When in space, the purpose of a propulsion system is to change the velocity, or "v", of a spacecraft. Because this is more difficult for more massive spacecraft, designers generally discuss spacecraft performance in "amount of change in momentum per unit of propellant consumed" also called specific impulse. The higher the specific impulse, the better the efficiency. Ion propulsion engines have high specific impulse (~3000 s) and low thrust whereas chemical rockets like monopropellant or bipropellant rocket engines have a low specific impulse (~300 s) but high thrust.