The Manned Maneuvering Unit (MMU) is a propulsion backpack which was used by NASA astronauts on three space shuttle missions in 1984. The MMU allowed the astronauts to perform untethered EVA 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. Following the third mission the unit was retired from use. A smaller successor, the Simplified Aid for EVA Rescue (SAFER), was first flown in 1994, and is intended for emergency use only.
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 (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 was used as the propellant for the MMU. Two aluminium tanks with Kevlar wrappings contained 5.9 kilograms of nitrogen each, enough propellant for a six-hour EVA depending on the amount of maneuvering done. Typical MMU delta-v (velocity change) capability was about 80 feet per second (24.4 meters/sec).
There were 24 nozzle thrusters placed at different locations on the MMU. To operate the propulsion system, the astronaut used his fingertips to manipulate hand controllers at the ends of the MMU's two arms. The right controller produced rotational acceleration 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.
The MMU was used on three Shuttle missions in 1984. It was first tested on February 7 during mission STS-41-B 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, null its rotation rates, and fly it into the Shuttle's payload bay for stowage. But the satellite rotation rates made it impossible for the astronaut's grapple fixture to snare the SMM. 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). 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, which flew in November 1984. The propulsion unit was used to retrieve two communication satellites, Westar VI and Palapa B2, that did not reach their proper orbits because of faulty propulsion modules. Astronauts Joseph P. Allen and Dale Gardner captured the two satellites and brought them into the Orbiter payload bay for stowage and return to Earth.
After the Space Shuttle Challenger disaster, the MMU was judged too risky for further use. 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, with its retirement, NASA developed different tethered spacewalk approaches. Additionally, the MMU gas ejecta was considered too damaging for the only other ongoing task, upkeep of the Hubble Space Telescope.
NASA transferred the flight article #3 to the National Air and Space Museum in 1998. The remaining MMU are stored at NASA (location not known).
Related astronaut propulsion units
The MMU can trace its origins back to the EVA "zip" gun used by Edward White on the Gemini 4 mission in 1965. The hand-held gun held several pounds of nitrogen, and allowed limited movement around the Gemini spacecraft. This was also used by astronaut Michael Collins on the Gemini 10 mission in 1966.
The U.S. Air Force, which was planning to use the Gemini spacecraft as part of the Manned Orbiting Laboratory, designed the Astronaut Maneuvering Unit (AMU), which was a backpack, using hydrogen peroxide as the fuel. The total delta-v capability of the AMU was about 250 feet per second (76.2 meters per second), roughly three times that of the MMU. The astronaut would strap on the AMU like a backpack, and maneuver around using two hand controllers like that of the present-day MMU. Because of the fuel, which comes out as a hot gas, the astronaut's suit had to be modified with the addition of woven steel "pants" made of Chromel-R metal cloth. The AMU was flown aboard the Gemini 9 mission, but was not tested because the astronaut, Eugene Cernan, had difficulty maneuvering from the Gemini cabin to the AMU storage place, at the back of the spacecraft, and overheated, causing his helmet faceplate to fog up.
The Gemini AMU was not flown after Gemini 9. In 1973, the second AMU was flown aboard the Skylab 3 mission. Tested inside of the orbiting laboratory, the Skylab AMU was identical in appearance to the Gemini version, but used nitrogen gas instead of hydrogen peroxide - the nitrogen would vent harmlessly in the oxygen-nitrogen atmosphere used in the station, and allowing both unsuited and suited testing of the unit. The Skylab AMU was the closest to the Shuttle MMU, but was not used outside of the spacecraft because the EVAs were conducted with the astronauts attached to life support umbilicals, and to prevent damage to the delicate solar arrays on the Apollo Telescope Mount.
The Simplified Aid for EVA Rescue (SAFER) is a smaller backpack propulsion system intended as a safety device during space walks. It contains 1.4 kg of gaseous nitrogen, which provides much less delta-v capability than the MMU, roughly 10 feet per second (3 meters per second). However SAFER is less complex, less expensive and simpler to use than the MMU, and the limited delta-v is sufficient for the intended rescue-only task. Other Crew Self Rescue (CSR) devices of which prototypes have been developed include an inflatable pole, a telescoping pole, a bi-stem pole and a bola-type lasso device (astrorope) the drifting astronaut could throw to hook to the space station.
The former Soviet Union also used a cosmonaut propulsion system on flights to the space station Mir. The SPK (or UMK) was larger than the Space Shuttle MMU, contained oxygen instead of nitrogen and was attached to a safety tether. Despite the tether, the SPK allowed the cosmonaut, wearing the self-contained Orlan spacesuit, to "fly around" the orbiting complex, allowing access to areas nearly impossible to access otherwise. Though tested on Mir in 1990, the cosmonauts preferred using the Strela crane (equivalent to the Mobile Servicing System). The SPK, which was left attached to the outside to the Kvant-2 module, was destroyed when Mir re-entered the atmosphere after decommissioning.
Published in July 2009.
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