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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.
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