Ullage motor

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.

[2] Failure of the Agena's internal timer was also blamed for premature ignition of this ullage system in the failed launch of "Discoverer Zero" on January 21, 1959.

[3] The second stage (S-II) of the Saturn V rocket used in the American Apollo program used four (originally eight) ullage motors located on the aft interstage skirt.

In his book Lost Moon, Jim Lovell recounted a description of a course-correction burn of the LEM's main descent engine to re-enter a free return trajectory to Earth during the successful recovery of the Apollo 13 capsule: When the ship had stabilized in the proper attitude for firing, Lovell would deploy the LEM's landing gear, extending its four spidery legs to get them out of the way of the descent engine.

This procedure, known as ullage, was intended to jolt the spacecraft slightly forward and force the descent engine fuel to the bottom of its tanks, eliminating bubbles and air pockets.

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 engine on the side of Saturn V third stage
Three sets of ullage motors are shown in this schematic of the Saturn V rocket