Had been talking NSWRs on a spaceflight forum and a thought occurred to me. Lithium-6 fission can be triggered with relatively low-energy neutrons and releases 4.78 MeV, a helium-4 atom, and a tritium atom. Without a neutron flux, however, lithium-6 is completely stable. With a small electrically powered neutron source and a magnetic nozzle, a lithium-hydride-fueled thruster could have specific impulse approaching 3% of the speed of light, making it an extremely promising low-thrust engine. But that's not what I'm interested in. In order to generate significant thrust, you need a high neutron flux...on the order of what you get inside a nuclear reactor. So why not? Natural uranium cannot sustain a chain reaction without a neutron moderator. Thus, a mass of natural uranium inside a tungsten vessel (for neutron reflection and for containment) is quite safe. Add a neutron moderator, however, and you'll get a chain reaction and very high neutron flux. Lithium hydroxide dissolves in water at 129 g/L. If you dissolved enriched lithium-6 hydroxide in heavy water, you would have a propellant with a density of 1,239 kg/m3 which, when pumped through a natural uranium cylinder, would immediately moderate neutrons, trigger a neutron flux, and undergo clean fission from that neutron flux. The reaction is inherently self-limiting, as the water is converted to steam by the lithium fission energy and thus stops moderating the neutrons. Such a propellant mixture has a theoretical exhaust velocity on the order of 2,000 km/s. The propellant serves as the reactor coolant, the moderator, AND the fuel source: This design avoids the problems with a nuclear thermal rocket because the reactor pile only needs to get excited enough to generate a high neutron flux, not generate the energy to heat the propellant. It also avoids the "giant nuclear bomb tank" and "fissioning radioactive death spray" problems inherent in an enriched uranium saltwater rocket a la Zubrin. There are also no moving parts anywhere close to the nasty bits. If water proves too corrosive, then you could use lithium-6 hydroxide dissolved in ethanol with a graphite suspension. Varying levels of coolant performance, neutron moderation, thrust, and specific impulse could be achieved by mixing pure heavy water or pure light water in with the saturated saltwater; this could be done dynamically at the turbopump since none of the propellant components are particularly dangerous. I foresee a few major challenges. I chose natural uranium since it is cheap and safe, but it may require a very high mass to achieve moderated criticality, which would put a high threshold on engine size and thrust. A different fissile neutron source might be better; I don't know how far down you can scale something like the HFIR. Also, you don't really want your propellant directly in contact with naked fissile material, so you'd want to have the fissile material separated from the propellant by some material that is neutron-transparent and has high strength and heat resistance but is also thermally conductive. Refueling the fissile mass would be tricky if you wanted to reuse, though I could conceive of a replaceable fuel rod arrangement. The exhaust isn't entirely safe; it releases a good bit of tritium. But that's far, far better than the nucleotides released by a uranium NSWR. Basically, you don't want to be anywhere near this exhaust stream (not the least of which because it will melt any launch pad) but it shouldn't be prohibitive. It may be useful to include a supercharger to mix atmospheric air in with the exhaust stream to increase thrust and reduce the energy level of the exhaust. An overly-optimistic discussion of a lithium saltwater rocket was discussed https://www.linkedin.com/pulse/20140724165847-39571567-nuclear-salt-water-rockets-revisited [Broken], but with the unrealistic expectation that Jetter-cycle D-T fusion would take place within the exhaust stream and that a lithium deuteride suspension would be used rather than dissolved lithium hydroxide. This is unnecessary; lithium fission provides plenty of energy on its own.