>As I commented, it's not that there aren't cryogenic temperature pressure relief valves, just that they may not be what you want!
Yeah for sure, if you think about it even if the rocket was filled with inert gas venting it might be very tricky.
You have an erect rocket on a launch pad, you have to vent evenly as a pressure relief valve is effectively a "rocket engine" at high enough pressures.
If one of them freezes and you don't have even distribution it can topple over.
You also have to vent the tanks in order and at a certain rate I would assume for example you want to vent to top tanks first to prevent the rocket from becoming too top heavy, and you also want to vent it at a certain rate to prevent quick shifts in the balance and weight distribution of the rocket.
