Ullage motor

Ullage motors or ullage engines are relatively small, independently-fueled rocket engine

Rocket engine

A rocket engine or simply "rocket" is a jet engineRocket Propulsion Elements; 7th edition- chapter 1 that uses only propellant mass for forming its high speed propulsive jet. Rocket engines are reaction engines and obtain thrust in accordance with Newton's third law...

s that may be fired to accelerate the rocket

Rocket

A rocket or rocket vehicle is a missile, spacecraft, aircraft or other vehicle which obtains thrust by the reaction of the rocket to the ejection of a jet of fast moving fluid exhaust from a rocket engine. Chemical rockets create their exhaust by the combustion of rocket propellant...

 prior to main engine ignition, when the vehicle is in a zero-g situation.

These motors give an acceleration that moves the main engine liquid propellants to the bottom of their tanks so they can be pumped and possibly further into the pump and engine plumbing to initiate them, while the ullage

Ullage

Ullage refers to the unfilled space in a container of liquid.-Etymology:The word comes ultimately from the Latin oculus, “eye”, which was used in a figurative sense by the Romans for the bung hole of a barrel. This was taken into French in the medieval period as oeil, from which a verb ouiller was...

 gas floats to the top away from the engine inlets.

Such motors were first invented by Soviet engineers for the Molniya

Molniya (rocket)

Molniya 8K78 was a modification of the well-known R-7 Semyorka rocket and has four stages.This derivative of the original three stage Vostok rocket was especially designed to bring high flying satellites into orbit or to launch probes to other planets. The first launch of this rocket was on...

 interplanetary launch vehicle

Launch vehicle

In spaceflight, a launch vehicle or carrier rocket is a rocket used to carry a payload from the Earth's surface into outer space. A launch system includes the launch vehicle, the launch pad and other infrastructure....

 in 1960.

The firing of the ullage motors is used during stage separation of rocket and/or stabilization of a rocket.

The four ullage motors of the Saturn V rocket used in the American Apollo program were located on the second stage.

Ullage motors or ullage engines are relatively small, independently-fueled rocket engine

Rocket engine

A rocket engine or simply "rocket" is a jet engineRocket Propulsion Elements; 7th edition- chapter 1 that uses only propellant mass for forming its high speed propulsive jet. Rocket engines are reaction engines and obtain thrust in accordance with Newton's third law...

s that may be fired to accelerate the rocket

Rocket

A rocket or rocket vehicle is a missile, spacecraft, aircraft or other vehicle which obtains thrust by the reaction of the rocket to the ejection of a jet of fast moving fluid exhaust from a rocket engine. Chemical rockets create their exhaust by the combustion of rocket propellant...

 prior to main engine ignition, when the vehicle is in a zero-g situation.

These motors give an acceleration that moves the main engine liquid propellants to the bottom of their tanks so they can be pumped and possibly further into the pump and engine plumbing to initiate them, while the ullage

Ullage

Ullage refers to the unfilled space in a container of liquid.-Etymology:The word comes ultimately from the Latin oculus, “eye”, which was used in a figurative sense by the Romans for the bung hole of a barrel. This was taken into French in the medieval period as oeil, from which a verb ouiller was...

 gas floats to the top away from the engine inlets.

Such motors were first invented by Soviet engineers for the Molniya

Molniya (rocket)

Molniya 8K78 was a modification of the well-known R-7 Semyorka rocket and has four stages.This derivative of the original three stage Vostok rocket was especially designed to bring high flying satellites into orbit or to launch probes to other planets. The first launch of this rocket was on...

 interplanetary launch vehicle

Launch vehicle

In spaceflight, a launch vehicle or carrier rocket is a rocket used to carry a payload from the Earth's surface into outer space. A launch system includes the launch vehicle, the launch pad and other infrastructure....

 in 1960.

The firing of the ullage motors is used during stage separation of rocket and/or stabilization of a rocket.

The four ullage motors of the Saturn V rocket used in the American Apollo program were located on the second stage. In the third stage (known as S-IVB

S-IVB

The S-IVB was built by the Douglas Aircraft Company and served as the third stage on the Saturn V and second stage on the Saturn IB. It had one J-2 engine...

), there was a more general Auxiliary Propulsion System that also had ullage functions.

Ullage motors are also commonly employed on deep-space missions where a liquid rocket

Liquid rocket

A liquid-fuel rocket or a liquid rocket is a rocket with an engine that uses propellants in liquid form. Liquids are desirable because their reasonably high density allows the volume and hence the mass of the tanks to be relatively low, resulting in a high mass ratio...

needs to do a burn after travelling for some time.

The terms ullage engine and ullage rocket are also used.

Ullage motors are typically very small; research shows that only milli-gee accelerations are needed to avoid excessive gas intake.

Some vehicles use other techniques to hold liquid at the inlet, as, once the main engines are lit, ullage motors are no longer required.