MatterBeam

I'm writing the next one!

This is correct. You eject one load of propellant per rotation of the tether. Of course, you could use very many tethers, up to a whole disk of them, to get you practically continuous thrust.

Just to be clear, the post is about spaceships having onboard tethers to travel like any other electric thruster.

A flat disk. A layer of explosives. ... stacked TWR should be in the millions.

This is from my latest blog post: http://toughsf.blogspot.com/2020/05/water-disk-rocket.html Some the images and tables here came out wonky so check them out at the original source. Hypervelocity Tether Rockets Rotating tethers can reach incredible velocities when they are built out of high strength materials. With some design features, they can greatly surpass the exhaust velocities of chemical or even nuclear rockets. They can become propulsion systems with impressive performance... and might look like the classic 'saucer' spaceship. How would they work? What performance could they achieve? Rotating Tethers Cover art by Mack Szbtaba. Rotating tethers are a fascinating topic that have been treated in depth by previous posts on ToughSF, such as using them to extract energy from planetary motion or make space travel much shorter. Two SpaceX Starships in a 1500m tether formation spun to generate artificial gravity. In summary, a tether made of high strength-to-weight ratio material can withstand enormous forces while remaining lightweight. If spun in a circle, usually many kilometers wide, it can support a load on one end as long as it is supported by a counter-weight on the opposite side. The tip velocity achievable before the tether breaks from centrifugal force will reach several kilometers per second. It can be boosted even further if the tether is tapered: wider at the base and thinner towards the tip. With this technique, tethers made of mass-produced materials like Kevlar can cover a significant fraction of orbital velocity, making it good enough to be used to build a skyhook. Skyhook principle of operation. The important factor here is how heavy of a tether we need to handle a certain payload mass spinning at a certain velocity. First we need to work out the characteristic velocity of a tether, which depends on its material properties: tensile strength and density. Characteristic velocity = (2 * Tensile Strength / Density)^0.5 Characteristic velocity in m/s Tensile Strength in Pascals Density in kg/m^3 For Kevlar, the values we have are 3,620,000,000 Pa and 1,440 kg/m^3. Kevlar’s characteristic velocity is 2242 m/s Then we need to find the ratio between the tether’s tip velocity and the characteristic velocity, which we’ll simply call the Velocity Ratio VR. VR = Tip Velocity / Characteristic Velocity If our tether is spinning at 3300 m/s, then the VR is 3300/2442 = 1.351 Finally we get to the Tether Mass Ratio. It is the ratio between the tether mass and the payload mass it can handle. Tether Mass Ratio (TMR) = 1.772 * VR * e^(VR^2) A tether with a VR of 1.351 will have a Tether Mass Ratio of 1.772 * 1.351 * e^(1.351^2) = 14.85. It means that a 1485 kg Kevlar tether can handle a 100 kg payload at its tip while spinning at 3300 m/s. The HASTOL concept relied on 3250 m/s tethers. The Tether Mass Ratio is square-exponential. It climbs extremely rapidly with increasing VR. Doubling the tip velocity to 6600 m/s, for example, raises the Tether Mass Ratio of a Kevlar tether to 7122. Now a 712.2 ton tether is needed for the same 100 kg payload; a nearly 48x increase. As a consequence of this scaling relationship, large rotating tethers are optimized for velocities only slightly above their material’s characteristic velocity. Then some safety margin has to be added on top. It is not practical to have a 10 ton capsule matched with a tether of several thousand tons. Hundreds of launches would be needed to justify the presence of the tether. Large tethers also have some additional complications limiting their performance, such as the need to add multiple redundancy against micro-meteorite strikes and shielding against solar radiation that would otherwise degrade their materials. All of these measures cut into the mass actually dedicated to supporting a payload. Hoytether multiple redundant tether lines. But that is not the only way to use tethers. We can design them for an entirely different role. Higher Velocities It is possible to imagine much smaller tethers, perhaps a few meters across, spinning at much higher velocities. They would be completely enclosed in a protective container. The idea of a smaller, faster tether launching objects is not new. In fact, it is being worked on at full scale by alternative launch companies like SpinLaunch today. The idea is that we can increase tether velocity to many kilometers per second, then release small masses from the tether tips. This can be water or dust grains or whatever can flow down the tether’s length. Their release generates recoil in the opposite direction: that’s thrust. Momentum is lost with each release, though it can be regenerated by an electric motor that spins the tether. Counter-rotating tethers ejecting water for propulsion. If we mount a tether like this on a spacecraft, it can be used as a rocket engine as propellant exiting in one direction and thrust produced in the opposite direction. As long as two counter-rotating tethers are used, there is no torque. Essentially, they become an electric thruster with an ‘exhaust velocity’ equal to the tether tip velocity. There are many advantages. The tethers can use nearly any propellant they can pipe to their tips. Whether it is dust gathered from an asteroid’s surface, nitrogen scooped up from the edge of Earth’s atmosphere or water derived from a lunar mining operation, it can all go in the propellant tanks with minimal processing. That means there is no need to haul a chemical factory with you to every landing site in the Solar System. An orbital gas scoop. The tether itself should be practically frictionless and have nearly 100% efficiency. It operates mechanically (no electric currents or coolant flows) so it should produce negligible heat even at extreme power outputs, which are in turn limited only by its RPM. A frictionless magnetic bearing is necessary to enable high efficiency rotating tethers. A tether rocket compares favourably in many ways to existing technology like Hall effect thrusters or MPD thrusters. They do not have to pay the energy penalty to ionize their propellant, nor do they have the pulsed energy storage concerns of mass drivers (railguns, coilguns). Further advantages will be described later in this post. These tethers can be spun to very high velocities at the expense of impressive mass ratios. The g-forces exerted at their tips would be immense, but it is acceptable as their payloads won’t be fragile spacecraft. Also, since they are on a much smaller scale, it becomes much more affordable to build them out of the best materials available. For example, Toray’s polyacrylonitrile fiber T1100G with a characteristic velocity of 2,796 m/s or new UHMWPE fibres (Dyneema) being tested to a characteristic velocity of 2900 m/s. These may seem like tiny gains over the characteristic velocity of widely available Kevlar, but remember that the Tether Mass Ratio is square-exponential. Small improvements lead to huge decreases in tether mass. Here is a table of the performance we can get: All of these materials make it possible to achieve tether