Good point about the shuttle, that is one compact engine! Of course, it was not fullfilling quite the same role as the Enterprises' impulse engines, which had to operate continuously and over far longer periods of time (2 months, in TPS
). It may well be that the E's setup had extra components and backup facilities that were simply deemed not neccessary to the standard operating practises of a shuttle.
Franz Joseph (as Shaw
pointed out) doesn't allocate a lot of space in his cutaway for impulse engines. But what clues are on the outside?
There's the "spine" (which I coloured blue for clarity), a red outline leading to the base of the "teardrop" and shortly after that a bright yellow rectangle, outlined in red. What to make of all these?
The spine could be an anchoring point for the saucer (I think it actually was on the model). Or it could be a reinforcing strut for the impulse engines. Or perhaps it's some sort of power transfer conduit. The long red outline carries on in this direction and looks even more conduit-like. Finally, the bright yellow rectangle screams "danger!" and so could be an engineering hatch as well.
Factor in the L-shaped hatches and that's a fair amount of engineering space overall. Is it all impulse engines? Who knows!