Rocketry
In liquid rockets, ullage is the space within a fuel tank above the liquid propellant. 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 rockets keep their propellant in tanks. These tanks are never completely filled in order to allow for the expansion of the cold liquid propellant. On the ground, the space between the top of the propellant load and the top of the tank is known as "ullage space".
In micro-gravity conditions, the liquid may float away from the engine intake, which is typically very undesirable. Small rocket engines are sometimes used to settle the propellant prior to the main engine ignition. These are called ullage motors. Used as a verb, "ullage" refers to the procedure of accelerating a space vehicle to settle liquid propellants to the bottom of their tanks before pumping the propellents into the primary engine.
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