The problem with the 'newtonian physics' for the impulse drive, TGT, is that Roddenberry ALSO says that they're fusion engines
, and fusion doesn't work that way
At any rate, even if there were, there's no reason at all that the Phoenix (indeed, any warp-based ship) couldn't just have the 'warp field' envelop the ship and keep the 'nuclear rockets' for thrust. Remember, relative within the warp field, the ship doesn't have to go ALL THAT FAST to worry too much about inertial dampening anyway - since the mass within the field is normalized and the inertia is only what the relative /non-warp/ velocity would be.
Yeah, still, we ARE talking insane numbers here, mind you, but SOMETHING has to normalize the cabin.