceship goes. For the same propulsive power, lower exhaust velocity translates into higher thrust. Because each 0.5 gram of target mass is matched with 0.5 kg of fuel pellets, we can say that accelerating up from 100 km/s to 900 km/s requires 5600 tons of fuel pellets. That’s 560 tons of deuterium and 5040 tons of frozen water. To slow down back to 100 km/s and with an additional margin on top, we used a mass ratio of 1.016, or about 11 tons of target masses. What sort of performance do we get out of this set-up? The spaceship has an initial acceleration of 0.6g. It takes 23.2 hours to complete its acceleration, with a peak acceleration at the final pellet of about 3.6g. It will cruise at 900 km/s, enough to get it from Earth to Jupiter in 10 days, or from Venus to Neptune in 2 months. As an interplanetary transport system, it does not require very advanced technology or huge amounts of rare fuels. It is rather easy to replenish the bulk of the fuel pellet material, and while the 35 million kilometre long tracks might seem excessive, they are only constellations of a few hundred satellites holding positions in interplanetary space. Today’s mega-constellations are far more complex! Replenishing thousands of tons of water and hundreds of tons of deuterium would be the bigger challenge, but there are a few months to accomplish that task while the boost track solar sails make their trip around the Sun. -Futuristic version For this second example, we use more optimistic assumptions and have no care for costs. We will use 10 kg fuel pellets that are 0.5% Deuterium and 0.5% Helium-3, surrounded by 99% frozen hydrogen. Burnup will be 25%, and the usable energy fraction is 95%, so each pellet is expected to release 8.38 TJ. We are smothering the fusion fuel in inert mass so that average energy density and therefore exhaust velocity is reduced, in favor of increasing thrust and acceleration. A large SDI-era railgun meant to shoot masses at many km/s. In the future, waiting around for months so that a booster track is ready might be inacceptable, as there will be a need for trips to be completed upon short notice. So, instead of propellant-free and very cheap solar sails, we use coilguns to shoot out a boost track. The coilguns will only need to achieve velocities of only a few km/s, but this will be sufficient (and energy/infrastructure costs remain low). The boost track will consist of fusion fuel pellets encapsulated in fissile material, such as Plutonium 239. Fission reactions can be ignited by high velocity impacts, and at much lower velocities than fusion reactions. A fission-fusion hybrid booster track will be very expensive, but it would mean that a spaceship can start impact ignition from an initial velocity of less than 1 km/s! Each fission-fusion pellet consists of 2 kg of Plutonium surrounding the 10 kg fusion fuel/hydrogen mix described above. They are struck by 1 kg frozen hydrogen target masses at multiple km/s. Average energy density after impact is 12.95 TJ/kg, so the plasma expands at 7090 km/s. The penalty from the retreating velocity is negligible. Effective exhaust velocity (1kg in, 13kg out at 80% efficiency) is 942,170 km/s. A mass ratio of just 1.004 is needed to accelerate from 0 to 200 km/s. The boost track would be about 3.6 million km long. The spaceship would accelerate at about 0.67g along this track and exit after 8.9 hours. The spaceship then switches from the booster track to the main Fusion Highway. It is going faster than what is strictly necessary to ignite an advanced fusion fuel pellet upon impact, but it will help enable the following setup: The Fusion Highway will consist of several lanes. The ‘high speed lane’ is composed of many 10kg fusion fuel pellets, intended to be consumed by large spacecraft trying to get to places quickly. Parallel to this are ‘service lanes’ that propel smaller ‘tender’ craft that replenish the high speed lane. A tender craft has its own magnetic nozzle and is loaded with fuel pellets. It accelerates up a service lane to 100 km/s, and then drops off the fuel pellets to replenish the high speed lane. This causes the high speed lane’s pellets to move outwards at 100 km/s. Since the spaceship coming off the booster track is travelling at 200 km/s, it can catch up to the moving high speed lane pellets at 100 km/s. Why have a moving lane? The Fusion Highway will have to be millions of kilometres long. Having tender craft travelling at 100 km/s means that its entire length can be replenished quickly. For example, a 1000 km/s Fusion Highway is 69 million km long, and the tender craft can get it ready for the next trip in about 8 days. A 5000 km/s Fusion Highway will be 495 million km long and be ready every 57 days. Even faster tender craft, and a longer booster track to catch up with them, would be necessary for the longest Fusion Highways. The tender craft can also correct the positions of the pellets that they have dropped on their return journey back up the service lane. Frequent resupply flights can provide near continuous adjustments to pellet positions. The spaceship we’ll use carries a 100 ton payload. It has a 1000 ton propulsion system that is 80% efficient and can handle 10 TW in the exhaust plasma. 100 tons are dedicated to other equipment, and 7.6 tons to target masses, adding up to a total mass upon departure of 1207.6 tons. The spaceship can choose to exit the Fusion Highway once it has achieved its desired velocity. This can range from 100 km/s to 12,000 km/s (in addition to the 100 km/s granted by the boost track). It has enough target masses to reach the maximum velocity listed in the table below, and slow back down again. Acceleration on the Fusion Highway starts off at 2.7g, peaking at 23.5g at the highest velocities. It is likely that pellets start getting skipped to reduce acceleration if there is a human crew onboard. If a middling velocity of 2000 km/s is deemed sufficient, then the spaceship needs to ride the Fusion Highway for 27.2 hours. The acceleration length is 148 million km. Departing from Earth, the spaceship can reach Saturn in just 7.4 days. The spaceship expends 704 kg of target masses altogether, matched by 7040 tons from both acceleration and braking tracks. Closer destinations are limited by the length of the Fusion Highway. A continuous line of pellets from Earth to Mars, if both planets are on the same side of the Sun, may span as little as 55 million km. In this case, we can treat the spaceship as a classical Torchship that maintains a constant acceleration and perform a Brachistochrone trajectory: accelerate up the mid-point and then slow down to a stop. With 2.7g of acceleration, such a short trip can be completed in 25 hours. But what if we want to blaze a trail across the Solar System at 12,000 km/s? The spaceship would need to spend 3 days on either end of the track, and the acceleration length becomes 8.34 AU long, so the minimum trip distance is 16.68 AU. One possible use for such a velocity is crossing from Saturn to Neptune if they were on opposite sides of the Sun… a 39.6 AU trip which could be completed in a mere 9.4 days from stop to stop. A total of 7.6 tons of target masses would be expended on the system-spanning dash, matched by 76,000 tons of fuel pellets, of which 380 tons is rare Helium 3. That would make it a pretty expensive endeavour for delivering just 100 tons of payload. Comparison with alternatives These performance figures stand out even more if we try to recreate them using alternative propulsion systems. Let’s work out how large a fusion rocket we would need, starting with the modest Fusion Highway example. Normally, fusion propulsion can manage to produce the same exhaust velocity as the expanding plasma within a Fusion Highway rider's magnetic nozzle. A deuterium-burning rocket would have a maximum exhaust velocity of 12,900 km/s or 4.3% of the speed of light (it’s the average velocity of the reaction products from a ‘naked’ reaction), so normally accelerating up to 900 km/s and back down again is no problem. However, needing to have an electricity generating loop and fusion ignition equipment would bring down the average power density of a realistic fusion rocket down to 300 kW/kg at best. The maximum average acceleration of a 300 kW/kg fusion rocket that aims to achieve 1800 km/s of deltaV is about 0.02g. This limit exists even if we increase the power of the propulsion system to 10 TW or even 100 TW. At this acceleration, it would take 53 days to reach the desired 900 km/s transit velocity, which is clearly insufficient. If we want the same trip times, acceleration must average 1.1g, which means that power density must be increased massively. This becomes unfair to the assumption made for magnetic nozzle the Fusion Highway rider uses... The traditional fusion-propelled spaceship will struggle to match a Fusion Highway rider's performance, and start to look like Project Daedalus-inspired designs. The futuristic Fusion Highway is even harder to match. To perform a 12,000 km/s dash, a total deltaV of 24,000 km/s is required. A ‘naked’ Deuterium-Helium3 fusion reaction manages an exhaust velocity of 26,700 km/s or 8.9% of the speed of light, therefore we would need a mass ratio of 2.45. If we insist on recreating the 4.63g average acceleration while having the propulsion system representing 99.9% of overall dry mass, then the fusion rocket would need a minimum power density of 2.1 GW/kg. It would deliver 420,480 TW of fusion power. If we add 100 tons of payload and 100 tons of other equipment, we get a 200,000 ton dry mass and a 490,000 ton wet mass. About 145,000 tons of rare Helium 3 would be needed to deliver the 100 ton payload, despite the unfair power density advantage this super-advanced fusion rocket has over the already futuristic magnetic nozzle of the Fusion Highway rider. Now let’s compare the Fusion Highway to the Laser Beamrider. We start with the modest Fusion Highway. Achieving the 1800 km/s deltaV is no problem for a laser-propelled sailcraft. Acceleration is instead the main challenge. A laser perfectly reflected by a mirror delivers 1 Newton per 150 MW. Accelerating at 1.1 g means that each kilogram of onboard mass is matched by 1.62 GW of beam power. But how much beam power can a sail really handle? Zubrin's ultra-thin aluminium sail. A simple solid aluminium sail, even with 90% reflectivity, can only survive a beam intensity of 86.8 kW/m^2 and if reduced to 30 nanometres thickness, the minimum thickness needed to achieve such a reflectivity, it would have an area mass of 81 milligrams per square meter, giving us a propulsion system with a power density of 1.07 GW/kg. It would provide an acceleration of only 0.36g, without payload. To accelerate at 1.1g, a very advanced laser sail design will be required. Jordin Kare proposes dielectric laser sails that can survive much higher beam intensities, but require nanoscale engineering across kilometres-wide surfaces. In one example provided, a sapphire sail that is 57 nanometres thick and able to operate at 1563 K can handle 34 MW/m^2 but only masses 226 milligrams per square meter. Alone, it can accelerate at 100g. Or put another way, 1 kg of this sail material can accelerate 89.9 kg of payload at 1.1g. 100 tons of payload and 100 tons of other equipment could be attached to 2.22 tons of sapphire laser sail. The sail would have a diameter of 3.56 km and receives 327.3 TW of beam power. It will be difficult to keep such a gigantic structure from collapsing under 1.1g acceleration. Solid state lasers would have an efficiency of about 60%, so the electrical input required to generate such a beam is a whopping 545 TW. That’s over 250 times more than the world’s entire electrical output today. A beam generator station would be needed at both departure and arrival ends of the spaceship’s trip, or something like a Laser Web is needed to relay the beam across interplanetary distances. It might be expensive. We can now try to estimate the laser sail performance needed to match the futuristic version of Fusion Highway. Acceleration rises to 4.63g on average. The 200 tons of payload and other equipment must sit at the center of a 7.22 km wide sapphire sail that masses 9.26 tons. It receives 1,425 TW of beam power, requiring perhaps 2,375 TW of electrical power... Consequences A Fusion Highway has some clear advantages over other methods of rapid interplanetary travel. It might not be as flexible as a rocket engine or as versatile as a beamed propulsion system, but it allows small, lightweight spacecraft to reach very high velocities with minimal use of expensive fuels or complex equipment. What multiple Fusion Highways waiting to be used might look like. It does require time to set up and replenish, but as described in the multi-lane futuristic example, the Fusion Highway can be used to replenish itself. Multiple departures in quick succession might have to be served by multiple Fusion Highways aimed in the same direction, while multiple travel windows would require Fusion Highways spaced radially along a departure point’s orbital path. These requirements suggest that busy travel routes would end up having many interconnected Highways, making the ‘road network’ analogy valid. Furthermore, it creates the possibility that small waystations in interplanetary space