Human Satellites

Also in pride of place on Vance Brand's patch was a snazzy, jet propelled spacesuit backpack, known as the Manned Manoeuvring Unit (MMU), together with the surname of an astronaut who had waited longer than most for his first orbital voyage. Mission Specialist Bruce McCandless joined NASA in April 1966, along with Brand, but his patient wait for space had exceeded by more than a decade that of many of the Thirty Five New Guys (TFNGs). Even old timers like Bo Bobko, Don Peterson and Story Musgrave had not waited quite as long as poor McCandless.

One of the reasons for his lengthy status as an astronaut-in-waiting was that he had been instrumental in the design and development of the MMU and had long been expected to test it on its first outing in space. His long wait - close to two full decades by the time Challenger lifted off at precisely 1:00 pm on February 3rd 1984 - would be worth it and his famous photograph, snapped by STS-41B Pilot Robert 'Hoot' Gibson, has since graced many a spaceflight book, magazine cover, wall poster and screensaver.

It also won NASA and the MMU's prime contractor, Martin Marietta of Denver, Colorado, the coveted Collier Trophy for 1984. Special recognition was also granted to McCandless, NASA's Charles Whitsett and Martin Marietta's Walter Bollendonk for a triumphant maiden mission.

In the wake of the Columbia tragedy in February 2003, it seems bitterly ironic that the original purpose of the MMU was to enable spacewalking astronauts to inspect and possibly repair damaged thermal protection materials on the Shuttle's wings and lower surfaces. Moreover, in the words of a NASA press release from October 1979, it would permit rescue operations and even the servicing and deployment of satellites. Despite the hype that encircled its first flight, however, it was actually the latest in a long line of manoeuvring packs whose heritage dated back to the early 1960s.

A decade before McCandless undertook his MMU sortie outside Challenger, astronauts had evaluated a similar device whilst wearing spacesuits and shirtsleeves inside the Skylab orbital workshop. Still earlier, in June 1965, the first American spacewalker, Ed White, had employed a hand-held pressurised gas 'gun' to move around the exterior of his Gemini capsule. Today, the MMU's own descendant -known as the Simplified Aid For Extravehicular Activity Rescue (SAFER) is used routinely by astronauts working outside the International Space Station.

"In retrospect, I probably lavished too much attention on scientific and engineering interests, as opposed to the flying, flying and more flying," McCandless told me in a March 2006 email correspondence. "At any rate, I became interested in manoeuvring units shortly after the Gemini 9 fiasco in June 1966, in which spacewalker Gene Cernan was overwhelmed by immature pressure suit technology and was unable to fly the US Air Force's Astronaut Manoeuvring Unit [AMU]. This led to a retrenching to develop EVA technology and, unfortunately, removal of the AMU from Gemini 12, in order to guarantee ending the Gemini programme on a positive note. At about that point, I, together with a civil servant called David Shultz and Charles Whitsett became interested in showing that the concept of a manoeuvring unit was valid and that useful units could be built. We collaborated on the M-509

Stunning image of Bruce McCandless during his first flight with the Manned Manoeuvring Unit.

experiment - a multi-mode manoeuvring unit - to be demonstrated inside the Skylab workshop. I hoped to be the first 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, as the first US space station was launched on May 14th 1973, a solar panel and micrometeoroid shield were torn off during its ascent to orbit. Temperatures inside the workshop soared and were only stabilised by the efforts of the first crew - that of Skylab 2, consisting of Charles 'Pete' Conrad, Joe Kerwin and Paul Weitz - in a series of complex spacewalks and emergency repair work.

"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 following the loss of its micrometeoroid shield on launch," continued McCandless. "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. Ultimately, those tasks were scrapped and it was built and tested to support the Solar Max repair mission."

Although the need to potentially repair the Shuttle's heat-shielding tiles 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 disinterest and lack of firm funding. Then, in the spring of 1979, as Columbia was being moved from California to Florida, several tiles were lost and renewed vigour was injected into developing the backpack. By the time STS-1 took to the skies in April 1981, most of the tile problems, seemingly, had been solved and no MMU was aboard. Nevertheless, on opening the payload bay doors, the astronauts saw that some tiles were missing from one of the Orbital Manoeuvring System (OMS) pods, and, in response to concern that there might be tile damage to the belly of the vehicle, it was reportedly inspected by an imaging spy satellite.

