“Buck and Flash”: Remembering the First Flights of the Manned Maneuvering Unit (MMU), Forty Years On

This iconic image, captured by astronaut Robert “Hoot” Gibson, shows Bruce McCandless participating in humanity’s first untethered EVA, aboard the Manned Maneuvering Unit (MMU). Photo Credit: NASA

When the crew of STS-41B, tenth flight of the shuttle and fourth by Challenger, released their official crew patch late in 1983, it included in pride of place a jet-propelled space suit backpack—the Manned Maneuvering Unit (MMU)—and the name of the singular astronaut who had waited nearly two decades to fly it: Bruce McCandless. He played a critical role in designing and developing the unique MMU and taking it for its first test-drive in space, on the first-ever untethered Extravehicular Activity (EVA), 40 years ago this week.

Video Credit: NASA, via National Space Society (NSS)/YouTube

Little could McCandless have envisaged that his spectacular pair of EVAs in February 1984 produced some of the most recognizable spaceflight images of the 20th century. Photographs from those unique spacewalks have been used time and again on spaceflight books, magazine covers, wall posters and screensavers to this very day.

More than a decade before his epoch-making spacewalks, McCandless served as backup pilot for the first Skylab mission and participated in “Experiment M-509”. This nitrogen-fed space suit backpack was tested aboard the space station in the summer of 1973 and served as a forerunner for the MMU.

Commander Vance Brand leads the STS-41B crew out to the launch pad on the morning of 3 February. Photo Credit: NASA

“In retrospect, I probably lavished too much attention on scientific/engineering interests, as opposed to flying, flying and more flying,” McCandless told this author in 2006. “At any rate, I became interested in maneuvering units shortly after the Gemini IX-A fiasco, in which Gene Cernan was overwhelmed by immature pressure suit technology and was unable to fly the U.S. Air Force Astronaut Maneuvering Unit (AMU).”

Together with civil servant David C. Shultz and Air Force Capt. Charles “Ed” Whitsett, McCandless began work on Experiment M-509, which he described as “a multi-mode maneuvering unit,” which would be “demonstrated inside the Skylab workshop for safety and simplicity…obviating the need for expensive redundant systems and vacuum qualification.” At one stage, McCandless hoped to fly it, “but that was not to be. I was named as backup pilot for Skylab 2 and waved goodbye to being on the prime crew.”

Video Credit: Smithsonian National Air & Space Museum

Unfortunately, during Skylab’s ascent to orbit on 14 May 1973, a solar panel and the micrometeoroid shield were torn away. Temperatures inside the station soared and were only stabilized by the stoic efforts of the first Skylab crew in a complex EVA and a grueling month-long campaign of emergency repairs.

“They, however, were prohibited from trying the manoeuvring unit out due to fears that its nickel cadmium batteries had been damaged by the high temperatures inside the workshop,” McCandless said. “The two subsequent Skylab crews did use the M-509 and gave it glowing reports, thus enabling us to sell NASA management on building an MMU in connection with the shuttle, initially planned for the conduct of tile inspection and repair.”

Challenger roars to orbit on 3 February 1984. Photo Credit: NASA

In time, the spectacular success of the jet backpack would win NASA and prime contractor Martin Marietta the coveted Collier Trophy for 1984. McCandless, together with Whitsett and Martin Marietta’s Walter “Bill” Bollendonk, were granted special recognition for their contributions to its development and orbital checkout. Whitsett paid particular tribute to McCandless’ work. “Nobody has left his stamp on any instrument in space,” he told the Washington Post, “like Bruce has left his mark on the backpack.”

In the wake of the STS-107 tragedy, it might seem ironic that the original purpose of the MMU was to enable spacewalking astronauts to inspect and possibly repair damaged Thermal Protection System (TPS) materials on the shuttle’s wings and lower surfaces. Moreover, as noted in an October 1979 news release, it would also permit rescue operations and even the servicing of disabled satellites.

Bruce McCandless peers through Challenger’s overhead windows during STS-41B. Photo Credit: NASA

Although the need to potentially repair portions of the shuttle’s TPS was one of the main reasons for the MMU, its development—which began in earnest in 1975—was still hampered for some years by management apathy and lack of firm funding. Then, in the spring of 1979, as Columbia was being moved from California to Florida, several heat-resistant tiles were lost from her airframe and renewed vigour was injected into developing the backpack.

