Technical Officer: All right. The Chief and I were supposed to tell you command trainees about the K-drive. Only he won the coin toss so it's just me this class. Everybody know what the letter 'K' stands for?
Cadet: Kinetic. (The duh is left unspoken.)
T.O: Ah we got a college man there. Right. The K-drive imparts kinetic energy to reaction mass. This exits the back of your ship allowing it to accelerate in the opposite direction. Since you want to use as little re-mass as possible you want it to go very fast. So we're talking rockets. What kind of rocket do we use?
Cadet: Tenth generation Vasimir nuclear rocket.
T.O.: What does Vasimir stand for, wise guy?
T.O.: Variable specific impulse magnetoplasma rocket.
Cadet: Where does the second 'I' come from? Shouldn't it be vasimAr?
T.O.: ... moving right on. In the basic structure it is very similar to the first engines that took mankind to the planets and the nearer stars. Modern technology has let us improve the engine's efficiency dramatically. We don't have to drag an ice asteroid around for propellant the way our grandparents did. Can you tell me what the exact improvements were?
Cadet: Well we use a series of gravity generator toroids to accelerate the plasma exhaust dramatically increasing thrust and fuel efficiency.
T.O.: Specific impulse, not fuel efficiency. Engines use reaction mass or propellant, not fuel. But right. The grav generators increase thrust and specific impulse. You can get by with as little propellant as you want at the cost of increased power drain for the grav boosters.
Cadet: I don't understand something. If we already use gravity generators to fling propellant out the back why don't we just use hydrogen, or water for that matter? Why lug a fusion reactor around to feed the rocket plasma? There can't be that much extra thrust provided.
T.O.: My gosh that's a good question. I may need to sit down. Those are good points. First of all the rocket's reactor can serve as a back up power generator in emergencies. That feature has saved a few lives in Fleet history. Second grav generators can lose power for a variety of reasons. Then your water propellant will not do you any good. With the reactor design you still have a rocket that will take you at least some places.
Now what about the other chief improvement over the old rockets?
Cadet: ... force fields?
T.O.: Not very specific is that? Anyone want to elaborate? Right. Forcefield technology and old style magnetic fields keep the hot plasma from contact with the rocket engine, which you want to discourage. If the fields buckle you'll have a melted engine or worse. Our engines also have a heat tap to the main defensive shields. Those monsters already deal with multi-megaton nukes and terawatt energy beams and disipate their energy. The engines use them to radiate their waste heat away instead of the old style physical radiator fins. You military types will try to aim that heat away from prying eyes to try for a stealthy approach. Sometimes you even get it right. Oh all right, what is it now?
Cadet: We do have a radiator fin on the Tesla and other ships. Why?
T.O.: Again back ups. When you're in a fight you want as much radiator area as you can. The shields operate at various radii, see. When you're cruising they're bigger to radiate heat more effectively. To stop a beam cannon or nukethey have to contract, sometimes to a few meters from the hull. Less room to radiate heat, more heat build up. Back up fins can save your life then. Except ... The shields work both ways. They absorb EM and heat from your ship as well as external energies. We have to open holes in the shields for our rocket exhaust, sensors, comms and radiators. If a lucky shot hits your radiator in the middle of a fight through the hole opened for it you'll lose the radiator fin and then the engine, a few seconds later.
Cadet: So do we have a back up for a radiator hit?
T.O.: Sure. Lots. They're called lifeboats.