Untethered in Space: Bruce McCandless and the Manned Maneuvering Unit


When the Space Shuttle began flying in 1981 one of its promised goals was to repair and service satellites in orbit that had malfunctioned. Since most satellites were extremely expensive at this time, it was reasoned that repairing one would be more economical than launching a new one. In order to aid the astronauts in their new repair and servicing role, NASA developed a new propulsion system that would be worn over the spacesuit backpack. This new device was called the Manned Maneuvering Unit, or MMU for short. The MMU was designed to permit astronauts to perform a variety of extravehicular activities, such as satellite retrieval, science investigations, in-space construction, and rescue operations.

MMU Patch
NASA Side view of Astronaut Bruce McCandless II in the MMU

The Manned Maneuvering Unit was designed and built by Martin Marietta Corporation. It weighed over 300 pounds on Earth and was powered by two batteries with 852 watts at full charge. It was designed for a maximum of 6 hours of operation. The MMU was propelled by 24 compressed gaseous nitrogen thrusters, each providing 1.4 pounds of thrust. The astronaut controlled the MMU with two hand controllers that were mounted on side armrests. The MMU also featured an attitude-hold mode, in which data from motion-sensing gyroscopes directed the firing of thrusters to maintain a desired orientation in space. When not in use, the MMU was stowed and recharged in the flight support stations located in the forward end of the orbiter’s payload bay.

On February 7, 1984, on Space Shuttle mission STS 41-B, astronaut Bruce McCandless II, became the first person to ever fly untethered in space using the MMU. McCandless had already been an astronaut since 1966, but this was his first flight into space. He served as mission control capsule communicator or “capcom” in Houston for the Apollo 11 moonwalk on July 20, 1969. He was the astronaut back on Earth that Neil Armstrong and Buzz Aldrin communicated directly with during their time outside the lunar module. He would later be assigned as a member of the astronaut support crew for the Apollo 14 mission and was backup pilot for the first crewed Skylab mission.

Bruce McCandless II
McCandless with MMU

McCandless backed away from the Space Shuttle Challenger using the nitrogen thruster system of the MMU. After flying 150 feet away and then returning, he then ventured out to 320 feet away from the shuttle. This is an orbital spacewalk record that still stands today. The MMU worked just as it was designed to and performed magnificently on this first test flight. It would enable astronauts to fly untethered, perform satellite repairs and return to the Space Shuttle. McCandless would log four hours of MMU flight time and over 312 total hours in space on two different shuttle flights. His second flight was STS-31 in 1990 where he helped deploy the Hubble Space Telescope.

The second mission to fly the MMU was in April, 1984. On STS-41-C, an MMU was used by astronaut George Nelson to fly out to the malfunctioning Solar Max satellite and attempt to grasp it with a special capture tool. Three attempts to clamp the tool onto the satellite failed. The Solar Max was later captured during the mission using the Shuttle Challenger’s own robotic arm in the payload bay. The final flight of the MMU was in November 1984 on mission STS-51-A. The MMU was used by astronauts to retrieve Westar VI and Palapa B2, two communication satellites that did not reach their proper orbits because of malfunctions in their propulsion modules. Astronauts Joseph P. Allen and Dale Gardner captured the two satellites and brought them into the Shuttle Discovery’s payload bay for stowage and return to Earth.

Westar retrieval
Westar VI Retrieval
MMU Untethered above Earth

In the end, the Manned Maneuvering Unit was only used on three shuttle flights before being retired, all in the year 1984. This was no failure on the part of the MMU though, it was a wonderfully designed machine that performed its duties well. The Space Shuttle turned out to be so maneuverable that it proved to be simpler to just fly it over to a satellite or object and then grab it, either directly with the shuttle’s own robot arm or by a space suited astronaut working on the end of the arm. The better than expected maneuverability of the shuttle may have ended the need for the MMU, but it remains a magnificent and historic piece of spaceflight engineering.

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