tip velocities exceeding the best performance of chemical rockets (460s Isp or 4512 m/s) with a moderate mass ratio. Kevlar struggles when going faster than that. T1100G or UHMWPE can get us 7500 m/s exhaust velocity with a Tether Mass Ratio in the thousands. An exhaust velocity exceeding that of nuclear thermal rockets (1000s Isp or 9810 m/s) is achieved with T1100G at TMR 2.27 million and UHMWPE at TMR 0.89 million. A Tether Mass Ratio in the millions sounds extreme but consider it in these terms: a tether of 1 ton mass would be handling 1 gram of propellant at its tip. If it is 1 meter in radius, and the tip velocity is 10,000 m/s, then it makes a complete rotation 1591 times a second 95,460 RPM). It is not so extreme: commercial hard-drive disks spin at 7200 RPM and ultracentrifuges manage 100,000 RPM. We could compare at them to uranium gas centrifuges spinning at 90,000 RPM. Rows of uranium gas ultracentrifuges. If this 1m long tether releases a 1 gram drop of water every time it completes a rotation, it will have a mass flow rate of 1.59 kg per second. Thrust is propellant flow rate times exhaust velocity, so multiplying that figure by 10,000 m/s gives us a thrust of 15.9 kN. Thrust power is equal to half the thrust times exhaust velocity, which in this case is 0.5 * 15,900 * 10,000 = 79.5 MegaWatts! Let’s try to design two realistic Hypervelocity Tether Rockets, one with T1100G aiming for an exhaust velocity of 6000 m/s which is ideal for travel between the Earth and Moon, and another using slightly more advanced UHMWPE aiming for 10,000 m/s which is better for interplanetary travel. The g-forces at the tether tips will exceed 1,000,000g, which is troublesome as there would have to be some moving part that controls the flow of propellant that can open and close thousands of times a second. A piezoelectric poppet valve that can open and close 2000 times a second. Putting as many components as possible on the external container (control electronics, magnetic actuators) rather than on the moving tip could help. Lunar Tether Rocket The Toray T1100G material is selected because you can order spools of it right now. The individual fibres have a tensile strength of 7000 MPa and a density of 1790 kg/m^3. With its characteristic velocity, 6000 m/s tip velocity means a Tether Mass Ratio of 380. Why 6000 m/s? Because it allows a rocket to make the 8400m m/s deltaV trip from Low Earth Orbit to Low Lunar Orbit and back with a propellant mass ratio of 4 (that’s 3 kg of propellant for each 1 kg of empty rocket). That is modest for an upper stage of a launch vehicle, let alone a lunar transfer stage. The tether here can have a length of 3.67 m. It would rotate at 15,607 RPM. If it aims to shoot off 10 grams of water with each rotation, then it will have a mass flow rate of 2.6 kg/s. The tether itself will mass 3.8 kg but we can bump that up to 5.7 kg to add a 50% safety margin. A counter-weight doubles that value to 11.4 kg. It will feel 60 Newtons of recoil with each release, which seems like it can easily be handled by a suspension mechanism. To counter torque effects, we must add a second tether rotating in the opposite direction, which adds another 11.4 kg for a total of 22.8 kg. Average thrust from both tethers is 31.2 kN. Thrust power is 93.6 MW. This power can be delivered by a high power density megawatt-scale electric motor. An example of this today would be the H3X HPDM-3000 that manages 2.8 MW of output with a power density of 12.7 kW/kg. It is already meant to be stacked in multiple units. 93.6 MW of power would need to be delivered by 7370 kg of these electric motors. The motors are 94% efficient, so there’s 5.97 MW of waste heat to consider. The motors operate at 60°C, so 4282 m^2 of double-sided radiator panels are needed to handle their waste heat. This may need 4282 kg of 1 kg/m^2 radiator panels based on carbon fibre heat pipe technology. In total, this propulsion system masses 11,675 kg. If we add a 10% mass margin for equipment like water pumps, tether container walls, coolant pipes, we arrive at a total mass of 12,843 kg. The tethers are by far the smallest component, representing only 0.178% of the mass total. Toray T1100G Tether Rocket Performance Tip velocity = 6000 m/s Total Mass = 12,843 kg Thrust = 31.2 kN Thrust-to-weight ratio = 0.247 Average power density = 7.3 kW/kg If you add a power supply, propellant tanks, structural components and a payload, you get the rough draft of an Earth-Moon spaceship. The Hypervelocity Tether Rocket here far exceeds the performance of most electric propulsion systems you could slot into its place on such a spaceship. Aerojet Rocketdyne’s Hall thrusters struggle to reach 0.26 kW/kg. NASA’s more advanced electric thrusters aim for up to 4 kW/kg, but at a reduced efficiency of 60 to 85%. They are superior in terms of specific impulse, but that is not particularly needed in cis-lunar space. Interplanetary Tether Rocket Now we look at a 10,000 m/s UHMWPE tether. It will be more advanced but still within the realm of ‘near future technology’. Tether Mass Ratio is 891,437. The tether is short: 0.95 m in radius. It spins at 100,000 RPM. The amount of propellant released with each rotation is 1 gram. That means a tether mass of 891.4 kg and a mass flow rate of 1.67 kg/s. With counter-weights and a second counter-rotating tether, the tether assembly adds up to 3566 kg. We bump this up to 5349 kg for a 50% safety margin. The average thrust produced from the two tethers is 33.4 kN. Thrust power is 167 MW. Fully superconducting electric motors can reach astounding kW/kg values At this power level, it is sensible to switch superconducting devices. NASA’s 2035 goals for turboelectric propulsion on aircraft uses high temperature superconductors to achieve 40 kW/kg at 99.99% efficiency. The electric motor mass would only need to be 4175 kg. The waste heat produced at 65 Kelvin would be 16.7 kW. A superconducting design. A 201 kW Stirling cryocooler of 300 W/kg, would raise the temperature to 300 Kelvin (30% of Carnot efficiency) and 670 kg of equipment. The radiators to handle the final heat load (16.7 + 201 * 0.7 = 157.4 kW) add another 171 kg. In total, this propulsion system masses 10,365 kg. If we add a 10% mass margin as before, we arrive at a total mass of 11,401 kg. UHMWPE Tether Rocket Performance Tip velocity = 10,000 m/s Total Mass = 11,401 kg Thrust = 33.4 kN Thrust-to-weight ratio = 0.298 Average power density = 14.65 kW/kg This design has even higher performance and better specific impulse. It is well suited for missions to Mars. Its performance is somewhat comparable to a solid-core nuclear thermal rocket using liquid hydrogen, as it has the same exhaust velocity but it does not need bulky cryogenic propellant tanks or a full electrolyzing ISRU plant to refuel it. If solar or beamed power is available, it could do away with nuclear technology altogether and still achieve comparable performance. Neither of these designs are optimized. There could be further performance gains to be had from selecting a better tip velocity or cooling solution. For example, the propellant water could first be used to cool the electric motors to save on the mass of radiators needed. Or, we could employ several tethers to multiply the thrust the engine could produce without having to also increase RPM or tip velocity. Staging tethers on tethers Rockets get around the problem of exponential mass ratio by using staging. Tethers can employ the same