would have a useful role. A comet full of water and deuterium could replenish routes bringing spaceship to it, and since so little target masses are needed for a spaceship to ride a Fusion Highway, the opportunity cost for using them to visit different destinations is low. This could result in chains of smaller bodies, from moons to asteroids, that can be visited one after the other at low cost to the spaceship. It might be entirely possible to have ‘road trips’ with many stops in space, which would be interesting to scifi authors. Another interesting consequence is that the consumption of large quantities of water and the deuterium it contains would favor the occupation of icy moons and outer Solar System bodies. Interplanetary colonization tends to neglect these sites for their poverty in terms of metals, minerals and solar power. With Fusion Highways, they instead become abundant sources of fusion fuel that are easier to keep connected to a wider interplanetary network of Highways than a dry inner Solar System body like a metallic asteroid. Different users will demand different types of Fusion Highways. The bulk of transportation would be done with the cheapest ices and fusion fuels, which is why we often mention water and deuterium, but there is a performance edge to be gained from using Helium 3 fuels. Some spaceships will have smaller magnetic nozzles that cannot handle as much fusion power, while others will want to maximize acceleration. This suggests that there might be Fusion Highways with small, infrequent pellets, other faster tracks with large, frequent pellets, and even military routes held in reserve that have the highest quality fuels. Uranus and its moons might become an attractive destination. Finally, it is important to consider that Fusion Highways won’t operate alone. They are best served in combination with other propulsion systems, whether it is solar sails that resupply the Highways or independent rockets that can complete the ‘last mile’ of a delivery. It creates the possibility that the typical interplanetary spaceship is actually a multi-modal craft, which uses many propulsion systems that complement each other. For example, a magnetic nozzle and a few target masses are not a major burden to a fusion-propelled spaceship that can also deploy lightweight sails to ride a laser beam.
  9. A most excellent development. I assume this will include a Raptor analogue?
  10. The trouble is power density. Linear generators, as I mentioned in the post, are easy to install inside the spring arms, but they are much heavier for the power they deliver than a rotating generator, by a factor 10+. The issue is where to get that nuclear fuel. There's plenty on Earth's surface, and little anywhere else.
  11. Thanks for the explanation. I'm loving your mods by the way!
  12. I was doing a few calculations and I got some funny numbers. Something like the 'NEXT Ion Thruster - 0.625m' engine. It has an Isp of 6380s and a thrust of 2.1 kN. Engine power in the real world is equal to Isp * 9.81 * Thrust /2, so I get 65.7 MW of power. However, the part mass is only 200 kg. This means the power density is a whopping 328.6 kW/kg. This continues for all the electric thrusters. The 'VASIMR - 1.25m' engine has an Isp of 6000s and a thrust of 24.9 kN, meaning that it outputs 732.8 MW of power. Power density is at 732.8 kW/kg. For comparison, real world electric thrusters have power densities on the order of 0.5 kW/kg, rising to 2 kW/kg with a large design like VASIMR. This means the in-game propulsion is 160 to 360 times more powerful than modern technology allows. Is this the intended balancing for NFT, @Nertea?
  13. This is from the latest ToughSF blogpost: http://toughsf.blogspot.com/2021/01/moto-orion-mechanized-nuclear-pulse.html Moto-Orion: Mechanized Nuclear Pulse Propulsion The Orion nuclear pulse propulsion concept has been around for over six decades now. It is powerful and robust, but lacks the flexibility and features we expect from many more modern designs. Can we give it those additional capabilities? That cutaway is one of Matthew Paul Cushman’s amazing pieces. Basic overview of Orion William Black has plenty of great Orion artwork. There is a lot of information on Project Orion, available mostly here and here. It is best to read through them to gain a complete understanding of how it works. We’ll only give a simple overview to start. Project Orion’s design for a nuclear pulsed propulsion system was pretty simple. A physical plate of steel, protected with a thin layer of oil, faced a plasma jet from a nuclear shaped charge. The force of that blast was translated into useful thrust for the Orion spaceship. In this manner, a propulsion system could tap into the immense power of a nuclear detonation while sidestepping the heat management issues that would normally come from handling such an output. Its thrust was huge, enough to lift thousands of tons into orbit, and so was its efficiency, with an effective Isp of 2,000 to 12,000s. That’s five to thirty times the specific impulse of a chemical rocket, with thrust and efficiency that only gets better as you scale it up. We call this combination of high thrust and high efficiency a ‘torch drive’; a term from ‘Golden Age’ science fiction where authors did not want to spend pages explaining things like deltaV limits and interplanetary trajectories to their readers. A torch drive lets you point at your destination and accelerate to get there. Even today, sci-fi loves this solution. It did have drawbacks though. The fissile fuel in each nuclear pulse charge is inefficiently used, with the majority being wasted. This was because each pulse had to be small, so as to not obliterate the pusher plate, and therefore could not produce the better burnup ratios of large nuclear charges. The rate at which these pulses were ignited could not be varied by much either. Timing the pulses with the motion of the pusher plate, so that the blast would meet the suspension system in the right position, was essential. There were three parts to the suspension system. The first is the pusher plate itself. When struck at a precise angle, it could be accelerated at 50,000g or more without being bent or twisted. It first slams into a gas bag, that acts similar to how a car’s airbags are used in a car crash, to turn a sharp shock into a more gradual shove. Momentum from the plate is then transferred to a set of pistons at a much slower rate. These pistons are connected to rigid springs that convert the series of pushes into a continuous acceleration. When the timing is right, the literally well-oiled machinery is very strong. When the timing is off, things break down. The suspension cycle, in short. If one charge ignited too early, then only a fraction of the suspension length can be used to absorb the blast’s momentum, so it gets translated into a hard jolt. Ignited too late, and it would further accelerate an already retreating pusher plate, with potentially devastating consequences. A complete misfire isn’t great either. The suspension arms would only be partially compressed, and so would not reach full extension on the rebound and it would become unsafe to receive another nuclear blast. The Orion spaceship would have to wait for the suspension to wobble to a full stop, and then use a half-powered charge to restart it from a fully compressed state. Waiting to restart the suspension cycle isn’t a nice position to be in when launching off a planet. Another drawback was the inability to convert any of the nuclear pulse drive’s immense output into electrical power. The two-step suspension system simply acts as a fancy spring to transfer momentum between the nuclear blasts and the spaceship. Most of the time, this is not an issue. Liftoff from a planet or moon’s surface does not take long, so stored power is sufficient. Cost-efficient interplanetary travel consists of short uses of the main propulsion system followed by long periods of coasting, during which solar panels can be deployed. An Orion warship accelerating, from the sadly incomplete sequence here. However, some of the more demanding applications require a lot of onboard power. Military spaceships especially want the ability to both accelerate out of harm’s way, while producing plenty of electrical power to feed lasers, RADARs and other energy-intensive equipment. Fulfilling this requirement means sacrificing payload capacity to mount an onboard nuclear reactor or some other heavy solution. It’s also a problem for very fast transports that want to use the Orion engine as much as possible; they can only extend tiny solar panels while accelerating as anything bigger would get burnt off by the nuclear blasts. Of course, there are many other problems too, that we won’t go into more detail this time. The fact that each nuclear charge is a fully functional nuclear warhead, for example, means that a crash-landing would spill out a full nuclear arsenal, worthy of arming a superpower. Or that the main propulsion system of an Orion ship cannot be used to turn, so huge Reaction Control thrusters would be needed for every single maneuver. We cannot ignore the existence of more modern and more refined nuclear pulse propulsion designs either. Orion was dreamt up in the 1960s and a lot has happened since then. Mini Mag-Orion. Most notably, Mag-Orion and variants thereof. Instead of a physical pusher plate, a magnetic nozzle is used to capture the momentum of nuclear-generated plasma. Fully self-contained bombs are replaced by subcritical masses of uranium. They have to be detonated by external compression devices, such as a Z-pinch or a magnetic pulse. The result; they are completely safe in storage and gain a not-bomb-like-at-all quality. Generating electrical power is a simple repurposing of coils in a magnetic nozzle into Magnetohydrodynamic generators, and turning is accomplished by unequally deflecting the plasma within the nozzle one way or another. However, these more advanced designs cut away at the awesome potential of an Orion drive. The need for large magnets, cooling systems for the nozzle, capacitor banks for the ignition system, all add a lot of weight. Designs of this type have much lower thrust than the original Orion design. They can’t take off from large planets or even operate inside an atmosphere. They move away from that brutal, simple and resilient character that a nuclear Orion engine has, to become something flimsier and more complicated. Perhaps that is an unacceptable compromise, especially for someone seeking specific capabilities, or a sci-fi author aiming for a special aesthetic. Ad Astra Game's RocketPunk, seeking that aesthetic. Could we solve some of the original Orion’s most glaring drawbacks without moving too far away from the image of an atomic piston engine from a bygone era? Moto-Orion We alter the 60 year old design by giving it a crankshaft. It won’t be directly connected to the pusher plate - it can be connected behind the main suspension arms, so that it doesn’t have to receive the shock from a nuclear blast directly and become unreasonably long and heavy as a result. The crankshaft is connected to a crank that turns a large wheel. Depending on the pulse rate of the Orion drive, this wheel will turn at 54 to 69 RPM. A gear train would be needed to increase the RPMs into the thousands, suitable for an electric generator. Also necessary is a counter-torque mechanism, such as a second wheel or even just a counterweight turning in the opposite direction. Please note that the depiction in the diagram above isn't perfect, as all these mechanisms have to find a place in between the springs, hydraulics and other machinery above the suspension arms. A different arrangement would take up less room, but be harder to read visually. The concept is similar to a wind turbine and its generator, except the blades are replaced by a nuclear pulse-driven crank. The power that can be extracted through the crankshaft will be a fraction of the mechanical energy delivered through the Orion drive’s suspension. This is already a small percentage of the nuclear energy released by the pulse charges. The USAF design for a 10m diameter nuclear spaceship has a fantastic 32.9 GW output, but this is only 0.78% of the energy released by 1 kiloton yield blasts every second. We’ll call this the Motorized Orion or Moto-Orion. In practice, the electrical power that can be derived from an Orion drive will depend on the mass of the electrical generator and the equipment needed to manage waste heat. A high performance generator would have an efficiency of over 95% and a power density in the tens of kW/kg. Waste heat will be the main obstacle to generating a lot of electrical power, especially as electrical generators tend to operate at lower temperatures. As discussed in a previous post, temperature is the biggest factor in allowing for lightweight heat management systems. A generator would typically want to operate at room temperature 300K, but this would mean huge (and heavy) radiators would be needed to handle their waste heat. We want the hottest generators possible. They are mainly limited by the decreased performance of their electrical insulators at higher temperatures. Commercially available motors are available at 570K, but applying research like this could create generators that operate at 770K. However, increased temperatures also increase electrical resistance and therefore cut into the efficiency of a generator. Based on some studies, high temperature efficiency can be held at above 90%. A generator is a motor in reverse, so we will use these same temperatures and efficiencies. Estimating the power density of an entire heat management system is quite difficult, but we can make some estimates. 