MMU

MMU 2-F2 [NASA]

The MMU (Manned Maneuvering Unit) was an astronaut propulsion device developed by Martin Marietta for NASA in the United States. It was flown on Space Shuttle missions in 1984 to support untethered extravehicular activity.

The MMU was designed to allow astronauts to maneuver freely in space without a physical tether to the Space Shuttle. Its primary purpose was to enable inspection, handling, and retrieval of payloads at distances beyond the reach of the Shuttle’s robotic arm. The system expanded EVA operational flexibility by permitting controlled translational and rotational movement in all axes.

In operational use, the MMU supported satellite servicing and recovery tasks. It was employed during missions aimed at repairing or retrieving satellites that could not be handled solely by the Shuttle Remote Manipulator System. After limited flight experience, its role was reassessed and not expanded further.

The MMU was manufactured by Martin Marietta, later Lockheed Martin, as a self-contained propulsion and control unit integrated with the Shuttle Extravehicular Mobility Unit. The design emphasized system redundancy to mitigate single-point failures during untethered operations. The unit mounted directly over the astronaut’s life-support backpack and interfaced mechanically and electrically with the EMU.

The propulsion system used gaseous nitrogen stored in two aluminum tanks wrapped with Kevlar. Each tank contained 5.9 kilograms of propellant. The nitrogen supply supported up to six hours of EVA, depending on maneuvering activity. Initial ground charging provided a delta-v of approximately 110 to 130 ft/s, with on-orbit recharge capability to a minimum equivalent of 72 ft/s.

A total of 24 cold-gas thrusters were distributed around the unit to provide balanced control. Translational acceleration for a nominal mass was approximately 0.3 ± 0.05 ft/s², while rotational acceleration was about 10.0 ± 3.0 degrees per second². Attitude stabilization could be maintained using an automatic inertial attitude-hold function.

Control was achieved through two hand controllers mounted on adjustable arms. The right-hand controller commanded roll, pitch, and yaw, while the left-hand controller commanded translational motion along three axes. The controller arms were adjustable over a range of approximately 13 centimeters to accommodate different astronaut body dimensions.

With a full propellant load, the MMU had a mass of 148 kilograms. When not in use, it was stowed in a support station on the payload bay wall near the airlock. Two units were carried per mission.

The MMU flew on three Space Shuttle missions in 1984.

• Its first on-orbit test occurred during STS-41-B on 7 February 1984, when astronauts Bruce McCandless II and Robert L. Stewart conducted untethered EVAs.
• The second mission, STS-41-C in April 1984, involved an attempt to capture and service the SMM (Solar Maximum Mission) satellite. Direct capture using an MMU-mounted device was unsuccessful, and the satellite was ultimately stabilized and recovered using the Shuttle’s robotic arm.
• The final flight occurred during STS-51-A in November 1984. During this mission, astronauts Joseph P. Allen and Dale Gardner used the MMU to retrieve the communications satellites Westar 6 and Palapa B2, which were returned to Earth after failed orbital insertions.

Following these missions, the MMU was retired. A smaller emergency-only successor, the Simplified Aid for EVA Rescue (SAFER), was later introduced and first flown in 1994.