Fuel used by a conventional rocket ship. A conventional rocket works by accelerating a small amount of matter at very high velocity out the rear of the vessel. The ship therefore moves forward, in accordance with Newton's third law. The material that is fired out the rear of the ship is called the reaction mass. The reaction mass, or "fuel", often makes up a significant amount of the weight of a vessel, especially in the case of low tech and inefficient propulsion. Some transpapientech vessels do not require reaction mass as such; these are the so-called reactionless drive ships. Solar, laser, and microwave sail vessels, as well as beamriders, do not require reaction mass, as the "push" in that case comes from an external source.
Drive Performance is determined by the exhaust velocity of the drive system and mass ratio. For a given drive power, the acceleration will be lower for a higher exhaust velocity. Therefore, in selecting an exhaust velocity there is a tradeoff between maximum Δv and acceleration. Drives, especially those of warships, often allow this balance to be adjusted to some degree to adapt to different operating conditions.