[RightWhale]

So long as the thrust is enough to lift the assembly off the ground, the overall performance will depend on specific impulse. A nuke powerplant would have a lot more specific impulse than the chemical powerplant, which results in much more payload. Specific impulse has units of seconds and is called I_SP or something similar. A way of thinking about I_SP is how long the powerplant can produce that thrust. The chemical rocket will run out of fuel while the nuke will still be blasting away for a lot longer.

[SunkenCiv]

Yeah, the amount of propellant can be smaller, provided the impulse is caused by the heat from the nuclear reaction. That could greatly cut the amount of propellant needed, which of course is reduces the overall weight, making the propellant used even more effective... Asimov's fictional rockets (the more or less conventional ones) used nuke-powered steam engines, and his explanation was that the mass*velocity of the propellant almost equals the mass*velocity of the craft (in any design using propellant), so less propellant going faster is the same as more propellant going slower. :')