Ullage thrusters are small rocket engines that play a crucial role in the successful operation of liquid-fueled rockets, especially those with multiple stages. While their thrust output is minuscule compared to the main engines, ullage thrusters are essential for ensuring that the rocket’s engines have a steady supply of propellant during key phases of the launch.
What is Ullage?
In the context of rocketry, ullage refers to the unfilled space inside a rocket’s propellant tanks. As the propellants are consumed by the engines, this empty space increases. Under the microgravity conditions experienced during a rocket’s coasting phase between engine burns, the liquid propellants can float freely inside the tanks instead of settling at the bottom where the outlet to the engines is located. This poses a problem when attempting to restart the engines.
The Role of Ullage Thrusters
To address this issue, ullage thrusters provide a small amount of acceleration to the rocket stage, typically in the direction opposite to the main engines’ thrust. This settles the propellants at the bottom of the tanks, allowing the turbopumps or propellant feed system to draw in liquid rather than gas. Without ullage thrusters, the engines could ingest gas bubbles which can lead to cavitation, unsteady combustion, loss of thrust, and even catastrophic failure.
Ullage thrusters are usually fired for a brief period prior to igniting the main engines on an upper stage. The acceleration they provide is very small, on the order of 0.001 to 0.1 m/s², but it is sufficient to ensure proper propellant positioning. The burn time required depends on factors such as the propellant tank dimensions, ullage volume, and propellant characteristics.
Types of Ullage Thrusters
Ullage thrusters come in different forms depending on the rocket design:
Dedicated Ullage Thrusters
Some rockets employ dedicated ullage thrusters that are separate from the reaction control system (RCS) thrusters used for attitude control. These are often small solid rocket motors that are fired once and then discarded. A famous example is the Titan II GLV rocket used in the Gemini program, which had two ullage rockets on its second stage.
RCS Thrusters
Many modern rockets use RCS thrusters for the dual purpose of attitude control and ullage settling. By carefully orienting the rocket and firing the appropriate RCS thrusters, the same effect as dedicated ullage thrusters can be achieved. This approach saves weight and complexity. The Falcon 9 rocket by SpaceX uses nitrogen cold gas thrusters on its upper stage to provide both ullage and attitude control.
Autogenous Pressurization
An alternative to ullage thrusters is autogenous pressurization, where a small amount of propellant is heated into gas and routed back into the propellant tanks. The gas pressure pushes the liquid propellants toward the bottom, achieving a similar effect as ullage thrusters. However, this system adds complexity and is not feasible in all rocket designs.
Summary
Despite their small size, ullage thrusters are indispensable for the complex sequence of events involved in a rocket launch. They enable multi-stage rockets to ignite their upper stage engines successfully after a coasting phase, which is critical for reaching orbital velocity and desired trajectories. As rocket technology continues to advance, clever designs like using RCS thrusters for ullage settling help to simplify and optimize rocket systems. Ullage thrusters may not be the most glamorous aspect of rocket science, but they are indeed the unsung heroes that help propel rockets and their payloads into space.