strategy. Instead of placing a payload on the tip of a tether, another smaller tether can be attached. Each tether would spin independently of each other, and at the right moment, their tip velocities would add up. Here is an example with Kevlar: We want a tip velocity of 10,000 m/s. As we calculated previously, this would require an impractical tether with a Tether Mass Ratio of over 139.1 million. If we instead break it down into tethers of 5,000 m/s velocity, and stage them tip-to-tip, we would obtain stages with a mass ratio of 240. Two stages would add their tip velocities to 10,000 m/s and multiply their mass ratios to 240 x 240 = 57,600. This is obviously much lower than one huge tether. There is very little literature available on this idea. The closest concept is the Tillotson Two-Tier Tether, as depicted here. There will be challenges to designing a two-stage tether for use as a rocket. There’s the issue of transferring propellant between the tethers, which could be very troublesome if you want solid particles as propellant. Designing a rotating joint that can work smoothly when under high g-forces can’t be easy. Then there’s the difficulty of restoring momentum to the second-stage tether. A second-stage tether also needs its own counter-weight, which could double the overall mass ratio. But, if all these challenges can be solved, then we would get much more impressive tether rockets. Here is a table for two-stage performance: The same material selection as in the previous section is given a second stage so that the total Tether Mass Ratio for both stages reaches 500, 50,000 and then 500,000. The final ratio is doubled to account for the second stage tether’s counterweight. In this arrangement, even Kevlar exceeds 11 km/s tip velocity. UHWPE manages 13.1 km/s with a final tether ratio of 1 million. Let’s update the two tether rocket designs with staged tethers: Toray T1100G Two-Stage Tether Rocket Performance Tip velocity = 7430 m/s Total Mass = 12,843 kg Thrust = 25.2 kN Thrust-to-weight ratio = 0.2 Average power density = 7.3 kW/kg We maintained the 380 final tether mass ratio from the Toray 1100G tether rocket. However, with two stages, we get an exhaust velocity of 7.43 km/s. Thrust power from the electric motor is identical so the thrust-to-weight ratio has to fall to 0.2. UHMWPE Two-Stage Tether Rocket Performance Tip velocity = 10,000 m/s Total Mass = 6095 kg Thrust = 33.4 kN Thrust-to-weight ratio = 0.56 Average power density = 27.4 kW/kg The UHMWPE tether rocket aims for the same tip velocity, but with two stages the final Tether Mass Ratio (x2) can fall from 891,437 to just 7128. The tether assembly is reduced from 5349 kg to 42.7 kg, raising the overall thrust-to-weight ratio and average power density significantly. Note that for both of these designs, we are only calculating the mass of the engine - the part that converts electrical power to thrust. A complete spaceship would have to include an electrical generator, be it an onboard reactor, solar panels or a laser-photovoltaic receiver. In a realistic study, you will find that high engine power densities means the average power density of the propulsion module of a spaceship approaches that of the power generating section alone. The overall performance of a spaceship won’t improve much if you have a terrible power generator (0.2 kW/kg solar panels) but excellent engines (20 kW/kg). Solar-electric spacecraft with football fields of photovoltaic panels might not benefit much. Two-stage tether tip velocities means we obtain a propulsion system that can make shorter interplanetary trips. 1200 seconds of specific impulse means that a spaceship that’s 75% water (a mass ratio of 4) has 16.3 km/s of deltaV. It can start in Low Earth Orbit and arrive in Low Mars Orbit in 88 days, or complete a trip to Io’s orbit around Jupiter in 1.73 years instead of the usual Hohmann transfer of 2.73 years. This is without the assistance of aerobraking and with the ability to quickly load up on propellant at the destination for the return trip. A relatively quick trip from Earth to Jupiter. Theoretically, a third tether stage is possible. It would push the potential performance of tether rockets well into the domain of electric thrusters (2016s Isp with UHMWPE) while retaining the upper hand in thrust-to-weight and power density. However, the problems mentioned above would all be exacerbated. Carbon extraordinaire So far we have restricted ourselves to materials available in bulk today. Better materials exist; we only need to learn how to manufacture them in large quantities. The most promising of these are carbon nanomaterials: nanotubes and graphene. Carbon nanotubes are being grown right now, up to lengths of 50 centimeters. Graphene flakes are regularly added to epoxy resins and nanocomposite materials to enhance their strength. In the future, we could see them being produced in much larger quantities, enough to use for tethers. In order of difficulty of manufacture, we have multi-walled carbon nanotubes, single-walled carbon nanotubes and then graphene. Here are their ‘perfect’ properties: The characteristic velocity of these materials can exceed 10 km/s. When used in a tether with a Tether Mass Ratio (TMR) of 100, they can achieve tip velocities approaching 20 km/s. In a TMR 10,000 tether, they approach 30 km/s and they can push beyond 60 km/s with a TMR of 1 million. That’s better than what most electric thrusters are capable of today. Of course, it is unlikely we will be able to form tethers of several meters in length with zero defects, errors or safety margins using these materials in the near future. The strength of a single perfect fibre is reduced when it has to be bundled with many other fibres, bringing down the ‘engineering strength’ to about half of the maximum with no other factors involved. Even at their weakest, carbon nanotubes far surpass other materials. If we assume that a half of the theoretical maximum could be achieved in bulk quantities, the tip velocities we would actually achieve would be reduced by 42%. Then, we could apply staging. A two-stage hypervelocity tether rocket with specific impulse of 2000 to 4800s seems achievable with these materials. The overall power density of the rocket is difficult to estimate because access to carbon nanomaterials would also affect the weight of components like electric motors or radiator panels. The final design could easily exceed 100 kW/kg. It does mean that the performance of the power generating source becomes critical to good overall performance. Even a nuclear reactor with radiators and a turbine that we consider excellent today at 10 kW/kg would become a performance bottleneck when paired with a 100 kW/kg carbon nanotube tether rocket. Mechanical Rocketry What’s it like to use hypervelocity tether rocket engines? The radiators are tapered to fit inside the reactor's shadow shield, with the water tanks serving as extra shielding. They can simply be mounted on spacecraft and used to travel by throwing propellant out. It would look rather weird: they have no nozzles, only need small propellant tanks and their most distinguishing feature might look like a wheel... or if the tethers are placed internally, the whole spaceship might be configured like a disk. Not aliens, a spaceship with equatorial tether-rockets (and fancy lighting)! Meaning, your diamond hard science fiction can have fully justified 'flying saucers' roaming the Solar System. The tethers can