The Manned Maneuvering Unit (MMU) is an
astronaut propulsion unit that was used by
NASA
The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the federal government of the United States, US federal government responsible for the United States ...
on three
Space Shuttle
The Space Shuttle is a retired, partially reusable launch system, reusable low Earth orbital spacecraft system operated from 1981 to 2011 by the U.S. National Aeronautics and Space Administration (NASA) as part of the Space Shuttle program. ...
missions in 1984. The MMU allowed the
astronauts
An astronaut (from the Ancient Greek (), meaning 'star', and (), meaning 'sailor') is a person trained, equipped, and deployed by a List of human spaceflight programs, human spaceflight program to serve as a commander or crew member of a spa ...
to perform untethered
extravehicular spacewalks at a distance from the shuttle. The MMU was used in practice to retrieve a pair of faulty communications satellites,
Westar VI and
Palapa B2
Palapa is a series of communications satellites owned by Indosat, an Indonesian telecommunications company (formerly by Perumtel and then by PT Satelit Palapa Indonesia/Satelindo). The first satellite was launched in July 1976, at which time ...
. Following the third mission the unit was retired from use. A smaller successor, the
Simplified Aid For EVA Rescue
Simplified Aid For EVA Rescue (SAFER) is a small, self-contained, propulsive backpack system ( jet pack) worn during spacewalks, to be used in case of emergency only. If an untethered astronaut were to lose physical contact with the vessel, it w ...
(SAFER), was first flown in 1994, and is intended for emergency use only.
Overview
The unit featured redundancy to protect against failure of individual systems. It was designed to fit over the life-support system backpack of the Space Shuttle
Extravehicular Mobility Unit
The Extravehicular Mobility Unit (EMU) is an independent spacesuit that provides environmental protection, mobility, life support, and communications for astronauts performing extravehicular activity (EVA) in Geocentric orbit, Earth orbit. Introd ...
(EMU). When carried into space, the MMU was stowed in a support station attached to the wall of the payload bay near the airlock hatch. Two MMUs were carried on a mission, with the second unit mounted across from the first on the opposite payload bay wall. The MMU controller arms were folded for storage. When an astronaut backed into the unit and snapped the life-support system into place, the arms were unfolded.
To adapt to astronauts with different arm lengths, controller arms could be adjusted over a range of approximately 13 centimetres. The MMU was small enough to be maneuvered with ease around and within complex structures. With a full propellant load, its mass was 148 kilograms (326 pounds).
Gaseous
nitrogen
Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...
was used as the propellant for the MMU. Two aluminium tanks with
Kevlar
Kevlar (para-aramid) is a strong, heat-resistant synthetic fiber, related to other aramids such as Nomex and Technora. Developed by Stephanie Kwolek at DuPont in 1965, the high-strength material was first used commercially in the early 1970s as ...
wrappings contained 5.9 kilograms of nitrogen each, enough propellant for a six-hour
Extravehicular activity
Extravehicular activity (EVA) is any activity done by an astronaut in outer space outside a spacecraft. In the absence of a breathable atmosphere of Earth, Earthlike atmosphere, the astronaut is completely reliant on a space suit for environme ...
(EVA) depending on the amount of maneuvering done. The two propellent tanks had an initial charge on the ground prior to the mission allowing for
Delta-v
Delta-''v'' (also known as "change in velocity"), symbolized as and pronounced , as used in spacecraft flight dynamics, is a measure of the impulse per unit of spacecraft mass that is needed to perform a maneuver such as launching from or l ...
of 110 to 130 ft/sec. The tanks could be recharged in orbit to a minimum equivalent
Delta-v
Delta-''v'' (also known as "change in velocity"), symbolized as and pronounced , as used in spacecraft flight dynamics, is a measure of the impulse per unit of spacecraft mass that is needed to perform a maneuver such as launching from or l ...
of 72 ft/sec. For a nominal mass, translational acceleration was 0.3±0.05 ft/sec
2 and rotational acceleration was 10.0±3.0 deg/sec
2.
