OK, I think I decided on the one tier engine, and I refined the shapes some more...Uhh. No. Impulse engines are fusion rockets. Those ARE thrust vents. (We are talking about the Impulse Engine assembly, correct?)
Nope. Two words make that impossible, at least for conventional usage... "Reverse Impulse!" To say nothing of the CoM problems that the Enterprise would have...
There are no COM problems on the Enterprise if you assume the nacelles are denser then the rest of the ship. Seeing as the secondary hull is mostly hollow that's not a huge leap to make.

Oh, and I turned them on this time.
