The 4090 actually only supports PCIE Gen 4.0, so those getting fancy PCIE 5.0 motherboards will not gain any performance over the GPU. That, and I believe (really not sure on this) 4090 is not capable of saturating the 4.0 bandwidth. OP could’ve had his 5600X in a PCIE 3.0 build, though.

The propellant mass of nuclear thermal rockets would vary based on the fuel chosen (ideally, hydrogen) and the specific mission requirements but generally an equivalent NTR will have twice the efficiency of a chemical rocket. So twice your delta-v for the same propellant and mission payload.