1 m^2 of double-sided 2mm thick carbon fibre radiator fins would be 4 kg and radiate away 8.3 kW of heat at 520K. Note how this is a slightly lower number than the operating temperature of the generators, as we need a temperature gradient throughout the heat management system to actually move heat from where it is created to where it is radiated away. With reasonable figures for a silicone oil pump, a microchannel heat exchanger and a +20% margin for assorted pipes, valves and backups, it all averages out to 1.2 kW/kg. This seems like a low figure, but it only deals with the <10% of power that becomes waste heat. 1 MW of mechanical energy coming through the crankshaft would become 900 kW of electricity, handled by 45 kg of generators, and 100 kW of waste heat, requiring around 83 kg of cooling equipment. Altogether, this makes for an average power density of 7 kW/kg. This ignores the mass of the crankshaft, counterweight and other mechanisms, but they will be small compared to the rest. There is also the complication of radiator placement; they want to extend out from the hull, but also must stay within the shadow cone of the pusher plate to avoid being disintegrated by nuclear plasma. The original USAF 10m Orion had a payload capability of up to 225 tons (on certain missions). If a quarter of this was dedicated just to producing electricity, we could expect it to output 393 MW. That is a respectable amount! Here’s what a Moto-Orion derived from that design, with fully scaled radiators, would look like: Though, it is only 1.2% of the drive power. You could imagine an Orion drive spaceship that extracts more of its output as electricity, but it is fundamentally limited by the difference between the power density of the propulsion system (on the order of 330 kW/kg) and that of the power extracting equipment (<10 kW/kg). Furthermore, equipment that consumes that electrical output will take up an outsized portion of the spaceship’s payload capacity, due to their even lower power density (<1 kW/kg). There are other ways to generate electrical power. A linear alternator should be an ideal option. A magnet is simply pushed through a series of conductive coils, producing current as it travels up and down. It is just as efficient as a rotating electric generator, and depending on the exact design used, can operate at the same high temperatures. Even better, it does not produce any sideways torque, is easier to fit in between the suspension arms and is more resilient to vibrations. However, their power density is far lower than that of rotating generators, with 1.49 kW/kg being the best figure mentioned anywhere. Another option still is to use a high temperature superconducting generator. NASA has designs that aim for 60 kW/kg at the multi-megawatt scale. Efficiency is 99%, meaning that 1% of the power becomes waste heat. Thankfully, this heat is produced not in the superconducting magnet, but in the non-superconducting stator. It can reach 570K, so we can use similar heat-management equipment as described above. 1 MW of input power becomes 990 kW of electricity and 10 kW of heat, which are handled respectively by 16.5 kg of generator and 8.3 kg of cooling equipment, for an average power density of 40 kW/kg. The downside to using superconducting devices is having to mount the bulky and sensitive equipment needed to keep them in that state. A high-temperature superconductor needs to be kept in liquid nitrogen, which boils at 77K. About 0.01 to 0.1% of the power that a superconducting device handles is expected to become waste heat inside the cryogenic part through ‘AC losses’, where alternating currents create magnetic vortices within a conductor. Progress is being made into megawatt scale superconducting generators/motors. This Honeywell 1 MW design achieves 8 kW/kg. The passive solution to handling this heat load is to just let the liquid nitrogen boil. It can absorb 198 kJ/kg during vaporization, so for every kW a superconducting generator outputs, 5 milligrams per second of liquid nitrogen needs to be expended. Using the expendable liquid nitrogen solution, we can have the USAF 10m Orion dedicate 40 tons to electrical production, and 16.25 tons to liquid nitrogen reserves (adding up to a quarter of its 225 ton payload, as before). It would be able to output a whopping 1.6 GW of electricity, but only for 33.5 minutes before liquid nitrogen reserves run out. It’s not too bad; the spaceship would likely run out of pulse charges before it uses up all this coolant. The active solution is to use a cryocooler. It raises the temperature of the waste heat to a level where it can be disposed of using radiator panels of reasonable size. If the high temperature superconducting material operates at 100K, then it takes at least 4.7 Watt of cryocooler power to move 1 Watt of waste heat up the temperature gradient to 570K. A realistic cryocooler will achieve 30% of maximum Carnot efficiency, so we increase the power requirement to 15.7 Watts. We choose the 570K temperature target to keep using the cooling equipment from previous calculations (all the better to compare each solution). Cryocooler power density for aerospace applications is about 133 W/kg, but 300 W/kg is cited as an achievable goal. Putting these elements together, we have 1 MW of input power becoming up to 1 kW of cryogenic waste heat, which requires 15.7 kW of cryocoolers that mass 52 kg. The active solution brings down average power density to 12.9 kW/kg. It is a respectable figure, better than the non-cryogenic design’s 7 kW/kg, and especially interesting for missions with prolonged engine use with no opportunity to refill on liquid nitrogen.. A USAF 10m Orion that used an actively cooled superconducting generator massing 56.25 tons would produce 725.6 MW as long as the engine is running. There is a ‘catch’ to these cryogenic designs though. Superconducting magnets are not known to be resistant to radiation or damage of any kind. It is especially concerning when a nuclear pulse propulsion spaceship bathes itself with penetrating neutrons and high energy gamma rays repeatedly. The magnets cannot be placed too far away from the pusher plate and suspension system either, so they can’t hide in the relatively safe environment the crew enjoys at the other end of the spaceship. Flexibility There are two other major benefits to the Moto-Orion. The first is during start-up. The original Orion design relied on the suspension system being pre-compressed before the first full-strength nuclear charge could be used. It was the job of a half-strength bomb to get the suspension ready. While this use of fissile material is not too wasteful when compared to the hundreds of bombs that are regularly used, it is very inflexible. Start-up would only be possible a limited number of times, and only when the pusher plate is standing still… not at all comforting when space travel involves must-not-miss burns. It is even worse for a warship that needs multiple successive starts and stops to effect dodges from enemy fire. A Moto-Orion can use its electric generator in reverse, to produce torque while consuming energy from battery reserves. It can draw in the suspension arms to a compressed position, or time its pushes and pulls to bring a wobbling plate to standstill more quickly. The batteries can even be charged from another power source, such as solar panels, if battery reserves are depleted. This gives the spaceship an unlimited number of restarts. It gains the flexibility to halt and ready its drive at any time. The second benefit is recovery after the pulse sequence goes wrong, whether it is late, early or missed completely. Accurate suspension cycle for an Orion craft, by ElukkaJ. A Moto-Orion might be able to react quickly enough to adjust the position of the suspension system in case of a late pulse. Once the nuclear shaped charge moves past its designated ignition point, the spaceship’s motors would draw power to slow down the retreating pusher plate. This could prevent it from being accelerated into the suspension arms at an excessive velocity. When things go wrong, unpleasant, up to destructive, g-forces are generated. An early detonation is especially troublesome. Not only does it erode the pusher plate, it cannot be predicted. The Moto-Orion’s crankshaft and generator can be turned into an additional suspension arm to absorb the unexpected shock, but it would usually be weaker than the massive steel springs the engine habitually relies upon. Still, it can assist in bringing the pusher plate velocity back in line and ready to receive nuclear plasma blasts again. When it comes to misfires, Moto-Orion can potentially add velocity to the slower pusher plate (as it did not receive the momentum from the missed pulse) and bring the drive sequence back into correct timing. It can avoid a complete halt by drawing energy from battery reserves, and if it is powerful enough, do so without skipping a beat. There are other forms of flexibility, gained indirectly from having access to huge amounts of electrical power. They might not be as flexible in this regard as a nuclear-electric ship could be, as power generation is tied to the use of the engine and not an independent reactor, but many possibilities open up. Orion nuclear spacecraft could deploy drones and beam power to them, by means of microwave emitters or laser beams. They could receive nuclear charges ‘on the fly’ using magnetic scoops. Electrical Reaction Control thrusters can be used, so that the spaceship can turn more efficiently. There are many more possibilities. Consequences An Orion spaceship staging off a aerobraking lander at Mars. Moto-Orions are safer and more flexible than the original Orions. For a simple transport ship that only uses its engines briefly and wishes to maximize payload, the extra weight is unwelcome. Any craft that carries people might instead find that the additional capabilities and securities are a worthwhile trade-off. Warships would absolutely desire Moto-Orions. The huge amounts of electrical power turn them into terrifying attackers that can both unload with weapons energized by hundreds of megawatts of power while also performing multi-g evasive maneuvers. In a science fiction setting, Moto-Orions can deliver the retrofuturistic aesthetic of spacecraft riding on nuclear blasts while also making possible the use of exciting hardware like lasers and coilguns. One setting, RocketPunk, is in development by Ad Astra Games (and by Rick Robinson, who inspired ToughSF). It features Orion-propelled warships battling for Mars in an alternate Cold War future. More engaging action could be made possible with these motorized variants. The fact that a Moto-Orion connects electrical output with drive power by a single-digit percentage ratio is an interesting feature by itself. We discussed how this avoids troublesome issues such as The Laser Problem, where overpowered lasers have excessive ranges and render maneuvering during ship-to-ship combat useless. Low electrical power and high drive power give room for dynamic combat that is more exciting for readers or viewers. Other types of ‘torch ship’, like a rocket with an immensely powerful fusion reactor, could have better performance than Moto-Orion, but would have proportionally more electrical power - this pushes combat ranges so far out that maneuvering is rendered pointless again. The military potential of Orion was always at the forefront. Another bonus towards dynamic and interesting space combat is an Orion drive’s ability to continuously accelerate and outrun missiles that have less potent propulsion systems. Due to how poorly nuclear pulse propulsion performs when scaled down (burnup ratio and thrust efficiency drop dramatically), a missile would not be able to keep up with a full-sized Orion drive unless it had its own large and expensive pulse propulsion system. They would be excessively expensive, so only smaller and less powerful engines would be available to missiles. Consequently, Orion warships have a good chance of outpacing missiles. It creates a situation where one side having more missiles than the other does not automatically guarantee a win. Instead, careful use of maneuvers and relative positioning to set up a shot with short-legged missiles is necessary. All the better to read about or play through! The Project Orion battleship. We suggest going out and applying these calculations to bring motorized variants to other Orion designs. Huge spacecraft like the 4000 ton USAF 'battleship' could benefit immensely from this concept. You could also think about how Medusa could extract electrical power from its tether strokes, or even more outlandish ideas, such as a propulsion system where high velocity kinetic impactors strike a lump of propellant to create a jet of plasma that strikes a pusher plate, like a non-nuclear Orion.
