> [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.
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.