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I'm absolutely fine with the idea, that a certain angle/position of the nacelles is better than others. And hence it's good (for whatever reason - be it energy efficiency or more speed...) to have them in that position. So my question is less: "Why do the nacelles rise when they go to warp?". The answer is simple: "Because in trek-physics it's better to have them there when going to warp." My question is more: "Why are they lowered when dropping to impulse?"
The nacelles are not involve in the propulsion of the ship when on impulse. Why are they not forged in the favourable position for warp? For some reason it must be better to lower the nacelles when you fly on impulse.
Maybe they are less exposed in combat-situations? Are they easier to access for reparations if they are lowered? Can the Bussard-collectors work more efficiently if they don't have the saucer-section blocking their ways?
Why can the warp-nacelles be lowered? Why are they not fixed in the position which is best for flying on warp?