It would instead be used, said NASA, for satellite repairs and maintenance and was rendered all the more useful by the provision of electrical sockets for tools, portable lights and cameras. The device was 1.2 m high, 81 cm wide and 66 cm deep and, according to astronaut Joe Allen, who flew it in November 1984, resembled "some kind of overstuffed rocket chair". On a typical mission, two MMUs were stored on opposing walls at the front end of the Shuttle's payload bay and spacesuited astronauts backed themselves into it and secured its two spring loaded latches into place.

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. The first two operational flight units, valued at around ten million dollars apiece, arrived at the Johnson Space Center (JSC) in Houston, Texas, in September 1983 to support astronaut training. Two months later, they were installed aboard Challenger. Each weighing 140 kg, they were painted white to achieve adequate thermal control in the harsh environment of low-Earth orbit and were fitted 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 six kilograms of high-pressure gaseous nitrogen. To operate them,

A view of the 'rear' of the Manned Manoeuvring Unit during vacuum chamber tests at the Johnson Space Center in March 1981.

the astronaut used hand controllers at the end of two armrests: one provided rotational acceleration for roll, pitch and yaw, while the other allowed him to move forward, backward, up, down and from left to right. 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 was provided by a pair of silver-zinc batteries, capable of supporting the unit for up to six hours of autonomous flight as far as 140 m from the Shuttle. In fact, one of the MMU's widely publicised 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, nor his spacewalking buddy, Mission Specialist Bob Stewart, ventured so far from Challenger that the pilots would not be able to rescue them if necessary.

"We didn't want to come back and face their wives if we lost either one of them up there," joked Brand.

Its controllability, though, was precise. "The minimal training and precision flying features," said one magazine editor, who flew a model of the MMU at Martin Marietta's Space Operations Simulator in Denver, "were demonstrated by my ability, with only a few minutes' practice, to manoeuvre safely in close proximity to fixed objects." Joe Allen, whose own MMU sortie in November 1984 salvaged an errant communications satellite, also remarked that, in space, it "glided" and displayed none of the idiosyncratic jerks, jolts, bumps and grinding sounds that were characteristic of Martin Marietta's simulator.

For Bruce McCandless, who backed himself into the device on February 7th 1984, securing himself with two mechanical latches and a lap belt, it represented "a heck of a big leap", in terms of spacewalking technology and his own personal odyssey. In a similar manner to the excursions conducted by Musgrave and Peterson almost a year earlier, preparations for the two STS-41B spacewalks began shortly after Challenger reached orbit, when her cabin pressure was lowered to 10.2 psi. This reduced McCandless and Stewart's 'pre-breathing' exercises from the three hours needed under 'normal', 14.7 psi conditions to less than an hour.

Another common thread between McCandless and Musgrave was that they were two of the most highly trained EVA specialists in the astronaut office at that time. "I am probably not a 'representative' EVA trainee," McCandless remembered years later. "I was grossly over-trained! I took every opportunity to get into a pressure suit, an altitude chamber or a water tank, commencing early in the Apollo programme. I helped design the Skylab M-509 experiment and made water tank runs on all of the Skylab and Hubble Space Telescope EVA tasks for development, validation and training. Concurrently, we used Martin Marietta's Space Operations Simulator (SOS) for manoeuvring unit development and, conversely, the manoeuvring units to drive enhancement and further improvement of the SOS. In discussing training for spaceflight, the first thing to recognise is there is currently no single, comprehensive device or system that gives a 'total' simulation. Eventually, training for spaceflight will consist of taking the trainees into space as passengers and conducting the training, in situ. Until that day arrives, however, training is accomplished on a 'part task' basis, leaving it to the individual to mentally integrate all of the pieces when the time comes. In the specific case of MMU training, the SOS allowed the pilots to 'fly' around inside a large 15 m long by 4.1 m wide by 4.5 m high room as though we were in space, as determined by computer software driving servo motors in all six axes in response to control inputs and the laws of physics. It was quite effective and could accommodate a fully suited astronaut and reasonable sized mock-ups of 'target' objects, such as the underside of the orbiter for tile repair. 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 spacesuits 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 told me, "it was noisy up there, thanks to two independent radio channels and plenty of people wanting to talk to me!" Then, just before leaving Challenger's airlock, Stewart reported a caution and warning alarm, which indicated the pressure of his suit's sublimator had risen to 4.0 psi. However, after being switched off and back on, it performed normally.

