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{{Short description|Small rocket engines that help fuel settle in 0g before main engine start}}
[[File:Saturn V third stage ullage (APS) motor.jpg|thumb|This [[Saturn IB]] second-stage Auxiliary Propulsion Unit is similar to one used on the [[Saturn V]] third stage, which performed as an ullage engine for [[translunar injection]].]]
'''Ullage motors''' (also known as '''ullage engines''' or ''' ullage rockets''') are relatively small, independently
==Description==
[[Cryogenic]]-liquid-fueled rockets keep their propellants in insulated tanks. These tanks are never completely filled to allow for expansion. In [[weightlessness|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.
[[Image:Saturn v schematic.jpg|thumb|right|Three sets of ullage motors are shown in this schematic of the [[Saturn V]] rocket]]▼
The firing of the ullage motors is used before engine reignition, during stage separation
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 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]]), there was a more general 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 Excursion Module]] (LEM). [[Jim Lovell]] in his book "Lost Moon" 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 ill-fated [[Apollo 13]] mission:▼
[[File:Saturn V Rocket, 3rd Stage – Johnson Space Center. 20-3-2017 (38889065350).jpg|thumb|left|Ullage engine on Saturn V third stage]]
▲[[
The [[RM-81 Agena#Agena-A|Agena-A]] was one of the first vehicles to make use of an ullage system in preparation for ignition after separating from its [[Thor-Agena|Thor]] booster.<ref>{{cite journal|url=http://www.nro.gov/foia/declass/WS117L_Records/133.PDF|title=Discoverer I System Test Evaluation Report|publisher=National Reconnaissance Office|access-date=15 March 2015}}</ref> 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.<ref>Philip Taubman, 2003, ''Secret Empire - Eisenhower, the CIA, and the hidden story of America's space espionage'', p721; {{ISBN|0-684-85699-9}}</ref>
▲The
<blockquote>When the ship had stabilized in the proper attitude for firing, Lovell would deploy the LEM's landing gear, extending its four spidery legs to get them out of the way of the descent engine. Next the computer, relying on other instructions Haise typed into it, would fire four of Aquarius's attitude jets for 7.5 seconds. This procedure, known as ''ullage'', was intended to jolt the spacecraft slightly forward and force the descent engine fuel to the bottom of its tanks, eliminating bubbles and air pockets. After that, the main descent engine would ignite automatically firing at 10 percent thrust for 5 seconds.<ref>Jim Lovell & Jeffrey Kluger, 1994;"Apollo 13 (previously titled: Lost Moon)", p176; ISBN 0671-53464-5</ref></blockquote>▼
▲<blockquote>When the ship had stabilized in the proper attitude for firing, Lovell would deploy the LEM's landing gear, extending its four spidery legs to get them out of the way of the descent engine. Next the computer, relying on other instructions Haise typed into it, would fire four of ''Aquarius''{{'s}} attitude jets for 7.5 seconds. This procedure, known as ''ullage'', was intended to jolt the spacecraft slightly forward and force the descent engine fuel to the bottom of its tanks, eliminating bubbles and air pockets. After that, the main descent engine would ignite automatically firing at 10 percent thrust for 5 seconds.<ref>Jim Lovell
== Soviet/Russian operations ==
Ullage motors were used by Soviet engineers for the [[Molniya (rocket)|Molniya]] interplanetary [[launch vehicle]] in 1960.
Russian [[Proton (rocket family)|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.<ref >[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]</ref>
==References==
{{commons}}
{{Reflist}}
[[Category:Rocket engines]]
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