thrust in different directions by selecting a different firing port for their exhaust. A disk-shaped spaceship with firing ports along its rim can accelerate in any direction. It just has to take care not to aim its exhaust at nearby objects. Docking might have to be done entirely using secondary propulsion (RCS thrusters). Water can drill holes through asteroids, space stations and other spacecraft when shot out at 10 km/s. Over long distances, it would disperse into harmless mist but at short distances it would be dangerous. Dust or other solid particle propellant would not disperse and would remain dangerous forever. Their use in the Outer Solar System or between asteroids might be justified by the vast distances involved, but not in cluttered low planetary orbits, especially if exhaust velocity is less than escape velocity (the dust would circle back around). Spaceship pilots might need to pay attention to how long it takes for their tethers to reach operational RPM. Thrust would not be instantaneous, which makes delicate or urgent manoeuvres troublesome. Thrust levels can be adjusted by firing more or less frequently. Theoretically, the tether can be spun down to a lower tip velocity to allow for more propellant to be fired with each rotation. The potential thrust would increase exponentially as the tether velocity is decreased. However, the other critical component in a tether rocket is the electric motor. Its output is tied to its RPM, so spinning slower might also mean less watts from the motor. The solution to this is a gearbox… but the practical details of building a MW-scale 100,000 RPM set of gears are best left to people in the future. It should be noted that electric motor power does not have to exactly match the thrust power of a tether rocket. The spinning mass of a tether can be considered a type of flywheel, so it can store energy. Energy can be accumulated gradually by a small motor (which enables some mass savings), then released quickly from the tether. This is most useful for spacecraft that aim to raise their orbit via multiple short burns at the periapsis of their orbit. It maximizes the contribution of the Oberth effect and was used by Rocketlab’s Photon stage for the CAPSTONE lunar mission. It’s possible to rely on rotating energy storage alone for propulsion. An asteroid mining spacecraft could land on a target, hollow it out for raw materials, build flywheels-tethers out of the leftovers and spin them up before leaving. Those tethers would then eject pieces of asteroid dust for propulsion until their energy ran out. RAMA proposed this architecture but with a different way of converting stored energy into thrust (using catapult sling arms). In fact, asteroid mining is one of the best applications of tether rockets. The ability to use any propellant, the decent exhaust velocity (for an electric rocket) and the ability to store energy then release it quickly combine to make tether rockets ideal for asteroid hopping spacecraft. The deltaV for travelling between asteroids can be very low, which suits the tether rocket perfectly. An asteroid mining spaceship. Perhaps the ring sections could be tether-rockets... Sunlight may be too weak to keep a powerful motor running continuously in the asteroid belt, so slowly accumulating energy into a flywheel is a good option to have. Being able to use asteroid dust as propellant means the mining ships can hop to very ‘dry’ targets without worrying about the availability of water to refuel themselves. The tether itself could be made of locally sourced materials, such as glass or basalt fibres that exhibit ‘good-enough’ characteristic velocities of 1.5 km/s to 2 km/s. Glass fibre tethers would be larger and heavier than carbon nanotubes, but that’s actually an advantage if they double as energy storage flywheels. Manufacturing basalt fibres. This creates a ‘low performance’ niche for tether rockets. They could excel here as well as they do in the ‘high performance’ role with super-materials and extreme tip velocities. Other Applications Beyond simple use as rockets, hypervelocity tethers can have a variety of further applications. Drilling and excavation A high pressure water drill. A series of high velocity impacts concentrated onto a small area can serve as an efficient drill. Water or dust at 10 km/s can overcome the mechanical strength of practically any material, so what the target is made of does not matter. The impacts can be tuned to bore a hole through a target, or create shockwaves that fracture it into smaller pieces for easy excavation. One idea is to have the spinning tether first serve as a rocket to bring a spaceship close to an asteroid, then become part of mining equipment to dig into the asteroid’s surface and expose the dense core potentially loaded with precious metals. Just make sure to anchor the tether well! Mass Streams 'Pellet beam' propulsion. A tether could launch those pellets. The hypervelocity tether can be used as a mass driver to shoot a series of projectiles to propel other spacecraft. This is known as mass stream propulsion. The spacecraft riding these mass streams only need a device to catch the projectiles - it can be as simple as an ablative pusher plate or as complex as a magnetic nozzle that drops solid targets into the path of the mass streams and pushes off the resulting plasma explosions. Either way, the riders are unburdened by propellant, reactors or radiators, so they can have fantastic acceleration. Mass drivers are usually fixed structures that do not have to worry about their weight, so the tethers can aim for extreme mass ratios. A two-stage T1100G tether with a TMR of 100,000 per stage would have a tip velocity of 17.5 km/s. Spacecraft riding these mass streams could achieve a good fraction of this velocity, perhaps 16 km/s. More mass streams headed in the opposite direction would be waiting for them at their destination for braking. Together, they enable fast interplanetary travel. Railguns or coilguns could also be used as mass drivers, but they are usually much less efficient and take up a lot more room than tethers. Stealth Drive Dark, non-radiating and doesn't even leave a trail of hydrogen behind it. You might imagine that a hypervelocity tether would make for a good weapon. It could drill through any target and its firing rate would allow for enough shots to ensure hits at long range. However, this is unlikely. Hypervelocity tethers have no barrel, so they are inaccurate. It would be difficult to put them in a turret. Their large rotating mass means they act like a gyroscope that resists turning. The way the tether mass scales with projectile mass means that only the smallest projectiles are possible. That removes the option of using ‘smart’ guided projectiles with sensors and RCS thrusters to track a target as these may have a minimum mass of several hundred grams. Worse, they would be extremely vulnerable to battle damage. A small cut on the tether might lead to it completely disintegrating… inside your spaceship. So spinning tethers are a bad weapon. Does that mean they have no military use? There is one final advantage that comes into play. The exhaust of a tether rocket can be cryogenically cold. The entire launch process does not release any heat. Even the electric motor can be of a superconducting design bathed in liquid helium at <4 Kelvin. So long as you have access to electrical power, the tether rocket can be a completely stealthy propulsion system.

