>>12516167SpaceX is mostly in the LEO/GTO buisness.
Chemical rockets are the only technology so far that gets us into LEO without major issues as they provide a good thrust/weight ratio, good enough specific impulse and good enough safety.
Nuclear propulsion on the other hand has a worse thrust/weight ratio, great specific impulse and usualy a radioactive exaust.
For lower stages, you obviously don't want nuclear engines, but reuseable chemical ones.
If you only need to go to GTO, there isn't realy a reason to include nuclear engines, especialy since one would need to dispose of the upper stage.
Instead we just use slightly larger upper stages to go from LEO to GTO.
So we use chemical rockets for stuff in earth orbit as GTO is the highest orbit we launch stuff into at a significant volume.
Only when you need to go far beyond earth orbit on a high energy trajectory, nuclear realy starts making sense.
But even then chemical rockets have a tendency to be cheaper while having more managable risks.
So:
>nuclear first or second stageNot going to happen.
>nuclear third or higher stageMaybe, but unlikely.