  14. Technically, it is infinite propellant and not infinite Isp. Antimatter definitely beats ion propulsion, but anything with less energy density than that cannot do so... The photon rocket will likely look like a solar sail, except glowing hot at 3000K+ on one side.
  15. This is the most recent post on ToughSF: http://toughsf.blogspot.com/2020/11/nuclear-photon-rockets-flashlights-to.html Nuclear Photon Rockets: Flashlights to the Stars In this post, we will have a look at the concept of using a nuclear photon rocket for interstellar travel. They are an old concept that should theoretically be the ultimate form of relativistic propulsion. However, today they are unknown or unpopular. Why might that be the case? The image above is by David A. Hardy. The interstellar challenge The Daedalus starship. Interstellar travel is on a completely different level than interplanetary travel. The distances involved are orders of magnitudes greater. The shortest distance between stars is measured in trillions of kilometres. To face such distances, high velocities are required. The closest stars. A robotic probe might not mind spending several centuries to reach a destination. A human crew would want the trip done in their lifetime. Taking longer than that means running into technical and ethical trouble. The closest star to our Sun is Alpha Centauri A, currently sitting 40 trillion kilometres away, or 4.2 light-years. It would take 4.2 years to reach it when travelling at the speed of light. If we want to complete the trip within 20 years, we would have to travel at 21% of the speed of light. We also want to slow down at the destination. This means that we need a way to accelerate up to 21% of the speed of light, and then slow down back to zero - the deltaV sum is 42% of the speed of light. So how do we go that fast? The Falcon 9's Merlin rocket engines. Rockets are the space propulsion system we are most experienced with. There are many ways to measure a rocket’s performance, but only some are relevant to interstellar travel. Thrust, for example, is much less important when the trip will take many years; taking one month to accelerate instead of ten months is no longer a significant factor. Instead, let’s focus on exhaust velocity. Using the Tsiolkovsky rocket equation, we can work out the ratio between propellant and non-propellant masses of the rocket we are using. Mass Ratio = e^(DeltaV/Exhaust Velocity) DeltaV in m/s Exhaust Velocity in m/s A chemical rocket consuming oxygen and hydrogen propellants has an exhaust velocity of 4,500m/s. We find that for a chemical rocket to achieve a deltaV of 42% of the speed of light, we would need e^28000 kilograms of fuel for each kilogram of equipment, structure, engines and payload. That is a number that lies between 10^8428 and 10^13359. For comparison, the entire mass of the Universe is estimated to be 10^53 kg. Chemical rockets for relativistic travel are beyond impractical. The needle array of Enpulsion's IFM nano thruster. How about a rocket engine with a better exhaust velocity? Something like one of our most efficient ion thrusters? The Ultra-FEEP thruster that accelerates liquid indium to nearly 1,000 km/s is the best we can expect for now. It would still not be enough for relativistic velocities. To achieve a deltaV of 42% of the speed of light, we would need 6*10^55 kg of indium for each kilogram of dry mass. If you run the numbers yourself and lower the deltaV target, you would still find ridiculously high mass ratios being required. A deltaV target of just 2% of the speed of light, which would turn the trip to the nearest star an endeavour that spans about half a millennium, would still require a physics-breaking mass ratio of 10^579 from the chemical rocket, and a mass ratio of 453 from the Ultra-FEEP thruster. The lower value for the electric thruster seems much more reasonable, until you consider that indium is found at a concentration of 0.21 ppm in Earth’s crust. At our current output of 700 tons per year, a 1,000 ton dry mass craft would require at least seven centuries of indium production to fill its propellant tanks. To get away from these extreme figures, a logical decision would be to increase the exhaust velocity all the way to the maximum. The maximum is the speed of light. Photon Propulsion When your exhaust is light itself, the mass ratios required for relativistic velocities become decidedly modest. Light, more specifically photons, can be produced indefinitely ‘out of nothing’. In other words, if you heat up a surface, you can create a photon rocket that spontaneously produces and emits light without ‘running out’ of anything. All that is required is a power source. The more energetic the power source, the more photons that can be produced and the higher the photon rocket’s performance. The theory fits together neatly. The concept of using a nuclear reactor to heat up a surface so that it emits enough photons to produce appreciable thrust is at least 50 years old. Nuclear photon rockets could solve our problem of interstellar travel by harnessing the greatest sources of energy and utilizing the exhaust with the highest velocity. All the fuel they would ever need would be loaded up at departure, so they do not have to rely on the existence of any infrastructure at the destination or any assistance along the way. Perhaps they would have enough to return to us without having to refuel! However, ‘photon starships’ are not a popular idea today. They are not featured in NASA’s NIAC programs, nor are aerospace engineers dreaming up modern designs for them. What ‘catch’ has them relegated to relics of the past? Fission Photon Rocket A nuclear photon rocket from Boeing's PARSECS study. Let us start with the most familiar of nuclear energy sources: the fission reactor. A fission reaction produces about 80 TeraJoules for each kilogram of maximally enriched fuel. 95% of this energy is in the form of gamma rays or fission fragments; they can be blocked by a thick wall and converted into heat. About 5% leaks out in the form of neutrinos. This reduces the ‘useful’ energy density of fission fuel to 76 TJ/kg. In a typical reactor, the fuel is in solid form. Only a fraction of its potential 76 TJ/kg can be extracted in one fuel cycle. The products of fission, such as xenon-135 and samarium-149, remain trapped next to the fuel. These isotopes have a high neutron cross-section, which means that they trap and absorb the neutrons needed to sustain a fission reaction. Nuclear engineers consider these products to be ‘poisons’. If enough poisons accumulate in the fuel, the fission reaction cannot be sustained. The result is that a single fuel cycle achieves very low burnup of the fuel, which is the percentage of fissile fuel that has undergone fission. Typically, this is 1% to 5% of the total fuel load inside a reactor. On Earth, nuclear engineers deal with this problem by shutting down a reactor, extracting the slightly used fuel and sending it off for reprocessing. This involves removing the poisons, mixing in a small quantity of fresh fuel, and then returning it all to the reactor. A spaceship does not have the luxury of regularly halting its reactor while also lugging around a nuclear fuel reprocessing facility. Instead, we need to use a type of reactor that grants high burnup with no reprocessing necessary. The best option seems to be a gas-core nuclear reactor. In this high temperature design, the fuel and poisons are in a gas phase. It becomes easy to filter out the poisons as they are chemically very different from the fuel. We can have the fuel circulate within the core for as long as needed to achieve near 100% burnup. With the burnup problem solved, we can convert those 76 TJ/kg into heat. From a physics perspective, only about 0.77 grams of matter in a kilogram of fissile fuel becomes energy. This leaves us with 999.23 grams of waste after consuming the fuel. With no further use for it, we eject it to lighten the spacecraft. Imagine a nuclear starship designed specifically to make our next calculations easier. It consumes 1 kg of fuel per second. The average power output is 76 Terawatts. Thrust = 2 * Power/ Exhaust Velocity Thrust will be given in Newtons Power is in Watts Exhaust Velocity in m/s Those 76 Terawatts should result in 506.6 kiloNewtons of thrust. With a 95% efficient photon emitter, we gain a real thrust of 481.3 kN. After producing this thrust, we eject 999.25 grams of waste. Effective Exhaust Velocity = Thrust / Mass Rate Effective Exhaust Velocity will be given in m/s Thrust is in Newtons Mass Rate is in kg/s The ‘effective exhaust velocity’ based on this thrust and the amount of matter being ejected is actually 481.7 km/s. The critical point we make here is that while the thrust comes from photons travelling at the speed of light, exhaust velocity calculations must take into account all the masses being ejected. So what can a fission photon rocket do with an effective exhaust velocity of 481.7 km/s? It certainly cannot reach our desired deltaV. Achieving 42% of the speed of light would require a mass ratio of 10^113. Unless we have access to multiple Universes filled with highly enriched fissile fuel, this is impractical. Even with an extraordinary feat of engineering so that we could load a starship with 100 kg of nuclear fuel for each 1 kg of dry mass (and not have it immediately go critical), the achievable deltaV is only 2,218 km/s or 0.74% of the speed of light. Fusion Photon Rocket What if we used the better nuclear rocket: the fusion rocket? There are many different fusion reactions involving different fuels, but we are interested in those that provide the highest energy density. Proton-proton fusion provides a whopping 664 TJ/kg. However, it is very slow, taking thousands of years to complete, and it is not realistic to ever expect to take place outside of stellar cores. Next down the list is Deuterium-Helium3. About 353 TJ/kg is on tap. We won’t dive into the details of the various reactor designs that could be used, but suffice to say that near-complete burnup of fusion fuels is possible, and all the energy released can be converted into heat. If we compare the mass of the Deuterium and Helium 3 before fusing, with the mass of the helium and proton particles after fusion, we notice that 0.39% of the mass is missing. That is the percentage of mass converted into pure energy. It is a much greater percentage than nuclear fissions’ 0.077%. The list of particles involved in fusion reactions, with their exact masses. Let’s repeat the previous calculation for the effective exhaust velocity of a nuclear photon rocket. 