Ullage motor
SMALL ROCKET MOTOR USED TO SETTLE LIQUID FUEL PRIOR TO IGNITION OF A LARGER ROCKET
Ullage engine; Ullage rocket
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.