[paulsutter]

The article covers this. Virtually all nuclear engine proposals have used H2 as propellant because of the specific impulse, but this article mentions methane as an alternative. Of course, Starship today uses methane and Elon plans to create methane on mars.

> Most significant for our purposes is methane as propellant. It is six times denser than liquid hydrogen, can be stored at 100K, which is compatible with liquid oxygen, and it can be produced using water and carbon dioxide. At high temperatures, it breaks down into hydrogen and carbon, turning it from a 16 g/mol molecule into a 3.25 g/mol plasma. That is how it achieves a specific impulse only mildly lower than what is achievable using liquid hydrogen. Zubrin lists its specific impulse as

>Previous calculations using hydrogen propellant revealed how volume-limited the Starship design was. There was no room for the bulky liquid hydrogen, and getting to orbit meant sacrificing the payload mass and volume advantages that the Starship is built around.

>These could be solved by using denser liquid methane as propellant for the nuclear propulsion system. The Isp will be lower, but the mass ratios become so much better that more deltaV is available overall.