posted
I heard someone say that impulse engines don't work in atmospheres. If that's true then it's settled, what with the 1701-A leaving under impulse in ST:VI.
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posted
It's not all that clear if impulse engines work in atmospheres or not. All we know is that in DS9 "The Siege", two impulse-powered interceptors chased Kira's "sub-impulse" (thruster-only?) craft, and Kira claimed that taking the battle down to the atmosphere of Bajor would even the odds.
Perhaps Kira was not speaking specifically of impulse engines. Perhaps she just meant that atmospheric stresses would impose a common speed limit for both types of vessels? Or perhaps the small sub-impulse raider would fare better when deeper in the gravity well of the planet?
Also, it seems that impulse engines can be used in some types of atmospheres at least. Starships dip into the upper atmospheres of planets now and then, and maneuver at impulse. The Defiant went down into a gas giant in "Starship Down", and I believe impulse travel was at least implied. And the Voyager has battled atmospheric storms at impulse in at least "Tattoo".
So perhaps one should just say that impulse engines suffer severe performance limitations in atmospheres: so if there is an alternate propulsion system available, it should be used. But if no alternative exists, impulse engines can be used, even though they no longer are a superior form of Newtonian propulsion.
Why won't an impulse engine perform well in a thick atmosphere? Well, one might surmise the engine uses an extremely high-speed jet in order to expend as little mass as possible, and thus to keep propellant tank size down. A low-mass-flow, relativistic-speed jet would have trouble pushing against atmospheric pressure, while a slower jet of greater mass flow would suffer less problems.
Also, the mass-reducing subspace thingamabobs in the impulse engines might not like atmospheres. One wonders if the common and robust antigravs of Trek work on some other principle than subspace mass masking - often they simply seem to lift objects without reducing their inertial mass. Perhaps mass reduction is not possible within an atmosphere at all?
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