No, I've read that it can't be removed at the shipwright. And (not to be too nerdy here), if that third nacelle was gone, you would also lose the lance. If you don't want the lance, then maybe the Galaxy-R is the ship for you instead?
As has often been rehashed in this forum and just about anyplace else on the Internet where the pragmatics of STO gameplay have been discussed in depth, the Gal-R has a very long way to go in terms of performance, or even being a terribly viable ship for anything other than a tank, and one with some fairly glaring problems that hinder its practicality in even that function. Frankly, a lot of the fanbase, and STO players, want the Galaxy to be able to perform as a battleship (not necessarily the 'Mary Sue' ship so many scream is being sought at, either). This is probably why the idea of a Galaxy class with a ridiculously huge gun strapped onto it was such a hit when it first came out, especially if nobody really thought about the pragmatics of having a third nacelle on it aside from a new glowy bit that looks cool.
Furthermore, there is nothing whatsoever that states that the lance (or any other fortification) on the Galaxy-X is dependent on the nacelle. There are a number of additional fortifications on the ship that do not seem to have anything to do with it. According to all technobabble we have on the subject, by all rights that third nacelle is essentially a few hundred thousand pounds more to lug around with little benefit, as it generates absolutely no power (that's what a warp core is for, nacelles consume power, not make it) and nacelles are best used in pairs, so unless it's for redundancy, which few to no ships have in this regard, then it's basically there to look cool - and that's controversial at best for many fans of Starfleet-oriented technologies.
While I got the Dread recently, and have come to have some grudging respect for the looks as I rather enjoy the rest of the ship, I wouldn't mind a way to get rid of the third nacelle. I don't know if I'd use it, but I'd certainly consider it.
Third Nacelle is part of the Neck, you can't turn it off, but at least it's centered. I think the ship looks better with the Venture costume anyway, gives it a sleeker look.
That's part of the problem. Structurally speaking the reinforcements sans the nacelle and pylon could easily still be there, and I think the ship would still look good with those reinforcements. I think what's being asked for is essentially to have options for the neck that would correspond to this, or possibly the unfortified look of the Gal-R for this particular component.
I think the reason for the 3rd necelle was that in the episode it appeared it could do warp 14. so the third necelle is supposed to be somehow related to this higher warp capacity. Since the art department was just gluing things on the origional model I imagine it made sense and looked cool to them at the time and no real technomagic reason was given.
At least thats the reality based non technobabble reason.
Now for some technobabble
Warp propulsion in startrek functions similarly to nuclear power in the modern navy.
A nuclear reactor produces heat, that heat is then used to boil water and create steam pressure. That steam pressure is then used to drive a turbine, and that turbine drives either your propulsion directly or creates electricity to drive an electric motor.
I would imagine that a warp propulsion unit behaves in a similar fashion, so the necelle is just the motive portion of the system and it would stand to reason that the more necelles you have the more capeable your propulsion system is.
In the case of a warp field this may mean that you can run your necelles at lower than peak output individualy but achieve the same overall output allowing you to run them longer at a higher speed than with fewer necelles.
In the case of 4 necelle ships I have seen it stated that these ships only run their necells in pairs but can switch between them at will to facilitate longer duration high warp travel, I don't remember where I saw this but I believe it was a non canon source.
Actualy reading things pefore posting will make you look smarter than yelling loudly. Reading comprehension is aparently a lost art.
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