These subtle problems did not distract from the triumph of McCandless' Buck Rogers-style flight that day. Despite the sci-fi analogy, said Vance Brand, "it 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 91m away, politely offering to clean Challenger's cockpit windows as he floated over the flight deck. Watching intently from inside, an admiring Brand declined the offer.

"Having the opportunity to actually fly the MMU, the handling characteristics were exactly like those of the SOS," remembered McCandless, "with one, initially puzzling, exception. With the unit in 'attitude hold' mode, whenever I inputted a +/— X translation command, I heard and felt a chugging sound and vibration. On reflection, we collectively realised that this was caused by our bodies' centre of mass not being exactly co-aligned with the MMU centre of mass, thereby displacing that slightly. Consequently, a translational thrusting command tended to cause a slight pitching motion, which 'attitude hold' counteracted by modulating one or more active thrusters to 'off, as it had been designed to do, and counteracting the pitch moment, ultimately holding attitude right where it was supposed to. We ignored this effect and added a noise maker to the SOS to enhance the training for the STS-41C crew."

During their tethered work in the payload bay, McCandless and Stewart removed a failed television camera for replacement with an in-cabin unit and later installed it during the second spacewalk on February 9th. The MMU, too, performed admirably, but ironically, Brand undermined its raison d'etre. The backpack was touted as being capable of achieving far more precise and intricate movements than the Shuttle, but on STS-41B and 41C the real value of Challenger's manoeuvrability and her Canadian-built mechanical arm were demonstrated - by retrieving one of McCandless' lost foot restraints.

"I scurried down the starboard handrail of the payload bay," McCandless remembered later, "and held up my right hand and Vance 'flew' my hand to the point where I could grasp, and retrieve the errant restraint."

"I don't recall now whether it was before or after he went out with the backpack," Brand added, "but he was trying to reposition his foot restraint, so that he could get into it to do work. Our EVA equipment was generally tethered, but it somehow got away from him. I looked back and saw it floating away. I thought about it for a second or two and decided that the ground wouldn't have time to come up with a decision whether we ought to chase it and go after it. It was going to get away from us very quickly, so I couldn't see anything wrong with going after it. We chased it, Bruce caught it and we didn't have to worry about encountering that as 'space junk' the next time we came around the world. I had one switch that was out of position when I first fired the Reaction Control System jets, which had the thrusters aligned to an axis system that was 90 degrees from what I needed at that time. After the first thrusting, I had to reposition the switch to get the proper orientation, so that the right thrusters would come on and I could accurately chase the restraint." By so doing, Brand showed that the Shuttle was capable of the same intricate motions as the MMU and, on the next flight, STS-41C in April, when a task involving the MMU was frustrated, the Remote Manipulator System (RMS) would prove itself equally capable. Despite the MMU's success during two satellite recoveries in November 1984, the manoeuvrability of the Shuttle contributed to its ultimate demise.

In fact, the year immortalised by George Orwell would be the only time the MMU was ever used in space. By the end of 1984, it had seen service on three Shuttle missions, flown by six astronauts - McCandless, Stewart, Allen, George 'Pinky' Nelson, James 'Ox' van Hoften and Dale Gardner - for a total of just ten and a half hours, spread across six spacewalks. Other assignments were expected but, in the wake of the Challenger disaster, safety upgrades imposed by the Rogers Commission proved costly and the units were mothballed to await further opportunities.

Sadly, as of 2002, by which time the smaller, backpack-mounted SAFER device had been operational for several years, no such opportunities had crystallised.

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