[QUOTE=Ulzgoroth;872486]
Nuclear rockets, except nuclear salt water, have a built in reactor (with how much endurance, anyway?) providing a lifetime supply of fuel.
About the same as any other nuclear reactor. Realistically somewhere around a year, give or take a factor of 10, for anything except a direct contact gas core system (which loses fissionables to the reaction mass stream)
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Fusion rockets could fuse their reaction mass...considering that they're plasma thrusters, they must be directly exposing the reaction mass to fusing material at least.
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Nah, they could be some sort of electric rockets run on a fusion plant, or injecting the fusion exhaust into the reaction mass stream.
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But then there's air ram mode. It still has to fuse something, but it doesn't burn off anything from its tanks. Is the fusion reaction fuel negligible compared to reaction mass?
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Yes, it is. Depending on the fuels used, fusion reactions liberate a few percent of the mass energy of their fuel. A fusion reactor uses something like 1/10th as much fuel as a fission reactor for the same power output. Conversely they use about 10 times as much fuel as an antimatter rocket with the same performance uses antimatter.
With the exception of low thrust high endurance designs with specific impulses up in the millions of seconds, fuel use is tiny compared to reaction mass.
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Antimatter rockets, of course, use the antimatter which is included in their tankage as fuel. That may be a negligibly small mass (except for the pion drives, of course), but it's essentially the entire cost of fuel. So how do antimatter air rams work? They can replace the reaction mass, but obviously that does nothing to stretch the antimatter supply. Maybe they should carry extra antimatter for air ram mode?
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Probably. They'd continue to use antimatter at the same rate in air-ram mode as they would in rocket mode, so yeah, counting the antimatter as part of the fuel tank rather than separately doesn't work well here.