1 kg/s of fusion fuels are consumed, for a power output of 353 TW. This produces 2,235.6 kN of thrust out of a 95% efficient emitter. We expel 996.1 grams per second of waste, so the effective exhaust velocity is 2,244.4 km/s. This is nearly five times than a fission photon rocket’s effective exhaust velocity. However, this is still not enough. Our desired deltaV of 42% of the speed of light comes at the cost of a mass ratio of 2.4*10^24. While we could gather enough galaxies together to fuel our fusion photon rocket, we want something more practical. The reality is that a plausible fusion photon rocket with a mass ratio of 100 would only have a deltaV of 10,335 km/s or 3.4% of the speed of light. Barely enough for a multi-century generation ship to cross the stars and certainly not enough for travel within a lifetime. Staging the fusion rocket will not help very much. Also notable is the fact that an effective exhaust velocity of 3.4% of the speed of light is actually lower than the exhaust velocity of direct drive fusion propulsion, where charged particles are directly released into space through a magnetic nozzle. DHe3 releases a 3.6 MeV helium ion and a 14.7 MeV proton. Their averaged velocity is 7% of the speed of light. A photon rocket is a very inefficient use of fusion energy. Antimatter Photon Rocket The ultimate fuel should give the ultimate performance. Nothing beats antimatter! There are many types of antimatter. There are antielectrons, antiprotons, antineutrons and their combined form, anti-atoms like antihydrogen. Antielectrons annihilate with regular electrons in a ‘clean’ annihilation reaction that produces high energy gamma rays and nothing else. They are however the hardest type to store. Antiprotons are much easier to store, especially in the form of frozen antihydrogen ice. The downside is that their annihilation is ‘messy’, as it releases a plethora of products. With solid shielding, enough of the energy of those multiple products can be absorbed and converted into heat. We set the efficiency at 85%. Each kilogram of antimatter contains a potential for 90,000 Terajoules of energy. It must be matched by another kilogram of regular matter, so the average energy density is halved to 45,000 TJ/kg. As we only capture 85% of that amount, the useful energy density is 38,250 TJ/kg. If we consume one kilogram of antimatter/matter mix per second, we would have a drive power of 38,250 TW. A realistic emitter would convert this into 242,250 kN of photon thrust. The effective exhaust velocity is 242,250 km/s or 81% of the speed of light. With such a high exhaust velocity, an antimatter photon rocket would be able to achieve the relativistic velocities we desire. A deltaV of 42% of the speed of light would only require a mass ratio of 1.68. That’s 0.68 kg of antimatter/matter mix for each 1 kg of rocket dry mass. We might even be able to go much faster with high mass ratios; travel times to the stars in single-digit years seems possible. However, antimatter is exceedingly difficult to collect or create. A mass ratio that seems acceptable for a conventional rocket would actually imply an unreasonable amount of antimatter. Existing accelerator facilities, if tasked with solely producing antimatter, would require about 3.6 ZettaJoules to produce 1 kilogram of antimatter. That’s 3,600,000,000 TeraJoules, equivalent to 286 times the total yield of all nuclear bombs today (1.25*10^19 J), or the total output of the United States’ electrical grid (1.5*10^19 J) for the next 240 years. If we were very serious about producing large quantities of antimatter, we could design a superbly optimized antimatter production facility, with very efficient antimatter capture mechanisms. Production efficiency can be increased to 0.025%. This means that 1 kg of antimatter would require ‘only’ 360,000 TJ to manufacture. An antimatter photon starship would ‘just’ need the combined output of all humanity (8*10^19 J/yr) for the next couple of millennia to fill it up. An antimatter production facility. In practice, the awesome performance of antimatter propulsion would be reserved for civilizations higher up the Kardashev Scale. Verdict and Consequences All the calculations so far have assumed nearly perfect use of the energy released by fission, fusion or antimatter reactions. We have also ignored the massive complications that arise from trying to handle the power of those reactions. Despite this best case scenario, nuclear photon rockets do not seem to be up to the task of rapid interstellar travel. Fission and fusion power are just not energy dense enough. Antimatter is far too difficult to produce in huge quantities. The ‘catch’ is that physics is not kind to photon propulsion. For this reason, this sort of starship will remain a bottom-drawer concept for the foreseeable future. What effect does that conclusion have? If we want to use rockets, we must accept that interstellar travel will be slow. Other techniques or technologies have to be employed to make crossings that last centuries. Cryogenic hibernation, life extension or digitizing the mind can enable the original crew to survive that long; generation ships or embryo seeding can allow another group of people to arrive at the destination. Robert L. Forward's Laser-propelled lightsails. If we instead want interstellar travel done quickly, we cannot rely on rockets. All the popular methods for interstellar travel depend on non-rocket propulsion, such as Robert L. Forward’s massive laser-propelled sails or the ‘bomb-tracks’ discussed in a previous post. The energy cost of relativistic travel is no longer derived from a fuel carried onboard a starship, but from an external source. This external source takes the form of large infrastructure projects and preparations that require many years to complete; we trade away the flexibility and autonomy of rockets to gain huge speed, efficiency and cost advantages. A consequence of non-rocket propulsion is that interstellar travel cannot be a whimsical affair. It has to be planned a long time in advance (which has implications for the stability of the civilization organizing it all) and it would be evident to all observers at the departure and destination what is going on. No ‘secret’ missions to other stars! Of course, a scifi writer might not like the sound of that. Their options lie in more exotic types of rockets, more advanced civilizations or speculative science. Examples of exotic rockets include a starship powered by a rotating black hole, where matter is converted into energy at 42% efficiency (an effective exhaust velocity of up to 252,000 km/s or 84% of the speed of light) or a Ram-Augmented Interstellar Ramjet, where the thin interstellar medium is added to the exhaust of a fusion reactor for a greatly improved effective exhaust velocity. More advanced civilizations handle enough energy to be able to produce large quantities of antimatter, overcoming the main difficulty with this fantastic fuel. Speculative science opens up the possibility of using ‘quark nuggets’ to rapidly and easily create antimatter, as well as wormholes and Alcubierre warp drives. Though, we must warn you, that these different options might be more troublesome than photon rockets!
  16. @Terwin Both the tether tip and the payload can extend their own cables with maneuvrable drones on each end to prolong the rendezvous. Thing of it like a 'grappling hook' that aims itself at the main tether. You can also have a tether station with multiple tethers, so you have three or four tether tips to choose from in the space of a few minutes instead of a single pass. The ultimate evolution of this is the bicycle-wheel configuration, where a very large number of 'spokes' carry a circular rim. Alternatively, you can have multiple whole tether stations, separated by a few minutes on their orbital path, giving you many more opportunities to rendezvous.
