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Ullage motors or ullage engines are relatively small, independently-fueled rocket engines that may be fired to accelerate the rocket prior to main engine ignition, when the vehicle is in a zero-g situation.
These motors give an acceleration that moves the main engine liquid propellants to the bottom of their tanks so they can be pumped and possibly further into the pump and engine plumbing to initiate them, while the ullage gas floats to the top away from the engine inlets.
Such motors were first invented by Soviet engineers for the Molniya interplanetary launch vehicle in 1960.
The firing of the ullage motors is used during stage separation of rocket and/or stabilization of a 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.
Ullage motors are also commonly employed on deep-space missions where a liquid rocket needs to do a burn after travelling for some time.
The terms ullage engine and ullage rocket are also used.
Ullage motors are typically very small; research[who?] shows that only milli-gee accelerations are needed to avoid excessive gas intake.
Some vehicles use other techniques to hold liquid at the inlet, as, once the main engines are lit, ullage motors are no longer required.
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