Manned Maneuvering Unit

The image of space-suited astronaut Bruce McCandless flying free high above Earth is one of the most famous in spaceflight history, yet the device that made it possible, the manned maneuvering unit (MMU), had a surprisingly short career. The MMU flew for only ten hours, twenty-two minutes during three space shuttle flights in 1984.

The MMU measured 1.25 meters (49 inches) tall, 0.83 meters (33 inches) wide, and 1.2 meters (47 inches) from front to back with hand controller arms fully extended. Without nitrogen propellant, it weighed 142 kilograms (312 pounds). The MMU attached to the shuttle space suit's backpack by two spring-equipped latches.

The MMU was a product of maneuvering device development spanning nearly thirty years, and it became a stepping-stone to the Simplified Aid For EVA Rescue (SAFER) unit carried today during International Space Station (ISS) space walks. The first U.S. astronaut maneuvering aid was the Hand-Held Maneuvering Unit carried by spacewalkers outside Gemini capsules (1965-1966). The MMU's immediate precursor was the Automatically Stabilized Maneuvering Unit, a maneuvering backpack successfully tested in 1973-1974 inside Skylab, the first U.S. space station .

The National Aeronautics and Space Administration (NASA) added the MMU to the space shuttle program in 1974 to allow an astronaut to fly under the shuttle orbiter prior to Earth atmosphere re-entry to inspect its crucial heat shield tiles for damage. Development was slowed, however, by management disinterest and lack of money. In 1979, however, space shuttle Columbia lost tiles during a test flight atop its 747 ferry aircraft, so NASA launched a crash program to prepare the MMU for flight. Engineers soon solved the shuttle's tile problems, however, and the first shuttle mission (STS-1, 1981) flew without an MMU.

NASA then decided to use the MMU for satellite servicing. Astronauts Bruce McCandless and Robert Stewart tested the MMU on mission STS-41-B (February 1984). On STS-41-C (April 1984), astronauts failed to capture the Solar Max satellite using the MMU; they succeeded, however, by using the shuttle's Remote Manipulator System (RMS) robot arm. Astronauts using the MMU and RMS worked together to capture the Palapa and Westar VI satellites during STS-51-A (November 1984). These flights showed that the RMS was easier to use than the MMU.

The January 1986 Challenger disaster led to a sweeping safety examination of NASA human spaceflight systems, and the MMU was found wanting. In 1988 NASA put the two flight MMUs into long-term storage until a purpose could be found for them that justified the cost of upgrades for increased safety. As of 2002, no such purpose has been found.

The experience gained through MMU development has, however, been put to vital use. NASA applied it to the Simplified Aid For EVA Rescue (SAFER) device now worn under the shuttle space suit backpack. SAFER acts as a "life jacket" permitting astronauts who drift away from the International Space Station to maneuver back to safety. SAFER development began in 1992, and Mark Lee and Carl Meade first tested the device in orbit on mission STS-64 (September 1994). SAFER was first worn outside a space station—Russia's Mir—during STS-76 (March 1996), the third shuttle-Mir flight, and was first tested outside the International Space Station during STS-88 (November 1998).

see also Challenger (volume 3); Space Shuttle (volume 3); Space Suits (volume 3); Space Walks (volume 3).

David S. F. Portree

Bibliography

Covault, Craig. "Skylab Aids Design of Maneuvering Unit." Aviation Week and Space Technology 100, no. 22 (1974):42-47.

McKenna, James. "Rescue Device Shines in Untethered Tests." Aviation Week and Space Technology 141, no. 13 (1994):25-26.

Portree, David S. F., and Robert C. Trevino. Walking to Olympus: An EVA Chronology. Washington, DC: NASA History Office, 1997.

Smith, Bruce. "Backpack Modified for Tile Repair Use." Aviation Week and Space Technology 113, no. 7 (1980):65, 67-68.

Internet Resources

Portree, David S. F., and Robert C. Trevino. Walking to Olympus: An EVA Chronology. 1997. National Aeronautics and Space Administration. <http://www.hq.nasa.gov/office/pao/History/mon7.pdf>.