What was the function of ring-shaped module in 22nd century Vulcan ships?

Question

All 22nd century Vulcan ships had a ring-shaped module. Not just starships, but small shuttle pods also had this module. As Vulcans do everything logically, there must be some purposes of those modules. A repeating structure in all ships can't be a coincidence.

Over half decade has passed since I last watched Star Trek: Enterprise TV Series. So, I'm unable to recall the functions if they were ever mentioned in the show. Do you've those things in mind? Other sources are warmly welcome.

Here's small Shuttle pod:

Answer 1

In an article for Star Trek : The Magazine ("Designing the Ti'Mur"), Doug Drexler; Senior Production Artist for ST: Enterprise described the circular hoop feature as warp nacelles:

'Ah,' I thought, as I mulled over the Vulcan ship design question for Enterprise. 'This is the perfect place to fit the hoop ship.' "The script stated that Trip would be ga-ga over this Surak-class starship. After laying eyes on it, there was no question in my mind that he [Matt Jeffries] went to bed that night puzzling out the exotic shape.

The other change involved eliminating any physical connection between the main body of the ship and the hoop, so they are actually separate elements. "We liked the defiance of conventional structural support," Drexler explains. "It makes the Vulcans look like they control powers beyond human ken. This was true of the original TV Enterprise. Those struts that support the nacelles defy what we understand today. It says that these people are masters of technologies that we don't yet understand. It speaks volumes for the technology at play."
_{^{- JULY 2002 ISSUE 39 STAR TREK: THE MAGAZINE}}

Rick Sternbach; Senior Production Illustrator for ST: Enterprise described the circular hoop feature as an annular warp ring:

Abandoning the preliminary design lines which echoed the design of the long range shuttle, Sternbach arrived at a final version in September 1991 and his notes on the final design read, "Vulcan Ship V Variant of Annular; No windows or other details; basic body shape." Later he recalled, "The commandeered Vulcan ships in "Unification" followed a pretty familiar approvals flow of initial idea, producer changes, and final concept to go to the model maker, in this case Greg Jein. Since we hadn't seen much in the way of Vulcan ship technology, beyond the motion picture shuttle, it was a bit daunting to home in on a true Vulcan style, and I can't say I'm terribly happy with the final result. Hindsight always invokes a desire for more design time, which might have helped. Perhaps different proportions on the annular warp ring, more curves, and more positive-negative surface detailing." _{^{- Star Trek: The Magazine Volume 3, Issue 8, page 104}}

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Michael Okuda, Art Supervisor for ST: Enterprise has offered an in-depth treknobabble explanation of how the rings create a warp field:

One of the most radical experiments in early Earth starship design was the Enterprise XCV. Unlike the traditional nacelle-and-saucer configuration, the XCV uses an annular propulsion system, based on Vulcan vehicle designs. This ship however, employed cyclotron accelerators to create a high-energy proton flux. The protons circled through the massive outer rings of verterium gallenide segments, generating a symmetrical subspace field. Each of the two coleopter ring structures contained two counter-rotating cyclotrons. The cyclotrons in each ring operated slightly out of phase with each other, generating the propulsive field imbalance that carried the ship through subspace at warp speeds.

Answer 2

The rings are warp nacelles.

Answer 3

The Vulcan warp rings look similar to the proposed Alcubierre drive, and may have been influenced by that concept. It was first proposed in 1994, and Enterprise premiered in 2001. Due to work by NASA's Dr. Harold "Sonny" White, a ring configuration came to be seen as more efficient than Alcubierre's original geometry; however, White's design ideas were not published until 2003 (White, H., “A Discussion on space-time metric engineering,” Gen. Rel. Grav. 35, 2025-2033) and did not become widely known until 2011 (Warp Field Mechanics 101), so the timing seems a bit problematic.

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