So in ENT and TOS "impulse" is some sort of fusion power - possibly even some form of pulse fusion like in the proposed Daedalus starship design. (I like how on book called it "InterMittent Pulse fusion"...or "I.M. Pulse".)
Then in the TOS-era cast films, we now have this "impulse deflection crystal", which would seem to channel warp power to the impulse drive, so I guess fusion power sources wouldn't be necessary(...?) (We also seem to possibly be able to use the warp drive to augment the impulse drive, so that we can travel something called "warp .5" and other fractions of warp speed...which makes sense if some theories about the impulse drive being more than a mere reaction-based propulsion system...which I like, since they have artificial gravity, they would also have the basis for a reactionless drive...in fact, warp-drive itself could be said to be a reactionless drive - but I mean they could use artificial gravity to propel - push, pull and stop - the ship. Anyway...)
Now by TNG's time, we are back to just fusion (according to the technical manual. Inertial confinement fusion, to boot!) I dunno, seems kinda like a step backwards to me.
Then again, I guess it makes sense to have a seperate systems - if the warp core goes off line, or is damaged, or has to be ejected - you won't be stranded. Maybe that's why it was fazed out and fusion brought back...?
Or am I wrong and the TNG-era impulse drives can also be powered by - or at least get a boost from - the warp drive plasma?
What do you techie Trekkies think?