By the time STS-1 flew in April 1981, most of problems had been solved and no MMU was aboard. It would instead be used for satellite repairs and maintenance, and its usefulness was enhanced with electrical sockets for tools, portable lights and cameras.

Bruce McCandless (left) and Bob Stewart work at the forward end of Challenger’s payload bay, prior to the first MMU trial. Note the open airlock hatch cover at lower center. Photo Credit: NASA

In June of 1982, the agency announced that the shuttle would retrieve, repair and deploy the crippled Solar Maximum Mission (“Solar Max”) in early 1984. But before the MMU could be committed to the repair, a thorough test of its performance in orbit was required and this was the task of McCandless—the MMU’s “project pilot”—and fellow astronaut Bob Stewart on STS-41B. The pair jokingly dubbed each other “Buck” and “Flash” during their time in space.

Physically, the MMU measured about four feet (1.2 meters) high, 2.7 feet (81 cm) wide and 2.2 feet (66 cm) deep. According to astronaut Joe Allen, it resembled “some kind of overstuffed rocket-chair.”

Bruce McCandless maneuvers along Challenger’s payload bay door sill in the Manned Maneuvering Unit (MMU). Photo Credit: NASA

On a typical mission, two MMUs were stored on Flight Support Structures (FSS) on opposing walls toward the front of the payload bay. An astronaut would back into it and secure a pair of spring-loaded latches and a lap belt into place, then release the unit from its support structure and float free.

After more than four years in the design definition stage, in February 1980 NASA awarded the $26.7 million MMU fabrication contract to Martin Marietta of Denver, Colo. The first two flight units, valued at $10 million apiece, arrived at the Johnson Space Center (JSC) in Houston, Texas, in September 1983 to support astronaut training.

McCandless works with a simulated docking mechanism and target for the Solar Max repair. Photo Credit: NASA

Two months later, they were installed aboard Challenger. Each weighed 310 pounds (140 kilograms) and was painted white to achieve adequate thermal control in the harsh environment of low-Earth orbit, with electrical heaters to keep their components above minimum temperature levels.

Affixed to the back of each MMU were two propellant tanks, which supplied 24 tiny thrusters with a total of 40 pounds (18 kilograms) of high-pressure gaseous nitrogen. To operate the thrusters, the astronaut used hand controllers at the end of two armrests: one provided roll, pitch, and yaw control, whilst the other allowed him to move forward, backwards, up, down, and from left to right.

McCandless works with a simulated docking mechanism and target for the Solar Max repair. Photo Credit: NASA

Furthermore, by using both in unison, he could achieve very intricate movements. Particularly useful for repair missions, when a desired orientation had been reached, he could activate an automatic, “attitude-hold” function to free his hands for work. Electrical power came from a pair of silver zinc batteries, capable of supporting the unit for up to six hours of autonomous flight as far as 460 feet (140 meters) from the shuttle.

In fact, one of the MMU’s widely publicized features was that its wearer did not need to remain attached to the spacecraft by a safety tether. Of course, in the event of problems, most of its systems were redundant and neither McCandless or Stewart would venture so far from Challenger that the pilots would not be able to rescue them if necessary.

Bob Stewart pilots the MMU to a distance away from Challenger. Photo Credit: NASA

In its STS-41B Press Kit, NASA stressed that the EVAs would be designed with conservatism in mind. “Both McCandless and Stewart will fly untethered from Challenger to distances of about 150 feet (50 meters),” the space agency explained, “then to 300 feet (100 meters) and return.”

STS-41B Commander Vance Brand only half-jokingly quipped: “We didn’t want to come back and face their wives if we lost either one of them up there!”

During his historic untethered EVA with the Manned Maneuvering Unit (MMU) on 7 February 1984, Bruce McCandless ventured as far as 300 feet (91 meters) from Challenger. Photo Credit: NASA

The MMU’s controllability was crisp and precise. “The minimal training and precision flying features,” said one observer, who flew a model of the MMU at Martin Marietta’s Space Operations Simulator (SOS) in Denver, “were demonstrated by my ability, with only a few minutes’ practice, to maneuver safely in close proximity to fixed objects.”

