Technology
Space shuttle tiles: a question of bonding Silica tiles that protect shuttle from re-entry heat aren't falling off, but both NASA, Rockwell are working hard to make sure they won't Mitch Waldrop C&EN, West Coast
The rumors have been flying for a year now: that the space shuttle's thermal protection tiles—the heat shields that will save it from burning up during re-entry into the Earth's atmosphere—keep falling off; that they won't stick on properly; that they are cracking; that observers both inside and outside the National Aeronautics & Space Administration are seriously worried that the space shuttle, already years behind, schedule and billions of dollars over budget, is a lemon. Well, maybe. But that's not quite how they tell it at Rockwell International, the prime contractor for the space shuttle. It's true enough that the tiles, along with the shuttle's giltch-prone main engines, are the principal reasons for the current round of delays in the shuttle's first launch, says Richard Barton, spokesman for Rockwell's Space Systems Group in Downey, Calif. (The first flight is now scheduled for no earlier than Nov. 30, and may even slide into early 1981.) But the heat shield has been considered one of the critical technologies from the beginning. The shuttle is the first space vehicle ever flown with a re-usable thermal protection system. The problems, he says, are no more than one would expect. "It's a new technology," Barton insists. "We're moving along a learning curve." That may well be true, but in the process, Rockwell and NASA have had a traumatic education. For more than a year now the space shuttle program has been buffeted by both technical and political embarrassments. In March 1979, for example, orbiter 102, the Columbia, which will be the
first shuttle orbiter to fly in space, was to be moved atop its Boeing 747 transporter aircraft from Rockwell's Palmdale, Calif., production facility to the Kennedy Space Center in Florida. But since only 20,000 of its full complement of 30,000 tiles had been mounted at that point, some of the critical gaps were filled with ersatz tiles of polyurethane to prevent aerodynamic damage to the real tiles. Tape was placed over some of the remaining gaps for waterproofing. "It looked just awful," Barton recalls. Indeed, in photos taken at that time, such as one recently published in Newsweek, Columbia looks like a refugee from a war zone. The famous and photogenic Enterprise, by contrast, is all ersatz tile. The vessel is purely a test vehicle, and will never fly in space. The 747-transported Columbia's takeoff was a fiasco. As the 747 was lifting off, a strip of the waterproofing tape pulled loose from the orbiter and took several ersatz tiles with it. The flight was aborted immediately. After landing it was found that a few of the real tiles, no more than a dozen, had been damaged as the tape flapped around in the slipstream. Strangely enough, no one could recall who had ordered the tape put on in the first place. Actually the problem was minor, and some time later the transfer was completed successfully—this time without tape and with the ersatz tiles
mounted more securely. But the damage was already done. Rockwell's public relations officers, who failed to make clear to reporters the difference between ersatz and real tiles, had unintentionally fostered the rumor that the shuttle's tiles were falling off. But trivial as the incident was from an engineering point of view, it did serve to focus NASA's attention on the tiles' behavior as a mechanical system, as opposed to a purely thermal system. The tiles themselves are lightweight, fragile blocks of sintered silica fibers, with a bulk tensile strength of only about 13 psi. Because of this fragility they aren't bonded to the aluminum skin of the orbiter directly. Rather, they are isolated from stresses and vibration by a thin, feltlike "strain isolator pad" (SIP) made of Du Pont's Nomex fiber, a hightemperature nylon material. The bond is made with a standard roomtemperature vulcanizing adhesive. Unfortunately, the bonding wasn't everything Rockwell had thought it was. Even before the ferry flight incident, pull tests were showing that many of the tiles broke away from the adhesive line at only about 60% of their textbook tensile strength. This had been of no particular concern in the early days of testing, when the design load for the tiles was only 2 psi. During the spring and summer of 1979, however, improved
NASA technician applies tiles to Columbia orbiter 102's body flap May 12, 1980 C&EN
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structural simulations of the orbiter began to indicate that some of the tiles were going to have to withstand stresses considerably higher than that. If the tiles were, in fact, bonded as weakly as the pull tests indicated, some of them might break loose, threatening the orbiter and crew with immolation during re-entry. Faced with this unhappy prospect, NASA decided to take no chances. The agency decreed that every single one of the Columbia's 30,000-odd tiles be pull-tested, by hand, to 1.25 of its new design load. "Negative-margin" tiles that didn't meet this criterion were to be removed, their inner surfaces "densified" and strengthened with a paint-on slurry of colloidal silica, and remounted on the spacecraft. The problem, it turned out, was practically trivial—or would have been if NASA had known about it ahead of time. The SIP pads are made of two or more battings of horizontal fibers, sewn together with vertical strands. When the tile is pulled outward, these vertical fibers snap taut