  17. From the original post: https://toughsf.blogspot.com/2020/07/tethers-all-way.html Space Tethers: Stringing up the Solar System All the methods we have used to reach space so far have been subject to the Tsiolkovsky rocket equation - propellant must be ejected and more and more of it is needed to go further. Art above is by Jullius Granada. What if we could break that equation with rotating orbital tethers? The tether I have worked with Kurzgesagt to write the following video on the topic of this post: https://www.youtube.com/watch?v=dqwpQarrDwk. It is highly recommended that you have watched it first before continuing, as it is an excellent introduction and explanation of momentum exchange tethers. The simple description is that a rotating tether, consisting of a strong cable with an attachment point at the tip and an anchoring counterweight at the center, will be able to catch and throw payloads without requiring a rocket engine. The process of hooking onto a payload to accelerate it into a new trajectory will transfer momentum from the counterweight to the payload, causing the tether to slow down. In reverse, a payload can be caught and slowed down, transferring momentum back into the counterweight and speeding it up. The ability to transfer momentum back and forth is why these structures are also called momentum exchange tethers. NASA has long studied this option. In this post, we will go into more detail on what is needed to create a functional rotating tether, how it can be used and what its potential effects are on space travel and industry could be. The mechanics Using a tether to move from one orbit to another, in this case LEO to GEO. The idea to use a long tether to climb into space without expelling propellant is an old idea. A huge tower extending up past the atmosphere was described by Tsiolkovsky. It is ironic that the person who first described how hard spaceflight by rocket is, due to the exponential nature of the deltaV equation, is also the person who described the best way to side-step that problem with non-rocket launch. The material requirements for a full space elevator are extreme. The only practical way to build it would be to use carbon nanomaterials, but extended to a scale of multiple kilometres instead of the micrometres we struggle to produce consistently in a laboratory today. It is why we must turn to something that provides some of the same benefits without the same stringent requirements. For this, there is the orbital tether concept. A large object in orbit, such as a satellite, space station, captured asteroid or similar, can serve as an anchor point to extend a strong cable down to a lower altitude. A payload can grab onto the lower end of the cable and climb up to the altitude of the anchor point. This climb does not require the use of propellant. The simplest design is a stationary orbiting elevator that provides a deltaV benefit based on the difference in orbital velocities at high and low altitudes. An LEO to GEO elevator. In the example above, an space station orbits at 2,000 km altitude, at an orbital velocity of 6.89 km/s. It performs one orbit in about 2 hours and 7 minutes. The lower tip extends down to an altitude of 200 km. It retains its orbital period but the distance it travels is much less, so velocity is reduced to 5.41 km/s. A circular orbit here is 7.78 km/s, so it provides a 2.37 km/s saving. The upper tip reaches up to 3,860 km altitude. It covers much more distance with the same orbital period, so velocity increases to 8.43 km/s, compared to the 6.24 km/s of everything else orbiting at that altitude. It is a 2.19 km/s boost. In total, we get a 4.56 km/s benefit. Huge altitude differences are needed to create the potential for significant deltaV savings. Because the lower tip of the tether is travelling at orbital velocity, it cannot extend too far down either; as it would encounter the atmosphere and burn up. A tether boost facility designed to be launched from a DeltaIV. A rotating tether does away with those limitations. The velocity of its tips and the speeds at which it can capture or release payloads can vary greatly from the orbital velocity of the anchor point. It can be much shorter too. At its lowest point, the tip of a rotating tether will be travelling at orbital velocity minus the rotation velocity. At its highest point, the two velocities will add up. The length of the tether itself will place the tips at very different altitudes at their highest and lowest points. Moving a payload between these altitudes is an additional benefit. Let’s imagine a modestly-sized tether orbiting at a high altitude above the Earth. It is 1,000 km long, orbiting at 1,100 km altitude and rotating once every 70 minutes. Its lowest point is 100 km above the surface of the Earth. Its highest point extends to an altitude of 2,100 km. Tip velocity is 1.5 km/s. It is tapered from base to tip to minimize its mass. Tapered tethers are the lightest design. Orbital velocity at 1,100 km is 7.3 km/s. At its lowest point, the tether tip will be travelling at 5.8 km/s relative to the ground. At its highest point, this value becomes 8.8 km/s. If a suborbital craft launched from the ground to try to catch up with the tip at its lowest and slowest, it would need to expend a deltaV of about 6.8 km/s. It can then quickly transfer a payload onto the tether. The payload then starts its 35 minute journey up around to the opposite end of the tether. It experiences an average acceleration of 0.23 g while doing so. At the top of the tether, it is released into a trajectory that forms an ellipse with its periapsis at 2,100 km altitude and its apoapsis at 13,500 km. It can then expend an additional 1.4 km/s of deltaV to reach the Moon, or about 1.6 km/s to escape the Earth entirely. If a typical 350s Isp kerosene-oxygen rocket is used, then it needs a total deltaV of about 8.2 km/s to ride the tether to the Moon. Meaning, it has an overall mass ratio of 10.9. However, if there is no tether available, then the deltaV requirement rises to 12.5 km/s and the mass ratio required balloons to 38! The tether is effectively saving 4.3 km/s of deltaV and leading to a much smaller rocket. The tether can also help with returning from the Moon. The spacecraft swoops down from lunar altitude (384,400 km) to a rendezvous with the tether at 2,100 km altitude. It would be travelling at 9.6 km/s, so it needs to spend an additional 0.8 km/s of deltaV to slow down enough to match the 8.8 km/s velocity of the tether's upper tip. In return, it avoids having to slam into the atmosphere and instead is swung down for much gentler aerobraking. The weight savings from having a thinner heatshield could more than make up for the propellant consumed, especially if this is a reusable vessel. Note that tethers do not have a single velocity for catching and releasing payloads. It is in fact a range of velocities, from zero up to the tip velocity. For capture at lower velocities, a payload can aim to intercept the tether at a point closer to the base of the tether. Halfway up the tether means a redezvous at half the tip velocity. The same goes for release; not releasing from the tether tip means a lower velocity. You can imagine a vehicle launching up from the ground to catch the tether tip at its lowest point, and instead of swinging around to the other side, just slowly climbing up the tether until it can hop off from the anchor station. This puts it in an orbit parallel to the anchor station, which is great if you are not trying to fly off to the Moon or beyond. However, making use of this flexibility means adding a way to prevent the unused length of the tether from striking the payloads coming in for a rendezvous, as well as providing structures that allow payloads to climb up and down the tether (although they can be as simple as a pulley and cables). The ISS regularly is regularly reboosted against the effects of drag. And of course, none of these deltaV savings are for ‘free’. Accelerating payloads means the tether will slow down. If it slows down too much, it will de-orbit itself. The momentum lost with each catch-and-release operation must be recovered either by absorbing momentum from payloads being slowed down, or by using its own propulsion system. A major advantage of an orbital tether is that you do not have to immediately recover that momentum - it gives time for slower but more efficient propulsion systems like a solar-electric thruster to gradually accelerate the tether. A chemical propulsion system limited to 450s of Isp is not needed as the acceleration can be done over time with something that has thousands of seconds of Isp. The propellant needed to run the tether’s engines is greatly reduced. Even more interesting is the possibility of propellantless propulsion, such as electrodynamic tethers that push off the magnetic fields around a planet. Electrodynamic tether reboost. Another advantage is that the tether can ‘store’ excess momentum. It can accelerate itself to a more energetic orbit with a higher velocity. For example, a tether in a 2,000x2,000 km circular orbit could accelerate by 1 km/s to reach a 2,000x9,565 km orbit. It can still capture payloads at the same 2,000 km altitude, but it will have an additional 1 km/s of velocity to use. The extra velocity can be used to accelerate the same payloads faster, more numerous payloads to the same speeds or larger payloads than possible before. Tether masses and velocities Tether structure and materials for the early TSS-1 experiment in orbit. The tether materials determine how fast the tips can rotate. Each material has a certain characteristic velocity, given by: Characteristic velocity = (2 * Tensile Strength / Density)^0.5 Characteristic velocity is in metres per second. Tensile Strength is in Pascals. Density is in kg/m^3. Steel is strong, with a maximal strength of 2,160 MPa for AerMet 340, but dense, at 7,860 kg/m^3. This gives it a characteristic velocity of 741 m/s. The aramid fiber Kevlar is stronger and lighter, managing 3,620 MPa with 1,440 kg/m^3. Its characteristic velocity is 2242 m/s. The strongest material we can mass-produce today is Toray’s polyacrylonitrile fiber T1100G. It can resist 7,000 MPa while having a density of 1,790 kg/m^3, so its characteristic velocity is 2,796 m/s. If we can describe the tip velocity as a multiple of the characteristic velocity, then we can use a much simpler equation to work out how much a tether will mass. We’ll call this the Velocity Ratio or VR. For example, 1.5 km/s is a VR of 2.02 for steel but only a VR of 0.54 for T1100G. The tether mass will be directly proportional to the payload mass. If it has to pull up a 1 ton payload, it will be ten times heavier than if it only needs to pull on 100 kg payloads. Using the VR, we can calculate the tether mass ratio using this equation: Tether Mass Ratio = 1.772 * VR * e^(VR^2) Tether Mass Ratio is a multiple of the payload mass, in kg. VR is the Velocity Ratio. Using the previous example, a 1.5 km/s steel tether will have to be 211.8 times heavier than its payload. A T1100G tether would only be 1.28 times heavier than its payload. This is a significant difference. The e^(VR^2) portion of the tether mass equation highlights just how important it is to use strong yet lightweight materials and to keep the tip velocity close to the characteristic velocity. Here is a graph showing how tether mass increases with the Velocity Ratio for different materials: It should be noted that all of these calculations are for a tether with no safety margins. Any sort of variation, such as vibrations from the counterweight or an imperfect capture of the payload, would snap it. A minimal safety margin might be 50%. Crewed spacecraft might demand a 200% margin or more. What this means in practice is that the maximum payload the tether could handle is reduced to create a safety margin. To overcome the limitations of the tether tip velocities, the tether can move into higher energy orbits. For example, a tether with a 1.5 km/s tip speed starts off in a circular 2000 km altitude orbit moves itself into a 2,000x1,000,000 km orbit. It can still capture payloads at the same altitude but it now does so at a velocity of 9,391 m/s instead of 6,897 m/s. This gives it 36% more momentum to give, and it can release payloads at a velocity of up to 10,891 m/s relative to the Earth. This is beyond the escape velocity at that altitude! If the tether had stuck to its initial 2000km circular orbit, its tip speed would have had to be 4 km/s instead, which would have meant an exponentially higher mass ratio. As the tether collects and releases payloads, it must adjust the distribution of its mass to maintain its center of rotation. Adjusting the tether with a moving counterweight on a 'crawler'. This can be done by shifting the counterweight, moving additional masses up and down the tether, changing the length of the tether using motors and/or having a dynamic