In "The Apple" (TOS), Kirk orders Scotty:
"Tie every ounce of power the ship has into the impulse engines. Discard the warp drive nacelles if you have to, and crack out of there with the main section, but get that ship out of there!"
(That
"main section" referred to the saucer can be observed from its description in the writers' guide:
"...the ship's main saucer-like section...the portion of the ship in which we will be and which we will use most...contains at the very top the ship's bridge and general operation facilities...also primary ship's departments, living accommodations, recreational facilities, laboratories, and is in fact a completely self-sustaining unit which can detach itself from the galaxy drive units and operate on atomic impulse power for short-range solar system exploration...")
We get another order to
"disengage nacelles; jettison if possible" (no saucer sep mentioned this time) in "The Savage Curtain" (TOS).
FWIW, there's also this graphic from "Day Of The Dove" (TOS):
Throughout TOS and TAS, on the internal side, we see engineering modified in its layout, with large chunks of machinery apparently being added and removed, etc. Then in the movie and TNG era we see ships that separate in various ways (TMP warp sled,
Enterprise-D saucer sep,
Enterprise-E captain's yacht [formerly a torpedo launcher!],
Prometheus multi-vector assault mode), ships within the same class seeming to have external components swapped in or out (various
Nebula aft pods,
Miranda rollbar and mega-cannnnnon thingies, AGT
Enterprise-D) and ships that seem to be cobbled together from components of different design lineages and scales (DS9 'Frankenstein fleet' kitbashes).
The runabouts on DS9 were always meant to be modular, too, beyond just the rollbar component we saw:
And of course, while not
necessarily applicable to the Prime setting, there's always...
What do you suppose would have happened if the
Enterprise's saucer had somehow made it back to Yorktown alone without the secondary hull and nacelles, or vice versa, or another permutation of the preceding constituents? Scrap or rebuild? Could probably go either way, depending on the circumstances.
The
Enterprise has been known to get into a scrape or two, after all...
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MMoM