Comment Re:It's almost hard to imagine what it really mean (Score 1) 104
Note that if it's nuclear thermal, however, you'll need to rethink your tanks. Nuclear thermal basically has to be hydrogen. You're limited by the temperature off your reactor, and the amount of thrust you get from a given temperature is inversely proportional to the atomic mass of the propellant. The downside is a poor thrust to weight ratio (although it's significantly improved since the days of NERVA). But yeah... your fuel needs to be hydrogen, and that means tankage changes.
Note that one could also use Starship for a *hybrid* or "boosted" nuclear thermal rocket. In this, you still have a LOX tank, but a much smaller one. You can now run in three modes, and any transitional state between them:
1) Purely chemical
2) Purely nuclear thermal w/hydrogen
3) Chemical + nuclear (significantly higher ISP than purely chemical, significantly higher thrust than purely nuclear thermal).
So you could for example have the reactor in a standby state during the launch, then start ramping it once you have significant altitude and velocity, so you haven't produced any meaningful waste at low altitudes / velocities. After separation, thrust steadily transitions from chemical / boosted to purely nuclear thermal.
Note that there are some (much less mature) designs to try to get past the thermal limits of conventional materials. One is the "nuclear lightbulb" concept. In this, you have a gas core, surrounded by fused quartz tubes, and "seeded" hydrogen flowing through the quartz tubes. The gas core itself is hottest in its interior, coldest near the tube walls. It kicks off an insane radiative flux, which mostly passes through the fused quartz without turning into heat. In the propellant, the inverse situation occurs - the hydrogen gets hottest away from the quartz walls, coolest by the quartz walls, and keeps the quartz walls cool - something that can be further biased by the seeding patterns (rather than pure hydrogen, you mix in a dust of opaque material like carbon or high-temperature ceramics).
Very immature, though. And limited lifespan, as quartz blackens over time under high radiation fluxes.