What kind of neutron source can generate a gram/sec of neutrons?What kind of power input would you need for that?



Even milligrams/sec is awesome. Micrograms/sec ...



How would you avoid, or at least mitigate, chemical ignition of the lithium fuel? Lithium will chemically react with both of the fission products.



Coating the fissile material on the inside of a nozzle dose not seem practical, the neutron bombardment will penetrate into the coating even if it is fairly thick and cause fission in the whole mass of fissile material. The resultant fissile byproducts will be generated inside a solid fuel mass and will collide with neighboring atoms and cause standard thermal interactions as they slow down, the whole fuel mass will simply heat up rather then emit a surface sputtering that I think your looking for.



Some form of tiny-pellet feed system inside a magnetic nozzle seems to be the only way to get a well controlled reaction that would produce manageable thrust. The pellets can also contain non-fissile materials preferably highly transparent to neutrons which will be vaporized and contribute the bulk of the reaction mass, this will lower the ISP down to usable ranges, different pellet mixtures could produce different thrust and ISP ranges for the engine allowing a kind of throttling which has been shown to improve overall trajectory planning. The neutron source would need to be pulse based and would only need to produce a single standard pulse both of which should make it simpler.

It might work to use a "dirty" fissile primary as your neutron source, like a subcritical plutonium sphere surrounded by neutron reflectors . A little unsafe, but at least you don't have to worry about carrying a power source, and fissile material isn't released into the environment. I don't know what kind of neutron count is generated in controlled criticality...Well, I had proposed using lithium hydride rather than pure lithium metal, as this increases the particle count and also prevents the lithium from reacting (since it has already reacted). Of course the helium won't react with anything. The monatomic hydrogen might react with itself but that's not necessarily a bad thing. Specific impulse is less important here so an increase in molecular weight is not really a bad thing.See below for more discussion of storage.I was actually thinking of a very thin coating with a beryllium neutron reflector on the other side to produce more of a sputtering reaction. You could imagine stacking a series of thin "fuel layers" alternating between lithium-6 hydride and beryllium, with each pair of layers being discarded once most of the lithium hydride had been used up.A magnetic nozzle is a possibility; lithium hydride is itself highly diamagnetic, but it's also an ionic conductor, so there are quite a few ways that could be used.Since reaction mass is a much bigger concern than specific impulse (seriously, weakly relativistic ion spray is NOT nice), what about using something like enriched lithium-6 hydroxide saltwater? Lithium hydroxide dissolves in water at 129 g/L; a single liter of such saltwater therefore has 5.4 moles of lithium-6, with a total potential energy of 2.5e3 gigajoules. For reference, that's about 4% the yield of the Little Boy bomb dropped on Hiroshima...absolutely insane. Lithium-6 has the highest specific energy of fission of any fissile material.If you used heavy water rather than light water, then you could use (cheaper) natural uranium as the neutron source. You'd want to cast natural uranium into a cylinder with a converging end and wrap it in a beryllium neutron reflector; put a tungsten diverging nozzle on other side. Give the inside some sort of neutron-transparent but very heat-resistant coating (because, for the love of all that is good, you don't want fissioning uranium spraying out with your exhaust stream); not sure what material might work for that. When you pump the enriched lithium-6 hydroxide heavy saltwater into the exhaust stream, the heavy water will slow the fast neutrons from natural decay down into thermal neutrons, causing the natural uranium to go critical and release a spray of neutrons. These, in turn, collide with lithium-6 nuclei and trigger the lithium fission.A liter of enriched lithium-6 hydroxide heavy saltwater masses 1.229 kg, so the lower bound for exhaust velocity is going to be on the order of 2e6 m/s. Of course, you can decrease this and increase thrust by simply decreasing the enrichment of the lithium hydroxide or decreasing the amount of lithium hydroxide dissolved in the heavy water, though doing the latter tends to decrease the density of your propellant which is less ideal. Another issue is that not all the lithium-6 will fission; I don't really know how to estimate the percentage though.The only power requirement is the turbopump to push the enriched heavy saltwater into the "combustion" chamber, and you can probably use a fuel coolant loop for that. I'd need to dig pretty deep into nuclear reactor design and criticality to see what the minimum possible size would be. Thrust to weight ratio is going to be respectable; if you were using full enrichment and full fission, matching the thrust of a SuperDraco would require a mass flow of just 36 grams per second.If this design isn't clear let me know and I will do a graphic mockup.Does anyone know of a material with very high heat resistance that is neutron-transparent?