Complex sentence: The acceleration and maximum attainable speed seem to depend on the mass of the ship vs. the volume of its engines, which do not use reaction mass and hence needn't be mounted on or even exposed to the surface of the ship.
Split into simpler sentences: The acceleration and maximum attainable speed seem to depend on the ratio of the mass of the ship to the volume of its engines. These en gines do not use reaction mass and hence Minds need not mount them on or even expose them to the surface of the ship.