Yeah, but we have no useful way to channel the energy from nuclear fission into propulsion, so that’s a moot point if we’re talking in practical terms. At least, we don’t without irradiating everything. My point was not that combustion-based propellants are energy efficient, merely that it takes a lot of energy to escape gravity, and while theoretically nuclear propulsion would be more efficient, in practice, burning shit is really the best we can do without giving someone cancer every time we want to put something in orbit, because gravity is a fuck.