Manned Maneuvering Unit - History

History

In 1966, the US Air Force developed an Astronaut Maneuvering Unit (AMU), a self-contained rocket pack very similar to the MMU. This was planned to be tested during Project Gemini on an EVA by Eugene Cernan from Gemini 9A on June 5, 1966. But the test had to be cancelled because Cernan tired and overheated, sweated so profusely that his helmet visor fogged, before he could get to the AMU mounted on the back of the spacecraft. Astronauts did not learn how to work during EVA without tiring until the final Gemini 12 mission, but no AMU was carried on that flight. Since there was no real need for self-contained astronaut EVA flight in the Apollo and Skylab programs, the idea had to wait for the advent of the Space Shuttle program, though several maneuvering device designs were tested inside Skylab.

The MMU was used on three Shuttle missions in 1984. It was first tested on February 7 during mission STS-41-B by astronauts Bruce McCandless II and Robert L. Stewart. Two months later during mission 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 plan was to use an astronaut-piloted MMU to grapple the SMM with the Trunion Pin Attachment Device (TPAD) mounted between the hand controllers of the MMU, null its rotation rates, and allow the Shuttle to bring it into the Shuttle's payload bay for stowage. Three attempts to grapple the satellite using the TPAD failed. The TPAD jaws could not lock onto Solar Max because of an obstructing grommet on the satellite not included in the blueprints for the satellite. This led to an improvised plan which nearly ended the satellite's mission. The improvisation had the MMU astronaut use his hands to grab hold of an SMM solar array and null the rates by a push from MMU's thrusters. Instead, this attempt induced higher rates and in multiple axes; the satellite was tumbling out of control and quickly losing battery life. SMM Operations Control Center engineers shut down all non-essential SMM subsystems and with a bit of luck were able to recover the SMM minutes before total failure. The ground support engineers then stabilized the satellite and nulled its rotation rates for capture with the orbiter's robotic arm, the Shuttle Remote Manipulator System (SRMS). This proved to be a much better plan. Their successful work increased the lifespan of the satellite.

The final MMU mission was STS-51-A, which flew in November 1984. The propulsion unit was used to retrieve two communication satellites, Westar VI and Palapa B2, that did not reach their proper orbits because of faulty propulsion modules. Astronauts Joseph P. Allen and Dale Gardner captured the two satellites and brought them into the Orbiter payload bay for stowage and return to Earth.

After the Space Shuttle Challenger disaster, the MMU was judged too risky for further use and it was found many activities planned for the MMU could be done effectively with manipulator arms or traditional tethered EVAs. NASA also discontinued using the Shuttle for commercial satellite contracts, and the military discontinued the use of the Shuttle, eliminating the main potential uses. Although the MMU was envisioned as a natural aid for constructing the International Space Station, with its retirement, NASA developed different tethered spacewalk approaches. Additionally, the MMU gas ejecta was considered too damaging for the only other ongoing task, upkeep of the Hubble Space Telescope.

NASA transferred the flight article #3 to the National Air and Space Museum in 1998. The remaining MMU are stored at NASA (location not known).