A very intriguing idea! I certainly like it better than them being hangar bays.
As far as I'm concerned, they're big bulky prototypes for the "modern" 24th century driver coil. I think all ships all the way back to Archer must have had some form of this, albeit no doubt very primitive, and that the deflection crystal on the ships that had them played a role in mass reduction. However, I think
Excelsior was so big that they overdid it; the impulse engines as installed could move the ship adequately, but always ran hot and were gigantic. This required lots of maintenance and the funny little fins on top of the model.
For the
Enterprise-B variant, the standard impulse engines were "geared down" to run cooler because they were supplemented by the bulky new impulse engines, and the two radiator fins were removed.. Together, all four engines could move the ship much more effectively, and all ran less hot. The direction of exhaust for the new engines didn't matter because on the "modern" engines exhaust wasn't the main propulsive force and could be expelled on the fly in any direction. Later refits would see modern versions of the impulse engines applied in the standard location, but ships that mounted the extra engines would keep them to retain the extra power generation capabilities (and slight edge in maneuverabilty) that they afforded.
I assume the
Enterprise-B nacelle fins had something to do with either warp streamlining or possibly radiator functions needed for the new Bussard collector assembly. Actually, trying to figure out how pre-TNG engine features correlate to TNG-era engine features is rather interesting.
Enterprise refit type engines kind of have almost a compound Bussard assembly at the front, but who is to say if that's really what those elements are? Plus the original
Excelsior definitely didn't have its own Bussards. And then the
Grissom had a solid plate where a Bussard should go... with those things on the saucer underside perhaps being some kind of ramscoops.
I think this all tells us there's more than one way to gather fuel, and Starfleet experimented for a while before "getting it right" in TNG. So, the impulse Bussard booster could still be an added benefit.
Nothing to show tonight, but hoping to have some progress to show this weekend. I'm basically starting over.
@yotsuya's plans are so perfectly precise that I need to conform to them. Those plans are a bit more proportional and definitely more detailed than my previous resources. It may be time to switch gears to working on the history text a bit more while I work that out, as I have to have accurate schematics and floor plans to really work on the technical portion.