Ullage (rocketry)
In rocketry, ullage is the space within a fuel tank but above the liquid fuel. This term derives from the same term 'ullage' in winemaking, where it referse to the space above the liquid in a container such as a barrel or wine bottle.
Liquid, cryogenic-fueled rockets keep their fuel in big tanks. These tanks are never completely filled, to allow for the natural expansion of the cold liquid fuel. On the ground, the space between the top of the fuel load and the capacity of the tank is the known as "ullage space".
Ullage motors are relatively small, independently-fueled rocket engines that may be fired to accelerate the rocket. By Newton's laws of motion, this forces the main engine liquid fuel and oxidizer to the bottom of their tanks where they can be properly pumped. In this way the ullage space remains at the top. Such motors were first invented by Soviet engineers for the Molniia interplanetary launch vehicle in 1960.
The firing of the ullage motors may also assist in separation of rocket stages and/or stabilization of the rocket.
The four ullage motors of the Saturn V rocket used in the American Apollo program were located on the second stage. In the third stage (known as S-IVB), there was a more general Auxiliary Propulsion System that also had ullage functions.
The term ullage engine or ullage rocket is also used.