and create point loads. The densification process, invented at Rockwell, solves the problem by stiffening the tile's inner surface and distributing the load. So now at Kennedy Space Center one group of Rockwell workers is busily mounting tiles on the Columbia, while another group is trying to pull them off. It's embarrassing for NASA, and the press has made much of it. Embarrassment aside, the whole process of mounting and testing the tiles is tedious, painstaking work. Each tile has to be positioned on top of a piece of spongy felt to tolerances as small as 17 mils. And the tiles are so fragile that an accidental tap with a wrench, a hard hat, or even a key chain can crack the glassy surface— which means that the damaged tile has to be removed and the process started again with a new one. The tile situation has come to its current pass in part because of something called "concurrent development." If you wait until all the research and development is finished to begin production, you'll never get a space shuttle, explains Rockwell's Barton. "Concurrent development" is a design strategy that started with the Air Force in about 1960. "It's a gamble, true," he says, "but, it's been the experience of the industry that you can do some development along with production." Perhaps the tiles shouldn't have been classified that way, but Rockwell was convinced of the thermal characteristics of the tiles, he says. The company just didn't have the final 28
C&EN May 12, 1980
Tile problems are in the bonding to the orbiter Coating ilfiPfiflil Tile
Adhesive y Aluminum skin of orbiter
Strain isolator pad (SIP)
word on the structural characteristics. And that's why 20,000 tiles were in place before Rockwell did get that final, unsettling word. But the shuttle program's real problem, all along, has been money—or rather, the lack of it. The full extent of this problem came to public attention in the spring of 1979, about the same time as the ferry flight fiasco. In March, NASA administrators found themselves facing a pileup of substantial cost overruns. Their decision was to handle the problem much as they had in the past: Borrow money from other agency projects and slide the work schedule. This time, however, the Department of Defense balked. DOD had put up half the money for the space shuttle, it needed the space shuttle to launch its new spy satellites, and it needed the space shuttle on schedule. So NASA was forced to confess to Congress that the shuttle was not really on schedule and ask for a supplemental appropriation of $220 million to its fiscal year 1980 budget. The supplement was passed, but some committee chairmen began asking very pointed questions about NASA's management of the shuttle program. The space agency responded by naming a five-man in-house investigating committee, which made its report on Sept. 12, 1979. Overall, the committee said, the space shuttle program has done a very good job from a technical standpoint. But in an attempt to live within the original $5.15 billion (1971 dollars) estimate for shuttle development, rather than ask Congress for more money to make up for inflation and cost overruns, shuttle managers have set a very austere fiscal environment. And in the process they have consistently set up unrealistic work schedules, demanding more performance than can be delivered. Thus, work
planned for one year has been deferred until the next year—a practice especially hard on subcontractors, who have had to lay off experienced personnel when funding was postponed, then start up again with inexperienced workers. These funding deferments were one reason that the updated stress analysis of the tiles came so late, says Barton. That's also why the Columbia was being flown from Palmdale to the Kennedy Space Center before it was finished. Rockwell's 1100-member ground crew there, the group that would refurbish the shuttle during its regular flights, was already trained and waiting. Rather than leave them idle, the company decided to let them finish the Columbia. Given all the problems with the tiles, people inevitably are asking why NASA chose such a fragile, complex thermal protection system in the first place. And the tiles are fragile. They can be flaked apart with a fingernail. To get an idea of what a typical shuttle tile is like, take a 6-inch by 6-inch block of styrofoam, about 2 inches thick. Round off the edges slightly. Then, with model airplane paint, color the top and the four sides a glossy dark gray. The final product, in heft, texture, and general appearance, will be very much like a shuttle tile. The tiles just don't look high technology. They look more like old chunks of packing material. But every piece of that "packing material" costs NASA some $500. The tiles actually cover only about 70% of the orbiter, including the bottom, the vertical stabilizer, and part of the sides. Much of the upper half of the vessel, where re-entry temperatures will stay below 700° F, is covered with white Nomex felt; the nose and wing leading edges, where temperatures will reach 3000° F, are protected with reinforced carboncarbon. A total of some 20 different thermal protection materials are used at various places on the orbiter—the shuttle's wing, after all, has to do everything a 747's does, notes an engineer, and every bit of it has to be protected somehow. But it's the tiles that are straining the state of the art. There was a certain inexorable logic to this form of thermal protection, a Rockwell engineer recently explained to C&EN. (The engineer spoke freely, but because NASA and Rockwell are rather sensitive on the subject these days, he asked not to be quoted by name.) Along the underside of the orbiter, the temperatures during re-entry will reach some 2300° F, he says. To ensure that the thermal protection material doesn't oxidize, make it out of
