pressurant

Noun

 * 1) A gas used to drive a fluid through a fluid system.
 * 2) * The helium pressurant supplies the energy for the propellant transport from the tanks to the combustion chamber.
 * 3) * During tank pressurization under some circumstances, rapid mixing of relatively warm pressurant gas with cryogenic propellant can lead to rapid densification of the gas and loss of pressure in the tank.