3 ms·

Helium is used to pressurize hydrogen and other gases to move them through the plumbing. This link says "before takeoff" but its also used after. https://www.u

• magicalhippo 2y ago
  As far as I understand it, the point isn't just to pressurize the propellant to get it out of the tank and into the engine, but to maintain pressure so that the propellant remains liquid. If the pressure in the tank drops, the propellant would turn into gas which would effectively be lost and potentially dangerous (by allowing flames to go the wrong way). As your link points out, helium is nice because it is inert and remains a gas even at liquid hydrogen temperatures.
  • crznp 2y ago
    > [Helium helps to] maintain pressure so that the propellant remains liquid. IANA rocket scientist, but this doesn't ring true to me. Vaporization of LH2 is related to the partial pressure of Hydrogen, not the absolute pressure of the gaseous phase. You're going to lose ≈PV/RT of hydrogen molecules to the gas phase regardless (P being the vapor pressure of Hydrogen at the tank's temperature and V the volume of the tank). You still need the absolute pressure for structural stability. He has ≈twice the molar mass of H2, so H2 is better for the dry mass in the rocket equation anyway: what is left in the tank isn't wasted, but used for rigidity. You just need Helium because it won't have phase transitions: it's always a gas so it is easier to regulate pressure.