suspension system that also helps dampen vibrations. In later sections, we will go through the various ways tethers can be used and combined to cover the entire Solar Systebm. Skyhook The skyhook process from Hoyt. The most immediately beneficial application of an orbital tether is the form of a Skyhook. This is a well-studied concept that dips the tether tip as low and slow as possible into the upper atmosphere, so that a suborbital craft can catch up to it, rendezvous, transfer a payload and then fly away. Getting off Earth and into orbit is a massive task. It requires that over 9 km/s of deltaV be delivered in one chunk, by a high thrust propulsion system. Chemical rockets can do this, but they end up as balloons of fuel with a small payload on the tip. A skyhook can help reduce deltaV requirements where they are hardest to deliver: at the end of a tiresome fight against Earth’s gravity. Because of the exponential nature of the Tsiolkovsky rocket equation, the last 1 km/s of deltaV costs much more than the first 1 km/s. The savings enabled by a Skyhook are therefore disproportionately high. Imagine a 200 km long tether anchored to a station orbiting at 400 km altitude. Its tip speed is 2.4 km/s. This means it travels over the ground at 5.3 km/s at its lowest point, and swings above at 10.1 km/s. A rocket trying to catch up with this tether at its lowest point must deliver 5.3 km/s of horizontal velocity, but also about 1.5 km/s to reach a 200 km altitude as well as make up for drag and gravity losses on the way up. Its deltaV requirement becomes 6.8 km/s. With kerosene and oxygen propellants delivering an average Isp of 330s, it would need a mass ratio of 8.17. This is well within the reach of a single-stage vehicle, even with margins to return and land vertically for reuse. For comparison, a kerosene/oxygen-fuelled vehicle that must make orbit would need 9.5 km/s and a mass ratio of 18.8. It would need multiple stages and it would be difficult to create deltaV margins for recovery. The tether-assisted rocket is 2.76 times smaller and lighter for the same payload! But that’s not all. The tether swings around and launches its payload into a 400 x 35,800 km orbit. This is also known as a geostationary transfer orbit (GTO) - an orbit where a rocket would only need an extra 1.5 km/s to turn into a 35,800 x 35,800 km geostationary orbit. The tether’s top-side boost is worth another 2.4 km/s. If it has to be delivered by the same vehicle that must reach orbit on its own, deltaV requirements would add up to 11.9 km/s. With 330s Isp propulsion, this means a staggering mass ratio of 39.5. Modern rockets get around this by fitting their upper stages with more efficient rocket engines, but they still take a huge hit to their payload capabilities when launching to GTO instead of LEO. ULA’s Delta IV Heavy could launch 28 tons into LEO but only 14 tons into GTO. We could do better. A faster tether that dips deeper into the atmosphere is possible, further reducing the deltaV requirements for meeting it and reducing the constraints on the vehicle we use. HASTOL. We don't really want the plane to exit the atmosphere. The lowest a tether tip could reasonably go is 50 km in altitude, making it 200 km long if it orbited at 250 km altitude. It could be pushed up to 6 km/s in tip speed, bringing its tip to a mere 1.7 km/s relative to the ground at its lowest point and to 13.7 km/s at its highest point. We can call this design a ‘Hypertether', inspired by works like HASTOL. 1.7 km/s corresponds to Mach 5 at this altitude. We have had aircraft reach these speeds and altitudes for decades, under rocket power. We have developed hypersonic scramjets that can sustain these speeds much more efficiently too. A large aircraft could meet a Hypertether using existing technologies reliably, without needing a lot of propellant or excessive thermal shielding. The exponential mass ratios that make rockets so expensive no longer come into play. Hypersonic rendezvous vehicles could climb to this altitude using engines with Isp exceeding 4000s (using hydrogen fuel), fly long enough to attempt multiple rendezvous with the tether (one attempt per tether rotation period) and land, ready to fly again within the hour. The downside to this approach is that the mass ratio of the tether itself becomes unwieldy. At 6 km/s, even T1100G tethers require a mass ratio of 379. The result is huge tethers in orbit needed to handle even the smallest of payloads. With a 200% safety margin, a 1 ton payload would need a 758 ton tether in these conditions. Launching such a mass into space and fitting it with an appropriately sized counterweight and anchor point would require hundreds of launches to break-even with the cost. A staged tether can get around some of these difficulties. Just like a rocket, a tether can be broken up into stages. Each stage uses the tip of the previous tether as its anchor point. If two 3 km/s tethers are staged, then they could achieve a combined 6 km/s tip velocity. However, each stage only needs a mass ratio of 6, with T1100G. A 1 ton payload would need 1x6: 6 ton first stage tether and a (1+6)x6 : 42 ton second stage tether. Add a 200% safety margin and it would still be an overall mass of 84 tons, which is much lower than the previous 758 tons for a single tether. Many difficulties must be overcome with this design. The first is the need to absorb any lateral movement which could cause tether sections to run into each other. The second is to create a stable joint that can operate under huge stresses. Using some of the mass savings from a staged tether design to alleviate these problems is recommended. Finally, each tether stage will be relatively short, leading to high centrifugal forces being imposed. If a 200 km long tether is divided into two 100 km sections, each rotating at 3 km/s, then payloads would be subjected to an acceleration varying between 9 and 18g. Much longer tethers would be needed for human travellers. Overcoming these difficulties would yield a flexible Hypertether with exceptional performance but low mass. A huge, slowly rotating skyhook would not look much different from a section of space elevator near the ground. The ideal skyhook, as originally conceived for science fiction, uses multiple stages so that its combined tether tip velocity matches its orbital velocity. It would become stationary relative to the ground with each rotation. This means a combined 7.7 km/s for a tether orbiting at 250 km altitude. No rendezvous vehicle is needed; payloads would simply sit on the ground and latch onto the descending hook from the sky. A huge number of additional challenges face this ‘perfect Skyhook’ design, ranging from the need to prevent unpredictable air turbulence from smashing tether stages into each other, to needing thermal protection for tethers that accelerate to multiple km/s while coming up through the thickest portions of the atmosphere. High performance skyhooks around Earth will mostly aim to lift payloads up from the ground and out into space. They are likely to run at a permanent momentum deficit; propulsion is essential. Obtaining propellant is an obstacle, as are the power requirements. The simplest solution is to sacrifice a portion of each payload using the tether to carry propellant. Low performance tethers that sit at high altitudes and with low tip velocities will make this a very expensive option. This is because they make rendezvous vehicles work hard to get to them. If a 1,000 ton tether station accelerated a 3 ton payload by 3 km/s from rendezvous to release, it would lose 9 m/s itself. Accelerating 1000 tons by 9 m/s using a 3,000s Isp engine requires about 305 kg of propellant. This means that, roughly, for every 9 payloads accelerated by the tether, a 10th launch is needed for refuelling. High performance tethers have it worse. They lose more momentum proportionally with each payload they accept, because of their higher tip speeds. Accelerating a 3 ton payload by 12 km/s slows down a 1,000 ton tether by 36 m/s, requiring 1,223 kg of propellant to recover! Thankfully, they make travel to space so much cheaper that sacrificing every third payload for propellant still makes for an overall saving over rockets. Extraterrestrial sources of propellant can be much more interesting. It normally takes less deltaV to move propellant from the Moon to LEO than it takes to move it up from the ground to LEO, at about 5.8 km/s vs 9.5 km/s. With aerobraking, the deltaV required to return from the Moon’s surface to Earth orbit is reduced to 2.8 km/s. Lunar sources of propellant remain interesting even when we adjust the deltaV requirements to account for the tether helping out. A tether with 3.1 km/s tip velocity would reduce the deltaV needed to lift off from Earth’s surface and enter into Low Earth Orbit to 6.4 km/s. It would also reduce the deltaV needed for a spaceship to launch off the Moon and enter Low Earth Orbit to 2.8 km/s. This keeps lunar sources of propellant the better option over terrestrial sources. Another advantage of extraterrestrial propellants for tethers is that capturing them ‘recharges’ the momentum of the tether. Catching 1 ton of propellant coming in at 3 km/s would accelerate a 1,000 ton tether by 3 m/s. Using that propellant for a 3,000s Isp thruster would further accelerate it by 29 m/s. That propellant is worth 10% more than expected! The absolute best propellant source of Skyhooks around Earth is the atmosphere itself. Atmospheric gas scooping is discussed in full detail here. A tether tip dipping into the atmosphere can ‘cheat’ the gas scooping retention equation by collecting gases at a lower velocity than the tether station’s orbital velocity. For example, a tether at 250 km altitude rotating at 3 km/s would collect gases at a velocity of 4.7 km/s. If a 3000s Isp engine running on nitrogen and oxygen is used, up to 84% of gases collected can be retained. The gases retained can then be fed to rockets using the tether, turning it into an orbital fuel depot. What’s more exciting is that it removes the restriction from the tether to have high Isp engines in the first place. They are bulky and power-hungry equipment. A tether that only aims to regain velocity would be satisfied with 0% gas retention. Lighter, simpler propulsion options like nuclear thermal rockets, with an Isp of just 480s, become acceptable. Alternatively, we could use hydrogen-oxygen chemical rocket where payloads coming up the tether provide 12% of the propellant and the remaining 88% is oxygen collected from the atmosphere. Better than any propellant source is not having to use any propellant. This is important for very high altitude tethers that do not meet the atmosphere. Electrodynamic propulsion pushes off the magnetic field around Earth. It only consumes electricity. Although the thrust per kW is very low, it is a reliable and already tested option. Powering all these propulsion options is another concern. Ideally, a tether station would want a compact and long-duration power source like a nuclear reactor. Solar panels are also available, but they require hefty energy storage solutions from the periods where the tether is in the Earth’s shadow, and the drag from the exposed panels adds to the momentum loss over time. Between these two options is the possibility for beamed power. Whether it is from the ground or a space station far above, energy can be transmitted over microwaves or a laser beam to the tether station, where it is converted back to electricity with high efficiency. Moonhook From Hop David's excellent blog. Getting off the lunar surface and into orbit involves much lower velocities than on Earth. There is no atmosphere imposing a minimum orbital altitude either. For these reasons, there are many proposals to install a rotating tether around the Moon first. Such a Moonhook would only need a tip velocity of about 1.5 km/s when orbiting at a 400 km altitude. Because of the lower velocities involved, it can be very lightweight, and easy to transport into a lunar orbit from Earth. There would be no erosion from passing through gases, and it would only have to avoid lunar mountains (up to 6km high) when coming down. This tether can help transfer payloads to the lunar surface, but also to other interesting locations, such as the L1 or L2 Lagrange points. It could be the centerpiece of a cislunar economy, and unlike the ‘lunar elevator’ concept, it does have to extend across hundreds of thousands of km to be useful. Phobos elevator suggested here. Reasons for a moonhook also apply to other moons. Phobos is a popular destination for small moonhooks, enabling access to the martian surface for 2.14 km/s. It could relay work with a tether around Deimos to enable a zero-propellant transfer into and out of the martian system. Interplanetary trajectories As mentioned in the previous section, tethers can easily fling payloads far beyond Earth. Here is a table of tether tip velocities