A significant improvement (above 30%) in specific impulse can be obtained by using cryogenic propellants, such as liquid oxygen and liquid hydrogen, for example. Historically, these propellants have not been applied beyond upper stages. Furthermore, numerous concepts for advanced propulsion technologies, such as electric propulsion, are commonly used for station keeping on commercial communications satellites and for prime propulsion on some scientific space missions because they have significantly higher values of specific impulse. However, they generally have very small values of thrust and therefore must be operated for long durations to provide the total impulse required by a mission.