Related: Is there some fundamental limitation that would prevent steam-powered rockets from reaching space?

Thermal rockets, including nuclear thermal rockets, work best with minimal molecular weight propellant gases. This is because, for a constant temperature or a constant amount of thermalized kinetic energy per unit mass, low molecular weight leads to a higher velocity of the particles and therefore a higher exhaust velocity.

The effect is, inconveniently, strong enough that there is almost no point in using anything but hydrogen as propellant for a solid-core NTR, and many common volatiles such as water manage the impressive feat of being worse than even low-performance chemfuel.

Are there methods that can be used to improve the issue or get good performance out of non-LH2 propellant or to operate nuclear thermal rockets with good performance on higher density propellants?

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    $\begingroup$ Seems doubtful that you'd find anything that wouldn't also make the performance of an H2 NTR proportionally better. The solution is probably "don't use an NTR". $\endgroup$ – Starfish Prime Feb 26 at 20:33
  • $\begingroup$ What else would you use? There's not much else we can build anytime soon that has both high thrust and Isp over 500. $\endgroup$ – ikrase Feb 26 at 20:37
  • $\begingroup$ Use for what, though? An NTR isn't going to give you a useful SSTO for use on Earth, and whilst it might be high-thrust it is still no torchship handwave engine so it isn't going to whisk you across the solar system in a blink of an eye. $\endgroup$ – Starfish Prime Feb 26 at 20:44
  • $\begingroup$ (that said though, there are some interesting modified NTR designs out there. I've just been reading about the Scorpion, which I particularly like, or for more handwavium there are pulsed NTRs... both will get the highest Isp with the lightest propellant molecules though cos that's just how physics rolls) $\endgroup$ – Starfish Prime Feb 26 at 20:48
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    $\begingroup$ The truth is, you're probably better off using the same fissionables to build a power reactor for propellant production. Using a NTR (with hydrogen) roughly doubles your available delta-v...so does refueling at your destination. One of these options requires hauling a lot of radiation shielding around and has you periodically doing maintenance on a highly radioactive engine. A NTR also ties up those fissionables for its working lifetime while only making very intermittent use of them. $\endgroup$ – Christopher James Huff Feb 27 at 0:56

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