Liquid hydrogen is stored at about -253°C. Any heat leaking through the insulated tank boils a little liquid into gas, raising pressure — this "boil-off" must be vented before it exceeds the tank's safety rating.
boilOff ∝ (T_ambient − T_LH2) / insulation
dP/dt = k1·boilOff − k2·ventFlow
transfer: dLevel_tank/dt = −flow, dLevel_aircraft/dt = +flow (until full)
- Relief valve setpoint — the pressure at which the safety valve automatically opens and vents boil-off gas.
- Transfer flow rate — how fast liquid hydrogen is pumped from storage into the aircraft's tank.
- Ambient temperature — hotter air increases heat leak into the cryogenic tank, raising the boil-off rate.
- Manual vent — forces the relief valve open to release pressure on demand, as ground crews do before maintenance.
Real-world relevance: airports piloting hydrogen-fuelled aviation (e.g. ZeroAvia, Airbus ZEROe trials) must manage exactly this boil-off/pressure/transfer balance to fuel aircraft safely and efficiently.