There were 24 nozzle thrusters placed at different locations on the MMU. To operate the propulsion system, the astronaut used their fingertips to manipulate hand controllers at the ends of the MMU's two arms. The right controller produced rotational
acceleration
In mechanics, acceleration is the Rate (mathematics), rate of change of the velocity of an object with respect to time. Acceleration is one of several components of kinematics, the study of motion. Accelerations are Euclidean vector, vector ...
for roll, pitch, and yaw. The left controller produced translational acceleration for moving forward-back, up-down, and left-right. Coordination of the two controllers produced intricate movements in the unit. Once a desired orientation was achieved, the astronaut could engage an automatic attitude-hold function that maintained the inertial attitude of the unit in flight. This freed both hands for work.
History
In 1966, the
US Air Force
The United States Air Force (USAF) is the Air force, air service branch of the United States Department of Defense. It is one of the six United States Armed Forces and one of the eight uniformed services of the United States. Tracing its ori ...
developed an Astronaut Maneuvering Unit (AMU), a self-contained rocket pack very similar to the MMU. This was planned to be tested during
Project Gemini
Project Gemini () was the second United States human spaceflight program to fly. Conducted after the first American crewed space program, Project Mercury, while the Apollo program was still in early development, Gemini was conceived in 1961 and ...
on an EVA by
Eugene Cernan
Eugene Andrew Cernan (; March 14, 1934 – January 16, 2017) was an American astronaut, United States naval aviator, naval aviator, electrical engineer, aeronautical engineer, and fighter pilot.
Cernan traveled into space three times and ...
on
Gemini 9A on June 5, 1966. However, the test had to be cancelled because Cernan, tired and overheated, sweated so profusely that his helmet visor fogged before he could get to the AMU mounted on the back of the spacecraft. Astronauts did not learn how to work during EVA without tiring until the final
Gemini 12
Gemini 12 (officially Gemini XII) With Gemini IV, NASA changed to Roman numerals for Gemini mission designations. was a 1966 crewed spaceflight in NASA's Project Gemini. It was the 10th and final crewed Gemini flight (Gemini 1 and Gemini 2 were ...
mission, but no AMU was carried on that flight. Since there was no real need for self-contained astronaut EVA flight in the
Apollo
Apollo is one of the Twelve Olympians, Olympian deities in Ancient Greek religion, ancient Greek and Ancient Roman religion, Roman religion and Greek mythology, Greek and Roman mythology. Apollo has been recognized as a god of archery, mu ...
and
Skylab
Skylab was the United States' first space station, launched by NASA, occupied for about 24 weeks between May 1973 and February 1974. It was operated by three trios of astronaut crews: Skylab 2, Skylab 3, and Skylab 4. Skylab was constructe ...
programs, the idea had to wait for the advent of the
Space Shuttle
The Space Shuttle is a retired, partially reusable launch system, reusable low Earth orbital spacecraft system operated from 1981 to 2011 by the U.S. National Aeronautics and Space Administration (NASA) as part of the Space Shuttle program. ...
program, though several maneuvering device designs were tested inside Skylab.
Active use in space

The MMU was used on three Shuttle missions in 1984. It was first tested on February 7 during mission
STS-41-B
STS-41-B was NASA's tenth Space Shuttle mission and the fourth flight of the . It launched on February 3, 1984 and landed on February 11, 1984, after deploying two communications satellites. It was also notable for including the first untethere ...
by astronauts
Bruce McCandless II and
Robert L. Stewart. Two months later, during mission
STS-41-C, astronauts
James van Hoften and
George Nelson attempted to use the MMU to capture the
Solar Maximum Mission satellite and to bring it into the orbiter's payload bay for repairs and servicing. The plan was to use an astronaut-piloted MMU to grapple the SMM with the Trunion Pin Attachment Device (TPAD) mounted between the hand controllers of the MMU, null its rotation rates, and allow the Shuttle to bring it into the Shuttle's payload bay for stowage. Three attempts to grapple the satellite using the TPAD failed. The TPAD jaws could not lock onto Solar Max because of an obstructing grommet on the satellite not included in the blueprints for the satellite. This led to an improvised plan which nearly ended the satellite's mission. The improvisation had the MMU astronaut use his hands to grab hold of an SMM solar array and null the rates by a push from MMU's thrusters. Instead, this attempt induced higher rates and in multiple axes; the satellite was tumbling out of control and quickly losing battery life. SMM Operations Control Center engineers shut down all non-essential SMM subsystems and with a bit of luck were able to recover the SMM minutes before total failure. The ground support engineers then stabilized the satellite and nulled its rotation rates for capture with the orbiter's robotic arm, the Shuttle Remote Manipulator System (
SRMS
Canadarm or Canadarm1 (officially Shuttle Remote Manipulator System or SRMS, also SSRMS) is a series of robotic arms that were used on the Space Shuttle orbiters to deploy, manoeuvre, and capture payloads. After the Space Shuttle Columbia disa ...
