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[Air & Space Magazine] Untethered
« dnia: Grudzień 25, 2017, 01:29 »
Untethered (1)

The MMU may have been the coolest space vehicle ever. So why did its career end as soon as it began?

By Andrew Chaikin
Air & Space Magazine
October 2014

Bob Stewart tests the MMU in 1984. No astronauts since have ranged farther from their ship. (NASA)

The picture is one of the great mind-blowers of the 20th century: Astronaut Bruce McCandless is a tiny, free-flying satellite, 300 feet from the space shuttle Challenger, on the first test flight of the Manned Maneuvering Unit in February 1984. Never mind the personal thrill for McCandless, who had waited 18 years for his first spaceflight, 16 of them spent developing the rocket-powered MMU. The test on shuttle mission STS‑41B was the realization of a dream dating to the early 1960s, before anyone had walked in space, when planners envisioned a jetpack that would let astronauts move effortlessly among space stations and satellites. McCandless’ untethered sortie seemed to herald a new era for spacewalkers. Yet by the end of 1984, the MMU had made its last flight, ending a brief, if spectacular, episode in the shuttle era. What happened?

As alluring as it still seems, the vision of a free-flying astronaut was never a sure thing. Air Force planners developed a prototype of a propulsive backpack in 1962, and a flight model called the Astronaut Maneuvering Unit (AMU), outfitted with hydrogen peroxide thrusters, reached orbit aboard Gemini 9 in 1966. But the AMU’s test flight by Gemini 9 spacewalker Gene Cernan had to be canceled when the exertions of working within a stiff, pressurized spacesuit overwhelmed his suit’s rudimentary cooling system. The AMU’s creators thought it would get another try on the final Gemini mission, in late 1966, but by that time, stinging from a series of troubled spacewalks and determined to end the program with a successful EVA (extravehicular activity), NASA managers refused to risk it. When Gemini ended, hopes for a maneuvering unit seemed to end with it.

Not for long. By 1967 a new champion of astronaut self-propulsion had emerged at NASA’s space center in Houston—a young Air Force officer with buzz-cut hair and a shy smile named Ed Whitsett. “The guy was a dynamo,” remembers NASA engineer Joe McMann. “Yet to look at him, you’d never think it. He just looked like a meek guy.” Before long Whitsett had an equally passionate collaborator in rookie astronaut McCandless, an ex-naval aviator who’d spent his first years at NASA playing supporting roles on Apollo. “He was a fanatic,” says McMann. “He ate, slept, and breathed it, just like Ed did.” Together, Whitsett and McCandless began working on a design for an astronaut jetpack.

At the Martin Marietta plant in Denver, Colorado, astronaut Anna Fisfer trains on the MMU flight simulator. (Roger Ressmeyer/Corbis)

NASA chose their invention as one of the experiments to be conducted inside its planned Skylab space station. Propelled by compressed nitrogen, the unit had 24 thrusters for “translating”—moving forward or backward, side to side, or up and down—as well as rotating around the roll, pitch, and yaw axes. Powerful gyroscopes inside the backpack could hold an astronaut steady without using fuel. Built by Martin Marietta Aerospace in Denver, the unit was known by its official designation: M-509.

Skylab also carried two other designs for maneuvering systems, including a hand-held “zip gun.” Although far less capable than Whitsett and McCandless’ backpack, the gun was favored by some in Houston because it was relatively inexpensive, and had been built in-house. NASA’s Langley Research Center in Virginia threw its hat in the ring with a foot-controlled maneuvering unit designed to leave the hands free. In a kind of orbital fly-off, Skylab crews tested all three units inside the station’s cavernous workshop in 1973-1974, and the M-509 emerged the clear winner. Even scientist-astronaut Owen Garriott, who hadn’t received any preflight training on the backpack, flew it with ease. With the Skylab success, Whitsett and McCandless focused on their ultimate goal—a maneuvering unit for spacewalkers aboard the spacecraft then in development, the space shuttle.

Ed Whitsset (with glasses) worked doggedly to deign a jetpack astronauts could use to maneuver freely in space. (Courtesy Lou Ramon)

There was just one problem: In those days, EVA wasn’t even planned as a routine shuttle capability, because, according to several NASA veterans, program managers thought it was too risky. Their worry—oft repeated in the shuttle’s early planning phase—was that a pressure-suited astronaut in the vacuum of space was “one system away from certain death.”

