Is there a reason that the Nebula-Class ship do not have Impulse engines? So far, they are the only Federation ships in game I have seen that do not have the red engine vents or travel tails.
It's accepted as canon that the Nebula class does not have visible impulse signatures. It was a partial oversight in the original Nebula design on the show where it first appeared, and was eventually conceptualized that the Nebula had a different type of impulse that would allow for partial stealth.
From Memory Alpha: "A careful inspection of the Nebula-class model reveals that it has no visible impulse engines. In a recent entry on Doug Drexler's weblog, Rick Sternbach indicated that he envisioned the ship's impulse drives as being hidden behind baffles to scatter their signature for stealth reasons. He added that "modern" Starfleet vessel impulse engines operated on little conventional rocket thrust anyways, being more of a sub-warp subspace drive."
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From Memory Alpha: "A careful inspection of the Nebula-class model reveals that it has no visible impulse engines. In a recent entry on Doug Drexler's weblog, Rick Sternbach indicated that he envisioned the ship's impulse drives as being hidden behind baffles to scatter their signature for stealth reasons. He added that "modern" Starfleet vessel impulse engines operated on little conventional rocket thrust anyways, being more of a sub-warp subspace drive."