ullage (rocketry)

In rocketry, ullage is the space within a fuel tank above the liquid fuel. This term derives from the term 'ullage' in winemaking, where it refers to the space above the liquid in a container such as a barrel or wine bottle.Liquid, cryogenic-fueled rockets keep their fuel in tanks. These tanks are never completely filled in order to allow for the expansion of the cold liquid fuel. On the ground, the space between the top of the fuel load and the top 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 in mid-flight. 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 pumped. In this way the ullage space remains at the top of the tank. Such motors were first invented by Soviet engineers for the Molniya interplanetary launch vehicle in 1960.
See more at Wikipedia.org...