I've always wanted to know about the steady emission of what looks like steam/smoke(?) from the upper body (not nozzle) of the rockets in the pre-launch stage. Anybody knows more about that?
Oxygen can be kept in a liquid state by either keeping the temperature extremely low or keeping it under high pressure. Since reinforcing the tanks to handle incredibly high pressures would require too much weight, the tanks are instead constantly being topped up with cryogenic fuel until the moment of launch.
As for the venting: the fuel starts heating up and evaporating as soon as it's pumped into the tanks causing the pressure within the tanks to rise. Since the tanks aren't rated for high pressure, they have to vent the excess oxygen to keep within safety margins.
