Most satellites, space probes, and launch vehicle upper stages utilize chemical liquid propellants for propulsion. These propulsion systems typically use pressurant subsystems, including pressure regulators, to control the delivery of the chemical propellants to the vehicle engines and sometimes to deliver gas to cold gas attitude and reaction control systems. The rationale for using regulators is that controlling the pressure at which the propellants are fed into the main engine combustion chamber significantly enhances the efficiency of the engine and produces a constant thrust value.
Expendable launch vehicles often use blowdown hydraulic thrust vector actuation control (TVAC) systems to determine their trajectories during launch. These systems alter the vehicle's thrust vector by tilting the rocket engine(s) with electrohydraulic actuators. The hydraulic oil employed in the actuators is delivered to them at high pressure by means of a pressurization subsystem.