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.

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.

You are most welcome

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.

This makes me really happy.

That's awesome. Contact me for any questions.

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.

Thanks for keeping this going.

@ItsJustLuciCould you please provide an estimate for how much RAM is needed to play with the 32k texture pack in RC1?

I am very impressed than an 8 year old mod is still being maintained. Kudos to @NathanKell

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.

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A most excellent development. I assume this will include a Raptor analogue?

Thank you for continuing this!

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.

Thanks for the explanation. I'm loving your mods by the way!

I was doing a few calculations and I got some funny numbers. Something like the 'NEXT Ion Thruster - 0.625m' engine. It has an Isp of 6380s and a thrust of 2.1 kN. Engine power in the real world is equal to Isp * 9.81 * Thrust /2, so I get 65.7 MW of power. However, the part mass is only 200 kg. This means the power density is a whopping 328.6 kW/kg. This continues for all the electric thrusters. The 'VASIMR - 1.25m' engine has an Isp of 6000s and a thrust of 24.9 kN, meaning that it outputs 732.8 MW of power. Power density is at 732.8 kW/kg. For comparison, real world electric thrusters have power densities on the order of 0.5 kW/kg, rising to 2 kW/kg with a large design like VASIMR. This means the in-game propulsion is 160 to 360 times more powerful than modern technology allows. Is this the intended balancing for NFT, @Nertea?

Hi everyone. Is there a way to edit the size limits on the procedural wings?

