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The Manned Maneuvering Unit (MMU) is a retired astronaut propulsion unit that was used on three space shuttle missions in 1984. An astronaut would ”sit” in the MMU and the nitrogen propulsion system would allow the user to maneuver in space without having to be tethered to a spacecraft. The MMU was used to retrieve a pair of faulty communications satellites, Westar VI and Palapa B2. The MMU has been replaced by the Simplified Aid for EVA Rescue (SAFER), which is for emergency use only.
The MMU was designed to fit over the life-support system backpack of the Extravehicular Mobility Unit (EMU). The MMU had a mass of 148 kg with a full propellant load. The small size was to facilitate ease of movement within and around complex structures. Nitrogen gas was used as the propellant. Two attached tanks contained 5.9 kg of nitrogen each. This was enough propellant for 6 hours of EVA with moderate use. The MMU was capable of delta-v (velocity change) of about 24.4 m/s. Maneuverability was accomplished by 24 nozzle thrusters placed at different locations. The astronaut used fingertip controllers to the propulsion system. The controller on the right hand controlled rotational acceleration(roll, pitch, and yaw). The controller on the left managed translational acceleration(forward-back, up-down, and left-right). Fine coordination of the two controllers produced intricate movement and, once orientation was achieved, the astronaut had use of an automatic attitude-hold function to maintain inertial attitude during flight, thus freeing both hands for whatever work was needed.
The MMU was based on a similar suit developed by the Air Force in 1966 called the Astronaut Maneuvering Unit (AMU). The AMU made one spaceflight, but was not used due to excessive exertion by the designated astronaut prior to reaching the unit. The project was shelved until the Space Shuttle program began. The MMU was first tested during mission STS-41-B by astronauts Bruce McCandless II and Robert L. Stewart. During 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 MMU functioned perfectly, but the satellite could to be captured. An MMU was used for the last time during STS-51-A. During this mission the unit was used to retrieve Westar VI and Palapa B2. Neither satellite had reached their intended orbits due to faulty propulsion systems. Astronauts Joseph P. Allen and Dale Gardner brought the satellites into the shuttle’s payload bay for return to Earth.
The MMU was judged as too risky for practical use after the Challenger disaster. It had potential for use during the construction of the International Space Station, but other tethered EVA solutions were employed. Thus, the MMU was retired after three missions.
We’ve also recorded an episode of Astronomy Cast about the space shuttle. Listen here, Episode 127: The US Space Shuttle.