For Bruce McCandless, who backed himself into the device early on 7 February 1984, it represented “a heck of a big leap,” in terms of spacewalking technology and the culmination of his own personal odyssey. Preparations for this excursion had begun soon after Challenger reached orbit, four days earlier. The shuttle’s cabin pressure was lowered from 101.3 kPa (“normal” atmospheric pressure at sea level) to 70.3 kPa in order to reduce McCandless and Stewart’s “pre-breathing” routine from three hours to less than one hour.

Equipped with an MMU, STS-41C spacewalker George “Pinky” Nelson approaches the slowly-spinning Solar Max ahead of retrieval and repair activities in April 1984. Photo Credit: NASA

It was familiar ground for McCandless, who admitted to this author in 2006 that he was “probably not a representative EVA trainee” and had been “grossly over-trained.” Throughout the 1970s and 1980s, he took every opportunity to get into a space suit, an altitude chamber or a water tank and participated extensively in EVA simulations on Skylab and the Hubble Space Telescope (HST).

At Martin Marietta’s Denver facility, he and Stewart “flew” mockups of the MMU in the SOS, which measured 50 feet (15 meters) long by 13.5 feet (4.1 meters) wide and 14.8 feet (4.5 meters) high. “It was quite effective,” McCandless concluded, “and could accommodate a fully-suited astronaut and reasonable sized mockups of “target” objects, such as the underside of the orbiter for TPS repairs. It also had the capability for introducing malfunctions for training purposes.”

STS-41C spacewalker James “Ox” van Hoften flies the MMU in Challenger’s payload bay in April 1984 during the Solar Max repair and retrieval. The gold-colored satellite is visible in the background. Photo Credit: NASA, via SpaceFacts.de

In spite of their complexity, McCandless and Stewart’s excursions proved successful and the space suits and MMUs performed admirably. The only “nuisances” were static on the communication channels and difficulties attaching checklists to the suits’ arms.

“In spite of the sound-does-not-travel-through-a-vacuum tenet of physics,” McCandless said, “it was noisy up there, thanks to two independent radio channels and plenty of people wanting to talk to me!”

Wearing an MMU, astronaut Dale Gardner extends a “stinger” mechanism, prior to capturing the errant Westar-VI communications satellite in November 1984. Photo Credit: NASA

Those subtle problems did not distract from the triumph of McCandless’ Buck Rogers-style flight that day. Despite the sci-fi analogy, said Brand, the MMU “didn’t have the person zooming real fast. It was a huge device that was very well-designed and redundant, so that it was very safe, but it moved along at about one to two miles per hour.”

At his furthest distance from the shuttle, McCandless was almost 300 feet (91 meters) away and politely offered to clean Challenger’s windows as he floated past the flight deck. Watching intently from inside, an admiring Brand declined the offer.

Video Credit: Smithsonian Channel

Also watching intently, camera in hand, was STS-41B Pilot Robert “Hoot” Gibson, the person who snapped the singular photograph which would make history as one of the top five most-requested images from NASA. In an interview for the Smithsonian in 2001, he recalled the astonishing sight of McCandless flying the MMU.

“Bruce first did a couple of brief test flights in the cargo bay, staying very close in case anything should go wrong,” Gibson said. “As we were approaching sunrise on one of our daylight passes, he was cleared to make the translation out to 300 feet from the shuttle.”

Robert “Hoot” Gibson acquired the photography of McCandless’ first flight with the Manned Maneuvering Unit (MMU) in February 1984. Photo Credit: NASA

Grabbing his Hasselblad camera, Gibson began shooting frame after frame. Since Challenger’s orientation was 30 degrees off-vertical, McCandless appeared at a similar angle with respect to Earth’s horizon. Gibson knew that his images might easily make the cover of Aviation Week—they actually made two—and remembered taking multiple light settings and tweaking the focus several times before squeezing the button.

The famous shot would come to be known as “Backpacking” and even years later, McCandless kept a goofy version in his home, in which his grown daughter poked her head through the cut-out visor in a life-size reproduction at a Seattle, Wash., museum. In 2005, McCandless explained that what he liked most about the image was its lack of identity; with his sun visor closed, it was impossible to see his face, “and that means it could be anybody out there…sort of a representation, not of Bruce McCandless, but mankind.”

In Memory of Bruce McCandless (1937-2017)

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