Glushko

I actually agree with you - Karl Schroeder actually had a piece about the advantages of a fusion reactor in space. My concern here is that material science developments were a factor in the documents on fusion drives I pulled off the NASA technical reports server - and the need for robust cryogenic stuff. It's gonna take significant work with reactors on the ground (trying real hard to not be biased with magnetic confinement configurations, I'm partial to Spheromaks). I don't see how a non-break even/high k value fusion system is going to be any better then a VASIMR running off of beamed power. The T/W should be superior, no neutrons to worry about, and it is higher on the TRL scheme. Granted I'm just totally mad about inertial/MTF pulse systems so it could just be my bias. And beamed propulsion - I do love my beamed propulsion.

Nuclear thermal rockets use pretty much whatever working fluid you want. Water, ammonia, hydrogen... IRL NTR designs are low-thrust/high ISP due to the mass of the reactor. This is why the majority of nuclear thermal rockets intended for launch vehicles are used to drive the upper stages. You can also dump liquid oxygen into the exhaust for a nice afterburner effect - more thrust but it halves your ISP. That is a really optimistic view. VASIMR is a complete power hog - which is why the fast mission reference design uses a gas core reactor for power. D-T fusion is gonna take more then the hundred kW power draw they needed to bust out the exotic nuclear engineering for. Plus you have to deal with the neutrons. Cue embrittlement, activation, and fitting additional shielding into your payload mass.