But certain malfunctions—for example, something preventing the shuttle’s payload bay doors from closing—could be fixed only during an EVA. And if a shuttle crew were stranded in orbit, a rescue would have to be attempted. In one scenario, each astronaut would be zipped into a pressurized fabric sphere to be transferred to a second shuttle—with help, Whitsett and McCandless hoped, from their maneuvering unit. But that plan was scrapped because keeping a rescue shuttle on call for every mission wasn’t realistic.

Whitsett and his team kept on the lookout for other problems that a maneuvering unit could solve. By late 1978, as concerns mounted over the shuttle’s fragile thermal protection tiles, the two were pitching the MMU as a way for astronauts to repair damaged or missing tiles in orbit. But new techniques for strengthening the tiles before flight undercut that idea. Still, McCandless says, Whitsett (who died in 1993) never seemed to lose heart. “This was more than just a job for him,” he explains, adding that “Ed was a genius at getting yet another few thousand or tens of thousands of dollars to study yet another aspect and keep the program alive.” Finally, in late 1980, just months before the shuttle’s first launch, they got the opportunity they’d been waiting for. NASA’s Solar Maximum Mission satellite, launched earlier that year to study the sun, had been crippled by a malfunction in its attitude control system. Solar Max was designed to be serviced, and in the summer of 1982, NASA made the decision to have astronauts repair the satellite in the shuttle’s cargo bay. For that to happen, one of them would have to capture Solar Max. And for that, he would need the MMU.

Bruce McCandless was the first to fly the mini-spacecraft he spent 16 years helping to design. Protruding from the front of his suit , a docking mechanism was designed to attach to the Solar Max satellite on the next flight. (NASA)

With the rescue slated for a shuttle mission less than two years away, the MMU effort kicked into high gear at Martin Marietta. Program manager Bill Bollendonk and his team had to tackle a host of technical issues, including how to safely contain the backpack’s nitrogen supply at a pressure of 3,000 pounds per square inch. They adapted an aluminum alloy tank used on 747 airliners, and wrapped it in Kevlar for extra strength. Bollendonk says that to test the tank’s safety at operating pressures, “we ended up firing a .50-caliber bullet through it. It made a hole, but it didn’t explode.”

To extend battery life, the big power-hungry gyroscopes used to maintain orientation on the Skylab version were removed. The MMU’s control system would use smaller gyros to sense rotation, then immediately fire thrusters to counteract it. Also, the M-509’s pistol-grip hand controllers, which were tiring to operate in pressurized spacesuit gloves, were replaced by small T-handles that needed just a nudge of the fingertips. Adjustable control arms would accommodate the tallest and shortest astronauts.

Calling the MMU a “backpack” was a little deceptive, since, in its final form, it weighed more than 300 pounds. It contained enough nitrogen to accelerate from zero to 45 mph—just once. To conserve that precious supply, astronauts would fly the MMU at a crawl, just a few inches per second. (George Clooney’s zipping around in the movie Gravity was preposterous: If an astronaut acted that recklessly, the MMU’s fuel tank would be empty in no time.)

Two MMU flight units were completed in time for a test on shuttle mission STS-41B in February 1984, with McCandless and fellow rookie Bob Stewart named as the first pilots. Before joining NASA, Stewart had been an Army test pilot, and had worked on development of the Apache and Black Hawk helicopters.

First the two astronauts had to persuade shuttle bosses to let them fly the MMU the way it was designed: untethered. Back in 1966, worried that Gene Cernan could end up a permanent Earth satellite, managers had insisted the AMU be operated with the astronaut attached to a 75-foot lifeline. Now, for the MMU, they were talking about a tether 300 or more feet. According to McCandless, NASA spent $3 million to develop what looked like an oversize fishing reel mounted on the front of a spacesuit: If the jetpack conked out, its pilot could simply crank a handle and pull himself back to the payload bay.