). This proved to be a much better plan. Their successful work increased the lifespan of the satellite.
The final MMU mission was
STS-51-A
STS-51-A (formerly STS-19) was the 14th flight of NASA's Space Shuttle program, and the second flight of Space Shuttle ''Discovery''. The mission launched from Kennedy Space Center on November 8, 1984, and landed just under eight days later on ...
, which flew in November 1984. The propulsion unit was used to retrieve two communication satellites,
Westar VI and
Palapa B2
Palapa is a series of communications satellites owned by Indosat, an Indonesian telecommunications company (formerly by Perumtel and then by PT Satelit Palapa Indonesia/Satelindo). The first satellite was launched in July 1976, at which time ...
, that did not reach their proper orbits because of faulty propulsion modules. Astronauts
Joseph P. Allen
Joseph Percival "Joe" Allen IV (born June 27, 1937) is an American former NASA astronaut. He logged more than 3,000 hours flying time in jet aircraft.
Early life and education
Allen was born in Crawfordsville, Indiana, in June 1937. His paren ...
and
Dale Gardner
Dale Allan Gardner (November 8, 1948 – February 19, 2014) was a NASA astronaut, and naval flight officer who flew two Space Shuttle missions during the mid 1980s.
Personal information
Born on November 8, 1948, in Fairmont, Minnesota, Gardne ...
captured the two satellites and brought them into the Orbiter payload bay for stowage and return to Earth.
Retirement
After a safety review following the
Space Shuttle ''Challenger'' disaster, the MMU was judged too risky for further use and it was found many activities planned for the MMU could be done effectively with manipulator arms or traditional tethered EVAs. NASA also discontinued using the Shuttle for commercial satellite contracts, and the military discontinued the use of the Shuttle, eliminating the main potential uses. Although the MMU was envisioned as a natural aid for constructing the
International Space Station
The International Space Station (ISS) is a large space station that was Assembly of the International Space Station, assembled and is maintained in low Earth orbit by a collaboration of five space agencies and their contractors: NASA (United ...
, with its retirement, NASA developed different tethered spacewalk approaches.
The two operational, flown flight units MMU No. 2 and No. 3 were stored by NASA in a clean room at Lockheed Martin in Denver through 1998. NASA transferred flight article No. 3 to the
National Air and Space Museum
The National Air and Space Museum (NASM) of the Smithsonian Institution is a museum in Washington, D.C., in the United States, dedicated to history of aviation, human flight and space exploration.
Established in 1946 as the National Air Museum, ...
in 1998, which now hangs suspended in the hall above Space Shuttle Discovery in the
Udvar-Hazy Center
The Steven F. Udvar-Hazy Center, also called the Udvar-Hazy Center, is the Smithsonian Institution, Smithsonian National Air and Space Museum (NASM)'s annex at Dulles International Airport in the Chantilly, Virginia, Chantilly area of Fairfax C ...
annex.
Flight article No. 2 is on display at the
U.S. Space & Rocket Center in Huntsville, Alabama. As of 2017, MMU No. 1 is on display in the
Space Vehicle Mockup Facility
The Space Vehicle Mockup Facility (SVMF) is a large open space area located inside Building 9 of Johnson Space Center in Houston. The SVMF houses mockups of most pressurized modules on the International Space Station (ISS). It is primarily used f ...
at Johnson Space Center.
See also
*
Single-person spacecraft
*
Soviet SPK
References
External links
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{{Authority control
Human spaceflight
Extravehicular activity
1984 introductions