wrog

Also, thankfully, impulse engines (particularly in TOS) are very specifically not rockets, for all the same reasons. (Roddenberry made a big point about this early in show production. No rockets. None.)

I don't know whether what they actually are has ever been stated. I've been kind of thinking some sort of gravity control (antigraviton plates?) since we know they have artificial gravity and it's apparently some sort flat source (since it's so even throughout the ship despite all the disparate parts) and so on.