This is from the latest ToughSF blogpost: http://toughsf.blogspot.com/2021/01/moto-orion-mechanized-nuclear-pulse.html Moto-Orion: Mechanized Nuclear Pulse Propulsion The Orion nuclear pulse propulsion concept has been around for over six decades now. It is powerful and robust, but lacks the flexibility and features we expect from many more modern designs. Can we give it those additional capabilities? That cutaway is one of Matthew Paul Cushman’s amazing pieces. Basic overview of Orion William Black has plenty of great Orion artwork. There is a lot of information on Project Orion, available mostly here and here. It is best to read through them to gain a complete understanding of how it works. We’ll only give a simple overview to start. Project Orion’s design for a nuclear pulsed propulsion system was pretty simple. A physical plate of steel, protected with a thin layer of oil, faced a plasma jet from a nuclear shaped charge. The force of that blast was translated into useful thrust for the Orion spaceship. In this manner, a propulsion system could tap into the immense power of a nuclear detonation while sidestepping the heat management issues that would normally come from handling such an output. Its thrust was huge, enough to lift thousands of tons into orbit, and so was its efficiency, with an effective Isp of 2,000 to 12,000s. That’s five to thirty times the specific impulse of a chemical rocket, with thrust and efficiency that only gets better as you scale it up. We call this combination of high thrust and high efficiency a ‘torch drive’; a term from ‘Golden Age’ science fiction where authors did not want to spend pages explaining things like deltaV limits and interplanetary trajectories to their readers. A torch drive lets you point at your destination and accelerate to get there. Even today, sci-fi loves this solution. It did have drawbacks though. The fissile fuel in each nuclear pulse charge is inefficiently used, with the majority being wasted. This was because each pulse had to be small, so as to not obliterate the pusher plate, and therefore could not produce the better burnup ratios of large nuclear charges. The rate at which these pulses were ignited could not be varied by much either. Timing the pulses with the motion of the pusher plate, so that the blast would meet the suspension system in the right position, was essential. There were three parts to the suspension system. The first is the pusher plate itself. When struck at a precise angle, it could be accelerated at 50,000g or more without being bent or twisted. It first slams into a gas bag, that acts similar to how a car’s airbags are used in a car crash, to turn a sharp shock into a more gradual shove. Momentum from the plate is then transferred to a set of pistons at a much slower rate. These pistons are connected to rigid springs that convert the series of pushes into a continuous acceleration. When the timing is right, the literally well-oiled machinery is very strong. When the timing is off, things break down. The suspension cycle, in short. If one charge ignited too early, then only a fraction of the suspension length can be used to absorb the blast’s momentum, so it gets translated into a hard jolt. Ignited too late, and it would further accelerate an already retreating pusher plate, with potentially devastating consequences. A complete misfire isn’t great either. The suspension arms would only be partially compressed, and so would not reach full extension on the rebound and it would become unsafe to receive another nuclear blast. The Orion spaceship would have to wait for the suspension to wobble to a full stop, and then use a half-powered charge to restart it from a fully compressed state. Waiting to restart the suspension cycle isn’t a nice position to be in when launching off a planet. Another drawback was the inability to convert any of the nuclear pulse drive’s immense output into electrical power. The two-step suspension system simply acts as a fancy spring to transfer momentum between the nuclear blasts and the spaceship. Most of the time, this is not an issue. Liftoff from a planet or moon’s surface does not take long, so stored power is sufficient. Cost-efficient interplanetary travel consists of short uses of the main propulsion system followed by long periods of coasting, during which solar panels can be deployed. An Orion warship accelerating, from the sadly incomplete sequence here. However, some of the more demanding applications require a lot of onboard power. Military spaceships especially want the ability to both accelerate out of harm’s way, while producing plenty of electrical power to feed lasers, RADARs and other energy-intensive equipment. Fulfilling this requirement means sacrificing payload capacity to mount an onboard nuclear reactor or some other heavy solution. It’s also a problem for very fast transports that want to use the Orion engine as much as possible; they can only extend tiny solar panels while accelerating as anything bigger would get burnt off by the nuclear blasts. Of course, there are many other problems too, that we won’t go into more detail this time. The fact that each nuclear charge is a fully functional nuclear warhead, for example, means that a crash-landing would spill out a full nuclear arsenal, worthy of arming a superpower. Or that the main propulsion system of an Orion ship cannot be used to turn, so huge Reaction Control thrusters would be needed for every single maneuver. We cannot ignore the existence of more modern and more refined nuclear pulse propulsion designs either. Orion was dreamt up in the 1960s and a lot has happened since then. Mini Mag-Orion. Most notably, Mag-Orion and variants thereof. Instead of a physical pusher plate, a magnetic nozzle is used to capture the momentum of nuclear-generated plasma. Fully self-contained bombs are replaced by subcritical masses of uranium. They have to be detonated by external compression devices, such as a Z-pinch or a magnetic pulse. The result; they are completely safe in storage and gain a not-bomb-like-at-all quality. Generating electrical power is a simple repurposing of coils in a magnetic nozzle into Magnetohydrodynamic generators, and turning is accomplished by unequally deflecting the plasma within the nozzle one way or another. However, these more advanced designs cut away at the awesome potential of an Orion drive. The need for large magnets, cooling systems for the nozzle, capacitor banks for the ignition system, all add a lot of weight. Designs of this type have much lower thrust than the original Orion design. They can’t take off from large planets or even operate inside an atmosphere. They move away from that brutal, simple and resilient character that a nuclear Orion engine has, to become something flimsier and more complicated. Perhaps that is an unacceptable compromise, especially for someone seeking specific capabilities, or a sci-fi author aiming for a special aesthetic. Ad Astra Game's RocketPunk, seeking that aesthetic. Could we solve some of the original Orion’s most glaring drawbacks without moving too far away from the image of an atomic piston engine from a bygone era? Moto-Orion We alter the 60 year old design by giving it a crankshaft. It won’t be directly connected to the pusher plate - it can be connected behind the main suspension arms, so that it doesn’t have to receive the shock from a nuclear blast directly and become unreasonably long and heavy as a result. The crankshaft is connected to a crank that turns a large wheel. Depending on the pulse rate of the Orion drive, this wheel will turn at 54 to 69 RPM. A gear train would be needed to increase the RPMs into the thousands, suitable for an electric generator. Also necessary is a