It is true. Basic gas theory stuff, the average molecular speed (close to the speed of sound) at a given gas temperature is, to first order, higher for a lower

It is true. Basic gas theory stuff, the average molecular speed (close to the speed of sound) at a given gas temperature is, to first order, higher for a lower molecular mass. Otherwise, why bother with such a difficult to store propellant which you’re not even extracting energy from (as the energy comes from the reactor, not the propellant as in chemical rockets)? Chemical rockets reach over 3500 Kelvin, but Nerva only got to around 2300 Kelvin.