needed to place payloads on Hohmann transfer trajectories to different planets: Injection DeltaV is the velocity increase in meters per second that the payload must receive to enter a trajectory that takes it near the destination. Another propulsion system is needed to actually slow down once it arrives. The mass ratio calculations are done for T1100G cables. You will notice that some destinations, like Mars or Venus, are well within the capabilities of reasonably sized tethers. Mercury or Ceres can be reached with very heavy tethers. Going beyond Jupiter strictly necessitates the use of staged tethers, with Neptune probably off-limits for an Earth-based tether. The DeltaV values listed above are for Hohmann trajectories. For the Outer Planets, minor increases in deltaV (8400 m/s instead of 8200 m/s for Uranus, for example) were selected to enable missions that took less than 10 years to perform. Tethers can speed up travel between planets, by entering payloads into higher energy trajectories. Here is another table showing how much travel times (in days) can be reduced by tethers with 4, 6 and 8 km/s tip velocities. Venus and Mars are the greatest beneficiaries of an extra boost from a faster tether. Mars sees up to 5 times shorter trips when using an 8 km/s tip velocity tether. When the injection deltaV becomes more demanding, the benefit is reduced. A good idea is to have spacecraft using tethers employ their own propulsion system. They can act as an additional ‘stage’ with their own mass ratio between propellant and payload. As we calculated before, staging massively reduces the difficulty of reaching a certain velocity. Here is an example: A spaceship using 450s Isp chemical rockets loads up 2 kg of fuel for each 1 kg of dry mass. This gives it a mass ratio of 3 and a total deltaV of 4850 m/s. It performs a rendezvous with a 6 km/s two-stage tether made out of T1100G cables. The first tether stage has a mass ratio of 12, to get a tip velocity of 3000 m/s and a 100% safety margin on top. The second tether stage also has a mass ratio of 12. Mass ratio of this system is 3 x 12 x 12: 432. The final velocity of 10,850 m/s enables trips to Jupiter in as little as 325 days, or to Uranus in 1551 days. A two-stage tether that tries to achieve this velocity would have had a mass ratio of over 22,000, while a single stage tether would have needed a ridiculous 23.8 megatons of cables for each ton of spaceship. Working through calculations like these really helps highlight just how similar a tether stage and its characteristic velocity is to a rocket stage and its exhaust velocity. Tether trains and interplanetary networks A tether can hand over a payload to another tether. These tethers can be in different orbits, and have different tip velocities, so long as the relative velocity falls to zero during a rendezvous. Three interesting scenarios for tether handovers can be considered: -Exchange between circular orbits A tether in a low orbit can fling a payload up to an altitude that intersects with a tether in a higher orbit. It is caught and further boosted from this higher orbit. Or, payloads can be sent down from the higher tether. Here is an illustrated example: It can work best when the higher tether is a geostationary space station, or these tethers are transporting payloads between different moons around a gas giant like Jupiter. The most interesting aspect is that the tethers can keep each other from losing momentum, so long as the masses they exchange are balanced. The lower tether is naturally larger, as it has to send payloads up with a greater velocity. It could set up a ‘train’ of many momentum-neutral exchanges with several tethers. -Exchange with eccentric orbits In this exchange, one of the tethers is in a low circular orbit and the second tether is in an eccentric orbit with the lowest point (the periapsis) intersecting the first tether’s orbit. Here is an illustrated example: The main advantage is that the tether’s own velocity is added to the boost it can provide a payload. Multiple tethers can be used in sequence, bridging the velocity gap between a tether in a low circular orbit and a very eccentric, near-escape orbit. Low Jupiter Orbit at 42 km/s and Jupiter Escape Velocity at 60 km/s are separated by an 18 km/s gap. Three tethers with tip velocities of 4.5 km/s can relay a payload between them. It does not have to be done all within the narrow window where tethers are all lined up at the lowest point of their respective trajectories. The transfer between orbits can be done one by one. The tether in the lowest orbit accelerates a payload at 4.5 km/s. It is received by a second tether with a tip velocity of 4.5 km/s. The combined boost is 9 km/s. This is done again, to reach a third tether station that is on a near-escape trajectory, with a periapsis velocity of just under 60 km/s. Any extra boost from this third tether would allow a payload to escape into interplanetary space. A full 4.5 km/s boost can put it on a trajectory that sends it all the way back to Earth. Using tethers like this will put the deep gravity well of Jupiter on the same level of accessibility as Mars or Venus. The energy-intensive transfer of crew or cargo up and out of Jupiter can be compensated for by slowing down equal masses of ‘junk’ such as iceball comets or discarded asteroids. We can also expand the use of ‘tether trains’ to interplanetary space. Stations orbiting the Sun on circular or eccentric orbits could pass payloads between them for ‘free’, so long as momentum exchanges are balanced. A tether attached to a small body, as envisioned here. These tethers can be anchored to asteroids, moons or mobile bases, much like the slow Aldrin Cycler concepts. Payloads can hop between tethers at these points gaining or losing velocity. A cycler station makes a trip between Earth and Mars on a regular orbit. Cyclers are most interesting as they perform orbits that take many years, but with tethers, they can send payloads between them much faster. Moving between cyclers in this manner can take on aspects of a train stopping between towns, especially if the cyclers gain large enough populations to become noteworthy destinations on their own. This can lead to a ‘Wild West’ aesthetic, or fulfil the need to visit new locations without having to cross interplanetary distances. A Solar System tethered together A switch in transport of payloads from expensive, slow, propellant-consuming rockets to rapid, low to zero-propellant tethers would have an outsized effect on human expansion into the Solar System. Human passengers will see great reductions in travel times. The combination of an initial boost from a tether, with deltaV provided by a spaceship’s propulsion system, will connect the Inner planets within a matter of weeks. Tethers provide the option to collect propellant more easily, which means those spaceships can afford to spend a lot more propellant than they otherwise could, in turn making travel even faster. Even the Outer planets could be reachable within a few months of travel time. That’s a great step up from multiple years. Enough perhaps to prevent distant colonies from becoming the destination for a ‘once-in-a-generation migration’. Cheaper, quicker travel for humans means that automation is not needed as much. Machinery doesn’t have to work for years on end without maintenance, as a repair crew could arrive regularly. A more mobile population means that space becomes open to less skilled, less experienced workers to fill in job positions wherever they appear, instead of every station or outpost having to rely on multi-skilled workers that can handle prolonged isolation. More people moving around means better chances that ‘extras’ like luxuries and personal services can be accommodated, improving living conditions and so on, in a positive feedback loop. Inert cargo will also benefit from tether transportation. High value goods can be exchanged quickly. A latest generation computer processor wouldn’t have to spend years being exposed to cosmic rays before it reaches a colony around Jupiter as an out-of-date and damaged product. Profits can be made on platinum ground metals a few weeks after they are mined; this means adventurous asteroid mining companies don’t have to hold onto cash reserves so that they can operate for months in between deliveries. They can be smaller, leaner and take more risks. On the other hand, larger payloads can be moved at the same speed with tethers for much less cost. An exchange between two tethers, one on Mars and one on Earth, can take the regular minimal-energy Hohmann trajectory. However, far less propellant would be needed (if any at all, with momentum-neutral exchanges). The payloads would not need any engines, heatshields or large cryogenic propellant tanks. With the use of tugs to maneuver the payloads into a rendezvous at either end, they won’t even need expensive guidance systems. Such cheap travel opens up many new possibilities. Asteroid mining usually considers elements like iron and aluminium to be ‘wastes’ as their value is too low to be worth moving around. Their only use would be at the site they are extracted from. This no longer has to be the case; a much larger fraction of an asteroid becomes exploitable. A beneficial side-effect is that accessing these low-value resources to build up a colony in a remote corner of the Solar System becomes even more affordable. Complete this scene with a spinning tether in the background. Large, slow payloads that can easily be outrun by tether-boosted spacecraft opens the door to piracy. A better transportation system helps with the methods discussed in these previous posts. A ‘pirate tether’ can fling spacecraft into intercepts with payloads in transit. Criminal ports would have higher performance tethers to catch diverted goods from odd angles and high velocities. This is especially useful for stealth craft that can use a tether to boost into a trajectory without announcing themselves, and don’t want to reveal the location of their safe haven by slowing down using rockets. Anything criminals can do, the military can do better. Tether boosts means warships are closer to targets than before. Reduced reliance on onboard propulsion for deltaV means that more mass can be dedicated to armor and weapons instead of propellant tanks. Also, as mentioned before, a secret network of tethers can be employed to move stealth craft around the Solar System. Munitions launched like this can be smaller and easier to hide too. Further developments Everything mentioned so far is only the start of what is possible with tethers. The use of tethers as aerodynamic devices is under-explored. Their use and performance can be expanded over time, as new ideas appear or better technologies are matured. We could consider a hybrid of a stationary and rotating tether. A rotating hub could be installed at the lower end of a very long stationary tether. It would collect a payload and transfer it to another rotating tether at the upper end by climbing up a stationary segment. The main advantage of this hybrid tether is that it can greatly extend the use of small, low velocity rotating tethers, while also not having to fully cover the distance to the destination like a simple stationary tether would have to. Supermaterials can also be considered. Tethers don’t need carbon nanotubes to function, but they can make great use of them. The characteristic velocity of graphene (130 GPa strength, 2267 kg/m^3) is 10,709 m/s. A tether to payload mass ratio of 10 enables a tip velocity of 12.3 km/s. A staged tether can get this up to 24.3 km/s with a total mass ratio of 100. That’s enough to fling a payload out of Low Jupiter Orbit with one single tether, or enable trajectories from Earth to Mars in 34 days, or to Saturn in 360 days. Between two tethers, we could see velocity gains of over 50 km/s… the main limitation would become human endurance. Even with a 6g tolerance limit, a tether tip velocity of 24.3 km/s means a minimum tether length of 10,000 km to reduce centrifugal forces! Going further, tether transport networks can be tied into the Inter-Orbital Kinetic Energy Exchange networks for transporting and generating energy, described here. Tethers can set up the exchange of masses, or even convert them into electricity themselves by using an electrodynamic tether in reverse: instead of consuming electricity to push against a magnetic field, using the field to generate a current while braking against it. Finally note that we haven’t considered the Oberth effect and that tethers can exploit it. Sending a payload down into a gravity well before rapidly accelerating it gives it an extra boost that does not match the momentum lost by the tether. The faster the tether tip, the greater the effect.
  18. I have added @zer0Kerbal as an additional author on the SimpleLife page on SpaceDock. Thanks for these up!
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