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Manned maneuvering unit

MMU is an acronym meaning Manned Maneuvering Unit.

This is a jetpack (propulsion backpack that snaps onto the back of the spacesuit) which has been used on spacewalks (EVAs) from NASA's space shuttle, allowing an astronaut to move independently from the shuttle.

The unit featured redundancy to protect against failure of individual systems. It was designed to fit over the life-support system backpack of the Shuttle 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 centimeters. 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.

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.

There were 24 nozzle thrusters placed at different locations on the MMU. To operate the propulsion system, the astronaut used his or her 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 acceleration without rotation 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 the mid-1980s. After its first test flight on STS-41B, on STS-41C, Astronauts James Van Hoften and George Nelson used the MMU to capture the Solar Maximum mission satellite and to bring it into the Orbiter's payload bay for repairs and servicing. Their work increased the lifespan of the satellite. The final MMU mission was STS-51A, which flew in November of 1984. The propulsion unit was used to retrieve two communication satellites that did not reach their proper orbit because of faulty propulsion modules. Astronauts Joseph Allen and Dale Gardner captured the two satellites and brought them into the Orbiter payload bay for stowage and return to Earth.

U.S. Space Shuttle astronaut using manned maneuvering unit
Public domain picture from NASA

See also Hand-Held Maneuvering Unit (HHMU), the MMU's predecessor, and Simplified Aid for EVA Rescue (SAFER), a smaller backpack propulsion system intended as a safety device during space walks.

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