Lockheed Martin designed, built, and tested the Manned Maneuvering Unit (MMU) at its space center near Denver and at NASA's Johnson Space Center in Houston. The MMU represents the culmination of more than a decade of research and development.
The MMU was designed to permit astronauts to perform a variety of extravehicular activities, such as satellite retrieval, science investigations and observations, in-space construction, and rescue operations.
The MMU has been flown during three separate Space Shuttle missions. It was flight tested in February 1984 during Space Shuttle flight 41-B by astronauts Bruce McCandless and Robert Stewart. Over two days of extravehicular activity, McCandless and Stewart recorded five hours, 10 minutes of flying time on the two on-board MMUs.
The MMU was used again on April 8, 1984, by astronaut George D. Nelson on a mission to repair the Solar Maximum Observatory satellite (STS 41-C), and its performance was flawless. However, a device used to dock with the satellite was unable to lock onto the protruding trunnion pin on Solar Max, and the satellite eventually had to be retrieved by the Shuttle's remote manipulator system. The subsequent repairs to the satellite were successful. During the mission, Nelson and astronaut George van Hoften logged one hour, 10 minutes of flight time on the two-on-board MMUs.
Lockheed Martin’s contract was through September 1991 to develop a plan for returning the MMUs to flight status and to study future MMU use in conjunction with the Space Shuttle.
The MMU is a self-contained astronaut backpack propulsion device that allows astronauts to venture untethered from an orbiting spacecraft. The unit is powered by 24 nitrogen gas thrusters, and its main structure is aluminum. Other elements include two 16.8-volt silver zinc batteries, a control electronics assembly, and two hand controllers.
To use the MMU, an astronaut exits the Space Shuttle crew compartment through an airlock into the cargo bay. There the astronaut dons the MMU and releases himself from the flight support station. To maneuver in space, the astronaut uses the hand controllers. The control electronics assembly translates the hand controller movements and fires the thrusters. The astronaut can activate an auto-pilot system which will hold his attitude.
When not in use, the MMU is stowed and recharged in the flight support stations located in the forward end of the orbiter's payload bay.