Ullage motor

In a barrel or wine bottle, for example, ullage is the space within the container but above the liquid.

Liquid, cryogenic-fueled rockets keep their fuel in big tanks. These tanks are never completely filled, to allow for the natural expansion of the cold liquid fuel. The space between the top of the fuel load and the capacity of the tank is the ullage space.

Ullage motors are relatively small, independently-fueled rocket engines that may be fired to accelerate the rocket. By Newton's laws of motion, this forces the main engine liquid fuel to the bottom of the tanks (thus ensuring ullage space above) where it can be properly pumped.

The firing of the ullage motors may also assist in separation of rocket stages and/or stabilization of the rocket.

External links

Description of Saturn V engines