You can’t replace Raptors with NTRs because of size/thrust mismatch. A NTR with the same vacuum thrust as a Raptor would weigh at least 80 times more, and likely be far larger. Then you have shielding and radiators, it’s never gonna be able to lift off the Moon.
And you aren’t saving space losing the oxygen tank. First you will need that space for hydrogen propellant which doesn’t have the density of Methane. Worse is it also offset by requiring expensive (in energy and cost) additional cooling for Hydrogen.
And you aren’t saving space losing the oxygen tank. First you will need that space for hydrogen propellant which doesn’t have the density of Methane. Worse is it also offset by requiring expensive (in energy and cost) additional cooling for Hydrogen.