McCandless and Stewart wanted no part of it. Even if nothing went wrong with the MMU, they argued, the tether could easily get tangled around the shuttle’s high-gain antenna, or the robotic arm, or an astronaut’s foot. Or suppose the MMU did fail, and you tried to reel yourself back—how would you keep from colliding with the orbiter? Furthermore, they pointed out, if you had any sideways or up-and-down velocity to start with, you’d probably end up wrapped around the shuttle, with what McCandless wryly terms “undesirable consequences.” In the end, the astronauts managed to convince the brass that it would be safer to fly without a tether.
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Odp: [Air & Space Magazine] Untethered
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Untethered (2)

Bob Stewart hovers above Challenger's cargo bay during the MMU's trial runs. T-handle controls propelled the astronaut a few inches per second. (NASA)

The MMU development team found the managers’ anxiety understandable but misplaced. Each unit was protected against failures by near-total redundancy—two sets of thrusters, fuel tanks, regulators, valves, electronics, wiring, and batteries. McCandless says that when it came to possible malfunctions, “I can’t think of a single worst-case thing.” Even a stuck thruster, like the one that had sent Neil Armstrong’s and Dave Scott’s Gemini 8 spinning wildly through space in 1966, would hardly be a nightmare scenario; the pilot could simply turn off the bad thruster bank and use the backup set to stop any unwanted motion. In the unlikely event that both sets went haywire and had to be shut down, the astronaut would still not be stranded, because the MMU moved so slowly. The 1.4-pound thrusters were so gentle (astronaut Joe Allen, who used the jetpack on a later mission, likens their force to “a child blowing through a straw”) that an MMU flier would never outrun the shuttle, which could easily fly over and scoop him up in the cargo bay. In fact, before the test flight, mission commander Vance Brand practiced doing exactly that.

Still, even some insiders couldn’t help feeling nervous about what McCandless and Stewart were about to do. NASA’s Paul Bailey, an orbital mechanics specialist who helped train astronauts to fly the jetpack, felt confident in both Martin and in Ed Whitsett, but as the STS-41B launch approached, he had a recurring nightmare: He would be sitting on the shuttle’s tail section, with Earth filling the sky, just as McCandless and Stewart floated out of the airlock. “I was looking down the cargo bay over at the MMUs. And I thought, Holy mackerel, these people think I know what I’m talking about! And they’re going to die!” Waking up in a cold sweat, Bailey would grab his MMU reference book, go through it one more time, and reassure himself he hadn’t forgotten anything.

On February 7, 1984, Bailey and his colleagues were at their consoles in Houston as McCandless prepared to test the jetpack for real. After a few minutes working the controls in the confines of Challenger’s payload bay, the astronaut was satisfied that the MMU flew just as expected in nearly every way; the only surprise was a noticeable chugging sensation when the forward or backward translation thrusters were firing. The vibration turned out to be harmless and had a straightforward explanation: The thrusters were designed to fire through the jetpack’s center of gravity, which was offset slightly from that of the suited astronaut. As a result, each firing produced a small rotational force, which the MMU’s sensitive attitude-control system had to counteract.

Finally, with the world watching on live television, McCandless backed away from Challenger and into space. While his crewmates tracked him with the shuttle’s radar, McCandless measured distance with his own low-tech device: a metal rod notched to show the apparent size of the payload bay from different distances. After flying 150 feet away and returning, he ventured out to 320 feet, an orbital spacewalk record that still stands. He’d planned to take a moment during the journey to turn away from the shuttle and look out at the universe, savoring the experience of being a separate satellite, but he was so focused on reporting to his crewmates and mission control that he forgot.

The image of McCandless, small and alone against the black sky, struck an emotional chord with the public, and for the last 30 years McCandless has had to explain why, contrary to many people’s expectations, he wasn’t scared. “I still have a memory of comfort,” he says, based on his intimate familiarity with the device he’d helped to create.

Pinky Nelson tries docking to the Solar Max satellite during mission STS-41C. The MMU worked fine, but the capture didn't. (NASA)

McCandless remembers another feeling, though: cold. The shuttle suit’s cooling system was designed to keep astronauts comfortable even when they worked up a sweat. But flying the MMU was nearly effortless, and as McCandless glided through the void, away from the warm cocoon of the payload bay, “at one point I was shivering and my teeth were chattering,” he says.

The chill did nothing to dilute the satisfaction he felt at seeing the MMU perform as well as anyone had hoped. Bob Stewart offered his own assessment: The only way the jetpack could have been easier to fly, he says, would have been to “wire it directly to your brain.” And for Paul Bailey, the sight of McCandless and Stewart flying as no one had flown before was enough to end his recurring nightmare.

