h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Actually, the impulse engines have their own coils which generate a miniaturized subspace displacement field. The warp coils in the nacelles are designed to move the ship relative to space at extremely high speeds ranging from warp 1 to (almost but never) warp 10. However, the calibration necessary for this cannot produce fine control at sublight speeds. The impulse coils are designed to give greater control over the ship at sublight speeds, but both drives are strictly subspace-displacement. The only engine which propels the ship through a propellant are the chemical thrusters, which are very rarely used.