NTR without hydrogen

NΘP works when the N does Θ on the P. The engineers (I am not an engineer) have been talking the P - the propellant. Historically and currently rocketeers like X-Energy have assumed liquid hydrogen. I am unsure that hydrogen (boiling point: 20 K) is a good idea this side of Neptune. I asked some stupid questions last December which I hereby take back. More seriously why canwenot use - say - water (373 K), which stores better. Luckily some (actual) engineers have been coming up with their own ideas. Here is a powerpoint (literally) from one Dennis Nikitaev, via ProjectRho.

Nikitaev is an expert in hydrogen propulsion, offering a paper just last year on diluting it (with argon). Since this is a powerpoint (and since I am no engineer) I find the argument hard to follow, and it assumes the Senate Lard System rather than the SuperHeavy. I'll do my best in what follows.

The PPT does note water has its own problems: in particular it is reactive at steam temperatures (I did not know this). But ammonia isn't! - at least not with our chassis. And its boiling point although chilly (195 K) is at least viable in-system with a modicum of reflective shielding. UPDATE 10/16: Matter Beam @ToughSF doubts they can make ammonia on the Moon or Mars. He agrees that water sucks; he prefers methane. That liquifies at 100 K - not as good as ammonia, but comparable to LOX which isn't too bad. Honestly I doubt his doubt on ammonia.

Ammonia does share with water a lower I_sp than hydrogen. That cost buys us twice the thrust, though.

I care most about reducing complexity overall. We are not using ANY of this stuff in space right now; best to start as simple as possible. Ammonia might get that simpler engine. That is to say: a lighter engine, so we can bring more payload.

One issue with all the above is that we've already weakened the temperature by insisting on LEU (around here), and now we're set to knock down the I_sp as well. I wonder if we should just give up and head straight to fusion.

PLASMOID 9/4: If the molecules do break up, can we do that VASIMR thing to the plasmoid on its way out? Asking for Ebrahimi.