Ullage motor

This Saturn IB second-stage Auxiliary Propulsion Unit is similar to one used on the Saturn V third stage, which performed as an ullage engine for translunar injection.

Ullage motors (also known as ullage engines or ullage rockets) are relatively small, independently fueled rocket engines that may be fired prior to main engine ignition, when the vehicle is in a zero-g situation. The resulting acceleration causes liquid in the rocket's main tanks to settle towards the aft end, ensuring uninterrupted flow to the fuel and oxidizer pumps.