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Those aren't necessarily all that massive, though. Enormous in size, yes, but the mass would be a lot less than the mass of the fuel for less efficient rockets like chemical or nuclear thermal.

Either way, any vessel travelling beyond Mars or Venus in a time short enough to be safe for humans is going to require in orbit assembly, so size in terms of volume becomes less of a constraint. But as long as we're boosting all of our mass from Earth, that's what costs the money.

I think STR is the best possible competitor to chemical rockets for trips inside Jupiter’s orbit. NTR is what we will need out past Jupiter unless a few brave souls use chemical rockets for the fame of being first to make those extremely long expeditions to Jupiter and beyond. Much like early explorers in the age of sail.

Or even the Age of the Black Friday Sale! ;-)

The hotter your reactor is, the higher is your efficiency, and the smaller radiators you need.