Cryogenic propellant tanks coasting in space must be vented periodically to prevent excessive pressure. This requires that the liquid be settled with auxiliary ullaging rockets prior to venting to ensure minimum propellant loss. Other problems associated with blowdown of saturated liquids, such as surface level rise and boilover, are severe. The S-IVB, third stage of the Saturn Launch Vehicle, eliminates these problems during the earth parking orbit by continuously venting the liquid hydrogen tank. The boiloff gas is vented in the aft direction providing a continuous settling thrust on the propellants. The philosophy behind the system design is explored and critical components in the design are examined. Evaluation of system performance on several flights is presented