Two months later, on shuttle mission STS-41C, scientist-astronaut Pinky Nelson was at the controls of an MMU heading for the crippled Solar Max. In Denver, where Martin Marietta had built a huge simulator that moved on rails, Nelson had trained to fly between the solar panels of the slowly spinning satellite, then use a special capture device to grab onto a trunnion pin on the satellite’s side. Once firmly attached, Nelson was supposed to trigger the MMU’s attitude-hold feature to stabilize the satellite so it could be captured with Challenger’s robotic arm.

Things didn’t work out as planned, though—not because of any problem with the MMU, but because a small plastic nub near the trunnion pin, something that wasn’t in any reference drawing, confounded the capture device. After three unsuccessful tries, Nelson’s collisions with the satellite set it slowly tumbling. In a last-ditch effort, he grabbed the end of a solar panel and put the MMU into attitude hold, its thrusters firing vigorously to steady itself and the satellite. For a moment, Solar Max seemed to have slowed enough to be captured by the robotic arm. “I thought, We did it,” remembers Nelson. “And then I made a stupid mistake…which was to let go.”

Without the 830-pound mass of Nelson, the MMU, and the capture device attached, the satellite again began tumbling. Low on fuel, a frustrated Nelson returned to his crewmates, who all thought they’d lost their mission. But in a heroic effort, controllers at NASA’s Goddard Space Flight Center in Maryland were able to stabilize Solar Max remotely, and a couple of days later it was steady enough for Challenger’s arm to grab. Although the repair was carried out without any help from the MMU, Nelson had made a solo flight he’d never forget.

The jetpack did not get a chance to do what it was designed for until the following November, when STS-51A spacewalkers Joe Allen and Dale Gardner used it to retrieve a pair of errant communications satellites for return to Earth. To MMU insiders like NASA engineer Cliff Hess, it seemed as if the jetpack had finally established itself as an essential part of the shuttle’s toolkit. “We designed the MMU to be used on one flight every three years,” observes Hess, “and it wound up being used three times in one year, 1984. So we thought, Wow, this is great, we’re really moving.”

It didn’t work out that way. The MMU would never fly again—in large part because it was upstaged by the shuttle itself. The vehicle was so maneuverable that it proved simpler to just fly the orbiter over to an object or person and grab it, either with the robot arm or a gloved hand. This became clear even during the MMU’s inaugural mission. During one spacewalk when McCandless was not wearing the jetpack, he accidentally let a foot restraint float out of the cargo bay. Commander Brand told him and Bob Stewart to hang on, then steered the orbiter to within a couple of feet of the object so McCandless could retrieve it. After Pinky Nelson’s failed attempts to grapple Solar Max on the next flight, STS-41C commander Bob Crippen, using the shuttle’s thrusters in a special low-impact mode, was able to fly Challenger and its robotic arm right up to the satellite without disturbing it.

Following the 1986 Challenger disaster, safety regulations were instated that would have required the jetpack to undergo an expensive re-qualification; program managers, grappling with the cost of returning the shuttle to flight, were unwilling to spend the money.

When NASA was in the early stages of planning what would become the International Space Station, the question of the MMU’s utility came up again. Stewart was among its fans. “To be able to go out and fly around and repair the space station—gosh, if they had the MMU, this would be a piece of cake.” There was one drawback: If the jetpack failed, there would be no shuttle coming to the rescue. Stewart saw that as a low risk, but it worried others, and support for using the MMU on the station flagged.

Ed Whitsett had always hoped a fleet of astronauts wearing MMUs would assemble the space station. At the time of his death he had been working on a small maneuvering pack designed to attach to a spacesuit backpack for emergency use. In September 1994, testing the Simplified Aid For EVA Rescue (SAFER), STS-64 spacewalker Mark Lee became the last human to fly untethered through space.

Today, some EVA specialists advocate a next-generation MMU to help astronauts explore low-gravity worlds like asteroids or the moons of Mars, but so far NASA hasn’t approved development. The astronaut jetpack remains a capable—some would even say compelling—technology in search of a mission.