counter-torque mechanism, such as a second wheel or even just a counterweight turning in the opposite direction. Please note that the depiction in the diagram above isn't perfect, as all these mechanisms have to find a place in between the springs, hydraulics and other machinery above the suspension arms. A different arrangement would take up less room, but be harder to read visually. The concept is similar to a wind turbine and its generator, except the blades are replaced by a nuclear pulse-driven crank. The power that can be extracted through the crankshaft will be a fraction of the mechanical energy delivered through the Orion drive’s suspension. This is already a small percentage of the nuclear energy released by the pulse charges. The USAF design for a 10m diameter nuclear spaceship has a fantastic 32.9 GW output, but this is only 0.78% of the energy released by 1 kiloton yield blasts every second. We’ll call this the Motorized Orion or Moto-Orion. In practice, the electrical power that can be derived from an Orion drive will depend on the mass of the electrical generator and the equipment needed to manage waste heat. A high performance generator would have an efficiency of over 95% and a power density in the tens of kW/kg. Waste heat will be the main obstacle to generating a lot of electrical power, especially as electrical generators tend to operate at lower temperatures. As discussed in a previous post, temperature is the biggest factor in allowing for lightweight heat management systems. A generator would typically want to operate at room temperature 300K, but this would mean huge (and heavy) radiators would be needed to handle their waste heat. We want the hottest generators possible. They are mainly limited by the decreased performance of their electrical insulators at higher temperatures. Commercially available motors are available at 570K, but applying research like this could create generators that operate at 770K. However, increased temperatures also increase electrical resistance and therefore cut into the efficiency of a generator. Based on some studies, high temperature efficiency can be held at above 90%. A generator is a motor in reverse, so we will use these same temperatures and efficiencies. Estimating the power density of an entire heat management system is quite difficult, but we can make some estimates. 1 m^2 of double-sided 2mm thick carbon fibre radiator fins would be 4 kg and radiate away 8.3 kW of heat at 520K. Note how this is a slightly lower number than the operating temperature of the generators, as we need a temperature gradient throughout the heat management system to actually move heat from where it is created to where it is radiated away. With reasonable figures for a silicone oil pump, a microchannel heat exchanger and a +20% margin for assorted pipes, valves and backups, it all averages out to 1.2 kW/kg. This seems like a low figure, but it only deals with the <10% of power that becomes waste heat. 1 MW of mechanical energy coming through the crankshaft would become 900 kW of electricity, handled by 45 kg of generators, and 100 kW of waste heat, requiring around 83 kg of cooling equipment. Altogether, this makes for an average power density of 7 kW/kg. This ignores the mass of the crankshaft, counterweight and other mechanisms, but they will be small compared to the rest. There is also the complication of radiator placement; they want to extend out from the hull, but also must stay within the shadow cone of the pusher plate to avoid being disintegrated by nuclear plasma. The original USAF 10m Orion had a payload capability of up to 225 tons (on certain missions). If a quarter of this was dedicated just to producing electricity, we could expect it to output 393 MW. That is a respectable amount! Here’s what a Moto-Orion derived from that design, with fully scaled radiators, would look like: Though, it is only 1.2% of the drive power. You could imagine an Orion drive spaceship that extracts more of its output as electricity, but it is fundamentally limited by the difference between the power density of the propulsion system (on the order of 330 kW/kg) and that of the power extracting equipment (<10 kW/kg). Furthermore, equipment that consumes that electrical output will take up an outsized portion of the spaceship’s payload capacity, due to their even lower power density (<1 kW/kg). There are other ways to generate electrical power. A linear alternator should be an ideal option. A magnet is simply pushed through a series of conductive coils, producing current as it travels up and down. It is just as efficient as a rotating electric generator, and depending on the exact design used, can operate at the same high temperatures. Even better, it does not produce any sideways torque, is easier to fit in between the suspension arms and is more resilient to vibrations. However, their power density is far lower than that of rotating generators, with 1.49 kW/kg being the best figure mentioned anywhere. Another option still is to use a high temperature superconducting generator. NASA has designs that aim for 60 kW/kg at the multi-megawatt scale. Efficiency is 99%, meaning that 1% of the power becomes waste heat. Thankfully, this heat is produced not in the superconducting magnet, but in the non-superconducting stator. It can reach 570K, so we can use similar heat-management equipment as described above. 1 MW of input power becomes 990 kW of electricity and 10 kW of heat, which are handled respectively by 16.5 kg of generator and 8.3 kg of cooling equipment, for an average power density of 40 kW/kg. The downside to using superconducting devices is having to mount the bulky and sensitive equipment needed to keep them in that state. A high-temperature superconductor needs to be kept in liquid nitrogen, which boils at 77K. About 0.01 to 0.1% of the power that a superconducting device handles is expected to become waste heat inside the cryogenic part through ‘AC losses’, where alternating currents create magnetic vortices within a conductor. Progress is being made into megawatt scale superconducting generators/motors. This Honeywell 1 MW design achieves 8 kW/kg. The passive solution to handling this heat load is to just let the liquid nitrogen boil. It can absorb 198 kJ/kg during vaporization, so for every kW a superconducting generator outputs, 5 milligrams per second of liquid nitrogen needs to be expended. Using the expendable liquid nitrogen solution, we can have the USAF 10m Orion dedicate 40 tons to electrical production, and 16.25 tons to liquid nitrogen reserves (adding up to a quarter of its 225 ton payload, as before). It would be able to output a whopping 1.6 GW of electricity, but only for 33.5 minutes before liquid nitrogen reserves run out. It’s not too bad; the spaceship would likely run out of pulse charges before it uses up all this coolant. The active solution is to use a cryocooler. It raises the temperature of the waste heat to a level where it can be disposed of using radiator panels of reasonable size. If the high temperature superconducting material operates at 100K, then it takes at least 4.7 Watt of cryocooler power to move 1 Watt of waste heat up the temperature gradient to 570K. A realistic cryocooler will achieve 30% of maximum Carnot efficiency, so we increase the power requirement to 15.7 Watts. We choose the 570K temperature target to keep using the cooling equipment from previous calculations (all the better to compare each solution). Cryocooler power density for aerospace applications is about 133 W/kg, but 300 W/kg is cited as an achievable goal. Putting these elements together, we have 1 MW of input power becoming up to 1 kW of cryogenic waste heat, which requires 15.7 kW of cryocoolers