Textual definitions of the tech levels of space craft propulsion systems.
I try not to look too closely at this issue but I have a bunch of goddamn geeks for players who constantly buy Tech 7 or 8 and tons of tech skills, and ask baskets of irritating questions (and then get pissed when the action slows down--I've been frustrated of late, can't you tell :), so I assume:
TL4--hah! Orion propulsion at best. Don't launch in atomosphere! :)
TL 5--"primitive" fusion, using tritium-enhanced fuel. Tritium's not a happy element and it decays rapidly (5.8Y half life IIRC). Most fusion fuel to be had in the KW at the time of Alexius' reign is tritium-deuterium since most reactors are TL 5 (and the higher TL reactors *can* use this fuel). Water is used as reaction mass, so it is not an unusual sight for a ship to heave-to on a comet out a ways and break off a chunk if necessary.
TL 6--"developed" fusion much more elegant than TL 5. Deuterium-deuterium reaction requiring moderately enhanced, purified heavy water fuel. Still requires reaction mass. Many ships have heavy water purification equipment on board.
TL 7--"advanced" fusion with reaction mass collector--an even more elegant version of the TL 6 variety. Don't ask me how this works but since there's all sorts of debris and crap in space, it uses this as reaction mass (and thus basically works like "impulse engines" or the like)
TL 8--matter/antimatter if you want it (and it helps you, probably not, unless you have a source of free floating antimatter around). More elegant version of TL 7 stuff in the fusion camp, pretty much "turn the key and it goes." Mr. Fusion if you recall Back to the Future....
TL 9 and up--shit that I cannot really comprehend, including drives that somehow tap into the intertial energy of the spin of tha galaxy or some such tripe.... (Plug: read Edward Llewellyn's book Salvage and Destroy for ideas and a good story, too.)
Don't push on any of this too hard, I'm not a physicist, but it at least gives you an idea of what's going on. .