Source : Untethered

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Odp: [Air & Space Magazine] Untethered
« Odpowiedź #2 dnia: Listopad 03, 2019, 17:21 »
'To Face Their Wives': 30 Years Since First Untethered Spacewalk (Part 1)
By Ben Evans, on February 1st, 2014 [AS]

This iconic image, captured by astronaut Robert “Hoot” Gibson, shows Bruce McCandless participating in humanity’s first untethered EVA. McCandless’ historic spacewalk with the Manned Maneuvering Unit (MMU) occurred 30 years ago this week. Photo Credit: NASA

Thirty years ago this week, in February 1984, NASA launched its 10th shuttle mission, with a five-man crew and a relatively “vanilla” pair of commercial communications satellites to be deployed from Challenger. However, that was where the “ordinary” nature of Mission 41B—the first flight to be officially redesignated with a somewhat clumsy nomenclature of letters and numbers—ended, for in pride of place on the crew’s patch was a jet-propelled space suit backpack, known as the Manned Maneuvering Unit (MMU). It would be flown by astronauts Bruce McCandless and Bob Stewart, during which time they would become the first “untethered” spacewalkers in history. And McCandless’ foray would yield one of the most famous space photographs ever taken; a photograph which ended up on the covers of National Geographic and Aviation Week and which, even 30 years later, still retains pride of place on many desktop wallpapers, screensavers, and pieces of wall art. It even sparked a bizarre copyright case involving the singer Dido.

For McCandless, who joined NASA in April 1966, alongside Mission 41B’s commander Vance Brand, this momentous voyage into space would mark the end of a long wait. 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 tapped to test it on its first outing in space. “In retrospect,” he told this author in an email correspondence in March 2006, “I probably lavished too much attention on scientific and engineering interests, as opposed to the flying, flying and more flying.”

It is often said that great men come from great families with great and illustrious backgrounds, and McCandless, born in Boston, Mass., on 8 June 1937, certainly fulfilled much of this criteria. His great-great-grandfather, David Colbert McCanles, was a Nebraska rancher, a former sheriff, and, as a member of the “McCanles Gang,” was infamously killed by “Wild Bill” Hickok in December 1861. The family subsequently changed their name to “McCandless” and moved to Colorado. McCanles’ grandson, Byron McCandless, rose to become a Navy commodore who helped to create two separate designs for the Flag of the President of the United States. His son, the “first” Bruce McCandless, also followed a military career and graduated from the Naval Academy, but made his name and reputation during a bloody naval battle off Guadalcanal in the Solomon Islands.

Emblazoned with the name of the most-flown shuttle in the early 1980s (Challenger) and the names of the five crewmen of Mission 41B, the official patch also bore the Manned Maneuvering Unit (MMU) at right. On this flight, Bruce McCandless and Bob Stewart tested the jet-propelled backpack, ahead of its high-profile role in the Solar Max repair. Image Credit: NASA

In the early hours of 13 November 1942, Lieutenant-Commander Bruce McCandless was serving as communications officer aboard the U.S.S. San Francisco when the ship’s navigation bridge sustained a direct hit from the Japanese cruiser Nagara. The incident killed the admiral, his captain, and most of the other senior officers. McCandless himself was rendered unconscious and, upon reviving, he and another lieutenant-commander, Herbert Schonland, took command. Steering and control of the San Francisco were lost and regained on several occasions, and the ship endured 45 direct hits and sustained major structural damage, yet survived to fight another day. McCandless and Schonland both received the Congressional Medal of Honor, with citations which praised their heroism, leadership, and bravery. Both continued their naval careers and both retired as rear-admirals; McCandless would die in 1968, two years after NASA selected his son as an astronaut. Both Byron and the first Bruce McCandless would have streets at naval bases named in their honor, as well as a frigate, the U.S.S. McCandless, launched in 1971.

Bruce McCandless II was barely 5 years old when his father fought through the night to save himself and his shipmates in November 1942, but a naval career beckoned and a parent with a Medal of Honor virtually guaranteed him an appointment to a prestigious military academy. McCandless completed high school in Long Beach, Calif., and entered the Naval Academy, receiving his degree in 1958 and graduating second in his class of 899 students. (One of his contemporaries was John McCain, later a senator and Barack Obama’s Republican opponent in the 2008 presidential election.) At NASA, McCandless first entered the headlines in July 1969 as the Capcom in Mission Control when Neil Armstrong first set foot on the Moon.

