(response to removed post removed)
Anyway, the cool thing about nuclear fusion is that when you get a small number of protons to bond together and become an atom of higher atomic number, you get energy out (a LOT of energy), up until iron I think, after which you lose energy. After that point you get energy out through nuclear fission again, which is the technology we're currently capable of, since nuclear fission in heavy elements is easy to induce/happening on its own. Nuclear fusion releases HUGE amounts of energy, but also takes huge energy to start. Stars are self-sustaining nuclear fusion reactions, mostly using hydrogen as fuel, with ignition heat coming from a critical mass of hydrogen heating under its own gravity (IIRC).
Anyway, it seems unlikely to me that the hydrogen thruster would be a fusion torch, because if we could do that we wouldn't be using fission reactors for most of our power. The nuclear thermal rocket makes sense, if these hydrogen reactors used power (which could be explained away as potential electrical output lost to heating hydrogen, or the hydrogen being heated electrically, which would be too inefficient for a real rocket but in this game hey, we can have huge space ships with tons of reactors onboard). If it ran on hydrazine, it would probably just be called "rocket fuel" in game and there would be some process involved in making it. Hydrazine is harder to make than plain hydrogen. It's not ion thrusters because, again, the energy in those is mostly from electrical charge so we would see power draw.
I think the hydrogen thrusters operate on the principal of "it looks like a big rocket, so it goes fast! And red ones go fasta!" Keen haven't ever been in this for precise physics anyway (see: thrust acts on center mass in the thruster's vector no matter where thrusters are placed).