A far fetched explanation, IMO...Picard wrote:Impulse engines are implied to be something akin to thrusters, yet we see no exhaust even at high impulse. So it might be that nacelles generate some sort of mass lightening field, helping impulse engines to do the work, but warp drive isn't power efficient enough to propel ship at STL on its own.
If Warp Drive is power efficient enough to get from low Warp to high Warp, then why can it not be efficient enough to go to STL speeds?
No, Impulse engines are used at STL because they're what's needed to move in "normal space", and Warp Drive is used only to move at FTL because of it's space altering capability...
Also, no time dilation would be mentioned when on high Impulse if there was a Warp bubble at STL...