Ever wondered was that steam coming out of the side of the Saturn V rocket is? If you look closely, you'll see vapor steaming out of the top of the fuel tanks hours prior to launch.
All the three stages of the Saturn V use liquid oxygen (LOX) as oxidizer. The LOX is kept liquid at about -183 degrees Celsius. To main this temperature it continuously boils off and is replenished through the fill and drain line between loading and prepressurization.
