>In a turbopump feed system it is still necessary to pressurize the tanks slightly (10 to 50 lb/in<sup>2</sup>) in order to prevent pump cavitation. Rocket Propulsion Elements, Sutton, 4th edition, p. 223 Different vehicles have used stored onboard gas (typically helium) or autogenous propellants to provide this pressurization. Further reading: - https://space.stackexchange.com/q/21246/6944 - https://space.stackexchange.com/q/27752/6944 - https://space.stackexchange.com/q/33983/6944 - https://space.stackexchange.com/q/20245/6944 - https://space.stackexchange.com/q/6323/6944 - https://space.stackexchange.com/q/20648/6944