This is a Saturn V. You probably recognise it, and its engines, but do you recognise these?

They are ullage rockets¹. While unknown to many, they are crucial to the success of a mission.

The term “ullage” is one that comes from wine making where it refers to the empty space of a container.

This applies to rockets too because they are containers filled with liquid (propellants) and in their case, the ullage space is the void in the forward end of the propellant tanks.

Ullage is important because it provides room for the thermal expansion of gases. It also forms a space for the dissolved gases to accumulate.

These dissolved gases include “ullage bubbles” which are bubbles of gas which form in the propellant, in the absence of gravity. In the best case, they form small bubbles in the plumbing which cause sputtering much like a car when a combustion engine has air bubbles in its fuel line. This will degrade the rocket’s performance, but it can be corrected for with mid course correction manoeuvres. It’s also possible they block the propellant outlets entirely meaning the engine(s) cannot operate. But then the worst of all these scenarios: they could lead to “combustion instability” resulting in “catastrophic self-disassembly” (A.K.A. “rapid unplanned disassembly”) of the vehicle itself.

As you can tell, these ullage bubbles are not desirable but having room to create ullage is not sufficient by itself. We also need some form of (forward) acceleration to accumulate the dissolved gases and remove the bubbles. Any form of forward acceleration will do and this is exactly what the ullage rockets provide.

By accelerating the vehicle forward, the denser liquids settle to the aft of the tank while the lighter gases accumulate in the ullage. Thus, mitigating issues of the bubbles blocking the propellant outlets, or accumulating in the plumbing. Allowing for smooth sailing.

Next time you see a multi-staged rocket keep an eye out for these unsung heroes. Who knows? You may just spot one.