McCandless was later assigned as backup pilot for the first Skylab mission and participated in the development of an astronaut maneuvering unit known as Experiment M-509. This nitrogen-fed backpack was tested inside the space station in 1973 and served as a forerunner of the MMU. In fact, the spectacular success of the MMU would win NASA and prime contractor Martin Marietta the Collier Trophy for 1984. McCandless, together with NASA’s Charles “Ed” Whitsett and Martin Marietta’s Walter “Bill” Bollendonk, were recognized for their contributions to its development and checkout. Whitsett in particular would pay tribute to McCandless: “Nobody has left his stamp on any instrument in space,” he told the Washington Post, “like Bruce has left his mark on the backpack.”

During Mission 41B, Bruce McCandless ventured as far as 300 feet (90 meters) from Challenger. Photo Credit: NASA

One of the MMU’s original aims was to enable spacewalkers to inspect and perhaps repair damaged thermal protection materials on the shuttle’s wings and lower surfaces. However, it had been hampered for some years by management apathy and lack of firm funding. That changed in early 1979, as Columbia was being moved from California to Florida. During the move, several heat-resistant tiles were lost from her airframe and renewed vigor was injected into developing the MMU. 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. It would instead be used, said NASA, for satellite repairs and maintenance, and its usefulness was enhanced by the provision of electrical sockets for tools, portable lights, and cameras.

The MMU measured 4 feet (1.2 meters) high, 2.6 feet (81 cm) wide, and 2.1 feet (66 cm) deep. According to astronaut Joe Allen, who flew it in November 1984, it resembled “some kind of overstuffed rocket chair.” On a typical mission, two MMUs were stored on Flight Support Structures (FSS) on opposing walls of the shuttle’s payload bay. An astronaut would back into it and secure two spring-loaded latches and a lap belt into place, then release the MMU 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 operational flight units, valued at around $10 million 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 308 pounds (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 a total of 39 pounds (18 kg) 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, backward, up, down, and from left to right. Furthermore, by using both in unison he could achieve very intricate movements.

McCandless checks out the control systems of the MMU, before departing the vicinity of Challenger’s payload bay on 7 February 1984. Photo Credit: NASA

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 MMU 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 tether. Of course, in the event of problems, most of its systems were redundant and neither McCandless or 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 Vance Brand.

As circumstances would transpire, the MMU trials would prove the singular greatest success of Mission 41B, as will be explored in tomorrow’s history article.


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Odp: [Air & Space Magazine] Untethered
« Odpowiedź #3 dnia: Listopad 03, 2019, 17:23 »
'Plenty of People to Talk to Me': 30 Years Since First Untethered Spacewalk (Part 2)
By Ben Evans, on February 2nd, 2014 [AS]

During his historic untethered EVA with the Manned Maneuvering Unit (MMU) on 7 February 1984, Bruce McCandless ventured as far as 300 feet (90 meters) from Challenger. Photo Credit: NASA

For Bruce McCandless, who backed himself into the MMU in Challenger’s payload bay, 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. It was familiar ground for McCandless, who admitted 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 had participated extensively in EVA simulations on Skylab, Solar Max, and the Hubble Space Telescope.

In spite of their complexity, McCandless and Bob Stewart’s EVAs 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 explained to this author in a March 2006 email correspondence, “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 a pressure increase in his suit’s sublimator. However, after being switched off and back on again, 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 Mission 41B’s commander, Vance 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 300 feet (90 meters) 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.

Also watching intently, camera in hand, was Robert “Hoot” Gibson, Challenger’s pilot, and the person who snapped the 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 explained. “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, Gibson began shooting frame after frame. Since Challenger’s orientation was some 30 degrees from the vertical, McCandless appeared at a similar angle with respect to Earth’s horizon.

Gibson knew that any one of his images could easily make the cover of Aviation Week (they actually made two) and remembered taking multiple light settings and tweaking the focus five or six times for each photograph, before squeezing the button. The famous shot would come to be known as “Backpacking,” and, even today, McCandless possesses a goofy version in his home, in which his grown daughter pokes her head through the cut-out visor in a life-size reproduction at a Seattle museum. In 2005, McCandless explained that what he liked most about the image was its lack of identity; with his sun visor closed, it is impossible to see his face, “and that means it could be anybody out there … sort of a representation, not of Bruce McCandless, but mankind.”

