Ullage motor

Description

Cryogenic-liquid-fueled rockets keep their propellants in insulated tanks. These tanks are never completely filled to allow for expansion. In micro-gravity conditions the cryogenic liquids are without a free surface, and exist in a slushy state between solid, liquid, and gas. In this mixed state, ullage gases may be sucked into the engines, which is undesirable, as it displaces useful propellant, reduces efficiency, and may damage the engines.

Small rocket engines, called "ullage motors", are sometimes used to settle the propellant before main engine ignition to allow the formation of a temporary free surface (with a distinct boundary between liquid and gas states). These motors provide acceleration that moves the main engine liquid propellants to the bottom of their tanks ("bottom" in this usage always meaning relative to the alignment of the main motor the ullage motors are serving), so they can be pumped into the engine plumbing.

The firing of the ullage motors is used before engine reignition, during stage separation and/or stabilization of a rocket when there are brief reductions in acceleration which could allow the liquid propellant to float away from the engine intakes. Ullage motors are also commonly employed on deep-space missions where a liquid rocket needs to start a burn after traveling in micro-gravity.

To perform this ullage pre-acceleration, reaction control system (RCS) thrusters are sometimes used. Larger ullage motors can also use solid propellant, which makes them generally single-use.

American operations

The Agena-A was one of the first vehicles to make use of an ullage system in preparation for ignition after separating from its Thor booster. 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.

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 the S-IVB third stage, there was an Auxiliary Propulsion System that also had ullage functions. Ullage is often a secondary function of the reaction control system such as on the Apollo Lunar Module (LM). 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:

Soviet/Russian operations

Ullage motors were used by Soviet engineers for the Molniya interplanetary launch vehicle in 1960.

Russian Proton rockets use ullage motors called SOZ motors. These have a bad tendency of exploding years after end of operations, contributing to space debris. So far, 54 such SOZ motors have exploded in orbit.

Indian operations

The PSLV rocket has integrated ullage motors in between its alternate solid and liquid stages, with 12 on the first stage, eight on the second, and four on the final two stages.

References

References

1. [https://www.space.com/russian-rocket-motor-breakup-space-debris SPACE.COM: Old Russian rocket motor breaks up in orbit, generating new cloud of space debris]
2. "Discoverer I System Test Evaluation Report". National Reconnaissance Office.
3. Philip Taubman, 2003, ''Secret Empire - Eisenhower, the CIA, and the hidden story of America's space espionage'', p721; {{ISBN. 0-684-85699-9
4. Jim Lovell and Jeffrey Kluger, 1994, ''Apollo 13'' (previously titled ''Lost Moon''), p176, {{ISBN. 0671-53464-5
5. https://www.isro.gov.in/media_isro/pdf/Publications/Brochures/pslv_c1_publication.pdf {{Bare URL PDF. (January 2026)
6. "What is the purpose of the pairs of cylinders on the interstage of PSLV?".