So if they weren't bussards what were they?
They are never directly addressed in the series, but we can surmise their purpose. For example we know that the antimatter was stored in the nacelles ("anti-matter pods" aka "pods"). We also know that the ship had were three (possibly M/AM) reactors. With the antimatter in the nacelles it would make sense for the m/am reaction to take place there. So to main reactor in the nacelles with probably a smaller controlling reactor in the secondary hull( ala "That Which Survives"). Additionally we have the Constellation which in its damaged condition had both nacelle domes exploded, indicating that there must be some large amount of energy at play. This is also emphasized by the fact that the nacelles are ou tthere on struts so that the huge dangerous energies our out there away from the main ship. Finally in TMP we see that there are no more swirly domes inside the ship. Instead we see a blue glowy dome on top and the swirly intermix shaft. It's as if the function of the domes was moved into the ship.
All of this circumstantial evidence indicates to me that the nacelle domes were not interstellar vacuum cleaners, but instead were m/am reaction chambers. They were glowy and swirly because of the intense reactions going on there.
Why the change in TNG? Or just a mistake and they ran with it?
In Universe:
Well, with the M/AM reactions now going on inside the ship. This left the front of the nacelles free. You'll notice very little in this area on the Enterprise refit/A, Excelsior, and Enterprise B. By the Enterprise-C time they added Bussard collectors into the nacelles.
Reality:
Probably to draw visual similarities between the Original Enterprise and the Enterprise-D they kept the glowing elements on front of the nacelles. Just look at the early Ent-D sketches. Then during TNG they devised that those red glowing areas were bussards. That was then retroactively applied to the original Enterprise, even though there is zero evidence to support that they were bussards in that series. As mentioned by
ZapBrannigan I'm not sure if the fandom had accepted them as bussard-like devices before TNG and the TNG production drew from that fan based knowledge, or if the TNG production devised it on their own. Probably the former.
That concept got into the Star Trek zeitgeist long before TNG, just without the Bussard name. In Franz Joseph's Star Fleet Technical Manual (1975), the front-end dome is labeled as "Space Energy / Matter Sink (Acquisition)."
Geoffrey Mandel's diagram of the nacelle interior followed closely in FJ's footsteps. This was first published in Star Trek Giant Poster Book Voyage 13 (Sep 1977). He labels the forward dome as "Matter Intake."
I was referring to on screen references, but you are correct in pointing that out.
Good globes think alike. The antimatter is injected into the forward orange globe (M/AM reactor) which has an excess volume of matter, and it superheats and ionizes the matter into warp plasma. The fully ionized warp plasma runs the warp engines. Part of the warp plasma flow is routed into the secondary hull to be run through the dilithium crystal converter assembly to generate main power for the ship and convert the matter into antimatter to refuel the warp engines. "Regenerative Fuel Cycle".
You can see my creation here:
https://www.trekbbs.com/threads/flight-of-the-phoenix.296684/
Interesting that you have warp plasma going to the dilithium crystal converter assembly? Because if I remember correctly the dilithium crystals themselves provided power without warp plasma. But energy from the m/am reactions was what was used to recharge the dilithium crystals when they were depleted. But I guess my comment might be too far off topic and there already is the separate thread for it
You are seeing the glowing M/AM reaction with the internal electromagnetic field generator rotating inside the domes to keep the ionized gas confined, compressed and to cause the fully ionized warp plasma to separate for extraction. Trust me, I'm an expert on glowing globes with hot gas in them.