This assertion gained a measure of irony in September 2010, when he sued the singer Dido for her unauthorized use of the image on the cover of her album, Safe Trip Home. Although NASA images are not bound by copyright, McCandless argued that the image infringed his “persona,” but the case was settled amicably early the following year.

Four days into Mission 41B, flying the MMU carried the experience of spaceflight to an even higher level; in fact, many spacewalkers have agreed that the sensation of floating above Earth with nothing between themselves and the fathomless expanse of the Universe is a decidedly ethereal one. During their tethered work in the payload bay, McCandless and Bob Stewart removed a failed television camera for replacement with an in-cabin unit and later installed it during their second EVA on 9 February. The MMU performed admirably, but ironically it was Brand who undermined its raison d’etre. The backpack had long been touted as being capable of more precise and intricate movements than the shuttle, but on Missions 41B and 41C the value of Challenger’s maneuverability and her Canadian-built Remote Manipulator System (RMS) mechanical arm were demonstrated … by retrieving a lost foot restraint.

“I don’t recall now whether it was before or after he went out with the backpack,” Vance Brand explained, years later, “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.”

Both MMU evaluations, read Martin Marietta’s post-mission report, “performed as expected and no anomalies were reported.” Overall, McCandless flew the MMU for 3.5 hours and Stewart for just under two hours. Yet, as has been noted, it was Challenger’s own maneuverability, demonstrated by Brand, which rendered its future much less certain. “We used the autopilot a lot,” Brand said later. “We had the capability to maneuver the ship in rotation with a hand controller, but more often than not, we just punched something into the computer and set up the digital autopilot such that we got an automatic maneuver. That saved fuel, as we could move at very slow rates. We tested the [Reaction Control System] jets on orbit for translation up or down, sideways or forward and back. On the night side of the Earth, when we translated the ship down, the upward-firing RCS jets were used to do that. At night, it looked like a Fourth of July display because you could look out over the nose and you could see these tubes of fire going up. They were fantastic visual effects.”

Following McCandless’ flight, Bob Stewart also evaluated the MMU’s performance, preparatory to the long-awaited repair of Solar Max in April 1984. Photo Credit: NASA

By so doing, Brand showed that the shuttle was capable of the same intricate motions as the MMU, and, on the next flight, Mission 41C in April, when a task involving the MMU was frustrated, the RMS would prove highly capable. Despite the MMU’s success during two satellite recoveries in November 1984, the superb maneuverability of the shuttle contributed to its ultimate demise. In fact, the year immortalized 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 for a total of just 10.5 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 two flight units were mothballed.

More tellingly, MMU veteran George “Pinky” Nelson doubted, even in the heady days before January 1986, that the backpack would have flown again, except “maybe for a vehicle-to-vehicle rescue in a Columbia-like scenario, but not for any operations that were envisaged in the pre-Challenger program.” Like McCandless, he stressed that it was “well conceived and engineered but, unfortunately, the planned uses of the MMU were superseded by other capabilities that we developed, but couldn’t anticipate.”

Finally, when the Rogers presidential inquiry into Challenger’s loss presented its findings, renewed emphasis was imposed on increasing the safety of other shuttle components. “The cost of recertification,” recalled McCandless, “eventually killed it off. In the fall of 1989, there was an effort to fly the MMU again. A proposal was solicited from Martin Marietta for recertification and refurbishment for one Shuttle mission. It came in at $6.1 million, which was deemed too expensive, and despite some small sums for clean room environmental storage, just in case, they were eventually retired.” Today, the MMU flight unit first used by McCandless hangs in the Smithsonian. The other backpack was loaned to NASA’s Marshall Space Flight Center for possible future use as a “flying testbed” for autonomous rendezvous and docking systems, “subject,” said McCandless, “to the constraint that it be maintained in a condition that could be restored to flight configuration.” Both units, therefore, were mothballed until such time as their unique capabilities were needed again.

They never were.

A device known as the Simplified Aid for EVA Rescue (SAFER) was developed for the International Space Station (ISS), and the only other remotely viable role for the MMU was to assist with repairing the Hubble Space Telescope. However, this was dismissed, due to fears that plume impingement from its nitrogen-gas thrusters could damage the telescope’s optics.