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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. Bruce McCandless was a key player in the development of the unique MMU. He also took the MMU on its first space test, the first ever untethered Extravehicular activity (EVA), in this week’s 40th anniversary.
McCandless had no idea that his spectacular EVAs from February 1984 would result in some of most recognizable spaceflight photos of the 20th century. The photographs from these unique spacewalks are still used today on spaceflight books and magazine covers, posters, screensavers, and wall posters.
McCandless was a backup pilot for more than a decade prior to his epochal spacewalks. Skylab 1: the first mission and participated in “Experiment M-509”. This nitrogen-fed backpack space suit was tested aboard the station in the summer 1973 and served as an early prototype for the MMU.
“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 Gemini IX-A: What happened?, 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 Air Force Captain David C. Shultz, McCandless worked on Experiment M-509 along with civil servant David C. 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.”
Unfortunately, during Skylab’s ascent to orbit on 14 May 1973, a solar panel and the micrometeoroid shield were torn away. The temperatures inside the station were soaring and only stabilised by the stoic efforts made by the first Skylab crew during a complex EVA, and a month-long campaign for 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.”
In the end, NASA and Martin Marietta won the coveted Collier Trophy 1984 for the spectacular success of their jet backpack. 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. As noted in a news release from October 1979, it would allow rescue operations as well as the servicing of damaged satellites.
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. As Columbia was being moved to Florida from California in the spring 1979, several heat resistant tiles were missing from its airframe. A renewed effort was made to develop the backpack.
By the time STS-1 was launched in April 1981Most of the problems had been resolved and there was no MMU on board. It was to be used for satellite maintenance and repairs. The electrical sockets were added for portable lights, cameras, and tools.
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.
The MMU was about four feet high (1.2 meters), 2.7 feet wide (81 cm) and 2.2 feet deep (66 cm). According to astronaut Joe Allen, it resembled “some kind of overstuffed rocket-chair.”
On a typical flight, two MMUs would be stored on Flight Support Structures in the front of the payload compartment. The astronaut would secure the spring-loaded locks and a belt in place by backing into it, then release the unit and let it float away.
Martin Marietta from Denver, Colo. was awarded the $26.7M MMU fabrication project by NASA in Feb. 1980 after spending more than 4 years in the design-definition stage. The first two flight units worth $10 million each arrived at the Johnson Space Center in Houston, Texas in September 1983 for astronaut training.
Two months later they were installed aboard Challenger. Each weighed approximately 310 pounds (140 kg) and were painted white for thermal control in low-Earth orbit. Electrical heaters kept their components at temperatures above the minimum levels.
Two propellant tanks were attached to the back of every MMU. These tanks supplied 24 tiny thrusters a total of forty pounds (18 kilograms), of high-pressure nitrogen gas. The astronaut used two armrests with hand controllers to operate the thrusters. One controlled roll, pitch, yaw, and the other moved him forward, backwards and up, down and left to right.
By using both simultaneously, he was able to achieve extremely 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.
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!”
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.
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.”
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!”
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.
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.”
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)