that mass 52 kg. The active solution brings down average power density to 12.9 kW/kg. It is a respectable figure, better than the non-cryogenic design’s 7 kW/kg, and especially interesting for missions with prolonged engine use with no opportunity to refill on liquid nitrogen.. A USAF 10m Orion that used an actively cooled superconducting generator massing 56.25 tons would produce 725.6 MW as long as the engine is running. There is a ‘catch’ to these cryogenic designs though. Superconducting magnets are not known to be resistant to radiation or damage of any kind. It is especially concerning when a nuclear pulse propulsion spaceship bathes itself with penetrating neutrons and high energy gamma rays repeatedly. The magnets cannot be placed too far away from the pusher plate and suspension system either, so they can’t hide in the relatively safe environment the crew enjoys at the other end of the spaceship. Flexibility There are two other major benefits to the Moto-Orion. The first is during start-up. The original Orion design relied on the suspension system being pre-compressed before the first full-strength nuclear charge could be used. It was the job of a half-strength bomb to get the suspension ready. While this use of fissile material is not too wasteful when compared to the hundreds of bombs that are regularly used, it is very inflexible. Start-up would only be possible a limited number of times, and only when the pusher plate is standing still… not at all comforting when space travel involves must-not-miss burns. It is even worse for a warship that needs multiple successive starts and stops to effect dodges from enemy fire. A Moto-Orion can use its electric generator in reverse, to produce torque while consuming energy from battery reserves. It can draw in the suspension arms to a compressed position, or time its pushes and pulls to bring a wobbling plate to standstill more quickly. The batteries can even be charged from another power source, such as solar panels, if battery reserves are depleted. This gives the spaceship an unlimited number of restarts. It gains the flexibility to halt and ready its drive at any time. The second benefit is recovery after the pulse sequence goes wrong, whether it is late, early or missed completely. Accurate suspension cycle for an Orion craft, by ElukkaJ. A Moto-Orion might be able to react quickly enough to adjust the position of the suspension system in case of a late pulse. Once the nuclear shaped charge moves past its designated ignition point, the spaceship’s motors would draw power to slow down the retreating pusher plate. This could prevent it from being accelerated into the suspension arms at an excessive velocity. When things go wrong, unpleasant, up to destructive, g-forces are generated. An early detonation is especially troublesome. Not only does it erode the pusher plate, it cannot be predicted. The Moto-Orion’s crankshaft and generator can be turned into an additional suspension arm to absorb the unexpected shock, but it would usually be weaker than the massive steel springs the engine habitually relies upon. Still, it can assist in bringing the pusher plate velocity back in line and ready to receive nuclear plasma blasts again. When it comes to misfires, Moto-Orion can potentially add velocity to the slower pusher plate (as it did not receive the momentum from the missed pulse) and bring the drive sequence back into correct timing. It can avoid a complete halt by drawing energy from battery reserves, and if it is powerful enough, do so without skipping a beat. There are other forms of flexibility, gained indirectly from having access to huge amounts of electrical power. They might not be as flexible in this regard as a nuclear-electric ship could be, as power generation is tied to the use of the engine and not an independent reactor, but many possibilities open up. Orion nuclear spacecraft could deploy drones and beam power to them, by means of microwave emitters or laser beams. They could receive nuclear charges ‘on the fly’ using magnetic scoops. Electrical Reaction Control thrusters can be used, so that the spaceship can turn more efficiently. There are many more possibilities. Consequences An Orion spaceship staging off a aerobraking lander at Mars. Moto-Orions are safer and more flexible than the original Orions. For a simple transport ship that only uses its engines briefly and wishes to maximize payload, the extra weight is unwelcome. Any craft that carries people might instead find that the additional capabilities and securities are a worthwhile trade-off. Warships would absolutely desire Moto-Orions. The huge amounts of electrical power turn them into terrifying attackers that can both unload with weapons energized by hundreds of megawatts of power while also performing multi-g evasive maneuvers. In a science fiction setting, Moto-Orions can deliver the retrofuturistic aesthetic of spacecraft riding on nuclear blasts while also making possible the use of exciting hardware like lasers and coilguns. One setting, RocketPunk, is in development by Ad Astra Games (and by Rick Robinson, who inspired ToughSF). It features Orion-propelled warships battling for Mars in an alternate Cold War future. More engaging action could be made possible with these motorized variants. The fact that a Moto-Orion connects electrical output with drive power by a single-digit percentage ratio is an interesting feature by itself. We discussed how this avoids troublesome issues such as The Laser Problem, where overpowered lasers have excessive ranges and render maneuvering during ship-to-ship combat useless. Low electrical power and high drive power give room for dynamic combat that is more exciting for readers or viewers. Other types of ‘torch ship’, like a rocket with an immensely powerful fusion reactor, could have better performance than Moto-Orion, but would have proportionally more electrical power - this pushes combat ranges so far out that maneuvering is rendered pointless again. The military potential of Orion was always at the forefront. Another bonus towards dynamic and interesting space combat is an Orion drive’s ability to continuously accelerate and outrun missiles that have less potent propulsion systems. Due to how poorly nuclear pulse propulsion performs when scaled down (burnup ratio and thrust efficiency drop dramatically), a missile would not be able to keep up with a full-sized Orion drive unless it had its own large and expensive pulse propulsion system. They would be excessively expensive, so only smaller and less powerful engines would be available to missiles. Consequently, Orion warships have a good chance of outpacing missiles. It creates a situation where one side having more missiles than the other does not automatically guarantee a win. Instead, careful use of maneuvers and relative positioning to set up a shot with short-legged missiles is necessary. All the better to read about or play through! The Project Orion battleship. We suggest going out and applying these calculations to bring motorized variants to other Orion designs. Huge spacecraft like the 4000 ton USAF 'battleship' could benefit immensely from this concept. You could also think about how Medusa could extract electrical power from its tether strokes, or even more outlandish ideas, such as a propulsion system where high velocity kinetic impactors strike a lump of propellant to create a jet of plasma that strikes a pusher plate, like a non-nuclear Orion.

Technically, it is infinite propellant and not infinite Isp. Antimatter definitely beats ion propulsion, but anything with less energy density than that cannot do so... The photon rocket will likely look like a solar sail, except glowing hot at 3000K+ on one side.