oxide. To ensure that it doesn't crack perature areas receive a coating of escape from the porous tile as the under thermal stress (the tempera black, reaction-cured borosilicate shuttle ascends into the vacuum of tures range from 2300° F to 350° F at glass. The black color allows these space. The tiles' glass coating stops the orbiter's aluminum skin a few tiles to radiate away re-entry heat as just above the bond line to leave an escape route. inches away) use something with the fast as possible. Leaving gaps in the thermal pro lowest thermal coefficient possible. On the shuttle's upper surfaces the Those conditions led to the choice glass coating is white with aluminum tection system creates its own prob of pure amorphous silica, says the oxide. These surfaces will be kept lems, however. For example, at the Rockwell engineer. It has a thermal facing the sun while the shuttle is in bottom of the gaps, the orbiter's alu expansion coefficient of 3 Χ 1 0 - 7 per orbit. The white color will help keep minum skin must be insulated with an additional strip of Nomex felt, a degree Fahrenheit, the lowest the vehicle cool. known. Finally, the tile coating functions, "filler bar." On the other hand, as Now, don't make the thermal pro disconcertingly enough, as a water long as the "step error" between tiles tection out of solid silica because it proofing agent. The tiles are 93% dead is less than about 17 mils there will be weighs 140 lb per cu ft, he continues. space. The shuttle will be spending a relatively little problem with heated The whole idea of the shuttle is to get lot of time on a launch pad in Florida. air being forced down the gaps. Along some cargo off the ground. So you One sudden rainstorm and an un the forward part of the orbiter, the don't want it heavy, and you also protected set of tiles could get very, airflow will be smooth and laminar. don't want it to conduct heat. Both of very heavy. For added protection, Toward the rear, however, beyond the these requirements mean cutting Lockheed sprays the finished tiles transition to turbulent flow, the gaps with a silicone waterproofing agent, must be filled with flexible silica down the density. The technology to accomplish this and once the shuttle begins routine cloth. On orbiter 099, the Challenger, the grew out of a research program at flights, the orbiter will get a water Lockheed dating back to 1961. The proofing "bath" to replace burned- second orbiter scheduled to fly, the basic idea is to form thin fibers of away silicone almost as soon as it pure-silica tiles along the bottom surface will be replaced with a new silica into a light, open matrix that is lands. Since the orbiter's frame will be lighter, stronger type of tile. Called mostly dead space. At Lockheed's Sunnyvale, Calif., flexing and vibrating in flight, and fibrous refractory composite insula production facility, where virtually all since the aluminum of the orbiter's tion (FRCI), the material is made the the tiles are made, the silica fiber as skin has a thermal coefficient 40 same way as are the silica tiles, but received from Johns-Manville Co. times larger than that of the tiles, it is with 20% of the silica replaced with looks and feels like a mass of snow- necessary to leave gaps as large as γ^ aluminoborosilicate fibers that act white felt. The individual filaments inch between the tiles. These gaps like preshrunk concrete reinforcing D are between 2 and 4 micrometers also will allow air and water vapor to bars in the fiber matrix. across. The fibers are cleaned of the last bit of manufacturing residue, mixed with water to form a slurry, and pressed into wet blocks. After a colloidal silica binder solution is added, these blocks are dried and sintered at 2500° F. It is important that the silica be at least 99.56% pure, lest it revert to a crystalline form at this stage. When the block comes out of the sintering oven, says the Rockwell engineer, it is as much as 93% air. In 2 its lightest form it has a density of 9 lb per cu ft. Blocks of this material, sawed into quarters, are used to make tiles for the largest portion of the or biter's surface. A heavier and stronger form, 22 lb per cu ft, is used in areas that must withstand higher temper atures or mechanical stresses—the landing gear doors, for example, or the vertical stabilizer leading edge. After the tiles are cut to size, the I outer surfaces and four sides are coated with a layer of silica frit, baked on in 15-mil thicknesses. Only the side that will face inward to be bonded to the SIP is left free. In part, the coating protects the body of the Ethylenimine [El] is a molecular building block that re tile from handling damage and from acts with Acids, Acyl Chlorides, Isocyanates, Esters, aerodynamic abrasion during re Primary and Secondary Amines, Hydroxy Compounds, entry. Thiols, Double Bonds, Epoxides and Episulfides. By far the most important purpose For specific ways in which Aziridine chemistry can help of the coating, however, is that it al you, please contact Dr. David Roark, Technical Services, lows the tiles to absorb and radiate Cordova Chemical Company, Box 13400, Sac-éTjK heat properly. Tiles on the underside of the orbiter and in other high-tem ramento, California 95813, (916] 355-5000. \ j f
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May 12, 1980C&EN
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