Learning to Take the Heat: Insulation for the Space Shuttle

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The year 1979 was the planned launch year for the first shuttle prototype vehicle, Columbia. During that year, the vehicle shed some 40% of its "critical" ceramic thermal insulation in a flight riding piggy-back on a Boeing 747 from California to Cape Kennedy.

The environment of that flight was very benign compared to the mission ascent and reentry phases that the space vehicle would have to face. During reentry through the earth's atmosphere, dramatically high temperatures develop over the surface of the shuttle, ranging from 2300° F over large areas of the underbody to 2600° F on the nose and wing leading edges.

The failure of the shuttle tiles created a crisis period for the space program that lasted a full two years while solutions to the problem were pursued. UW engineering professor James Mueller and his research collaborators, along with undergraduate and graduate students, played a key role in the final solution to the tile problem, developing superior insulation material as well as methods of its attachment to the shuttle.

Mueller, who headed the division of ceramic engineering in the Department of Material Sciences of the UW College of Engineering, had been working for several years with his faculty colleagues on ceramic heat exchangers, and had expanded the activity to a multidisciplinary research program in high-temperature ceramics (see NASA Program in Ceramic Research).

In early design studies, NASA had determined that the thermal insulation for the all-aluminum shuttle should be made of silica--a ceramic material. Silica insulation for critical high-temperature use is constructed from very pure and fine silica fibers sintered together, bonded at high temperature, creating over a million temperature-welded joints per 1 cubic inch of material. The sintered silica is shaped into 6-inch by 6-inch by 3.5-inch blocks, covered with a black silica glass coating, and attached to the shuttle surface by a nylon felt pad, approximately one quarter inch thick, known as the strain isolation pad (SIP).

This pad isolates the brittle and weak silica block from the aluminum surface, which expands much more than the fragile tile, thereby causing the tile to crack and fail if attached directly. The tile base is glued to the felt pad (SIP) with a rubber bond, and the same "glue" is used to attach the SIP to the aluminum surface. The 1979 failure of the thermal shield was the loss of many of the "critical" tiles--some 5,000 tiles (of a total of 28,000 on the shuttle)--so critical that loss of even one would make reentry non-survivable.

When the tile failure occurred, NASA created a "crisis" committee--in this case a committee of 12 scientists and engineers from outside NASA--to investigate the cause of the tile surface failure and to propose "fixes." Mueller and UW professor of aeronautics and astronautics John Bollard were appointed to that committee, Mueller because of his background in ceramics and involvement in the choice of the fibrous tile, and Bollard because of his background in aeroelasticity and structural mechanics.

This committee met bi-weekly for the next two years, conducting research, preparing analytical models, and proposing test protocols to NASA. In the early days of the committee, Mueller and Bollard proposed an expansion of the ceramics design group activities to include assessment of tile failure mechanisms and studies of engineering remedies. NASA not only funded the expansion but also provided equipment and supplies to facilitate the effort, which involved about a dozen faculty and some 20 students at any one time. Mueller and Bollard managed the program while maintaining "a grueling pace" of crisis committee meetings, inspections of the shuttle tiles on the Columbia, inspection of tests, installations, and histories of tile manufacturing, analysis, and installation.

The two-year, sometimes frenetic research at the UW led to two very significant successes, says Bollard. First of all, the researchers discovered the fundamental initial cause of tile attachment failure and the resulting mechanics of detachment of the tiles from the SIP. Furthermore, they developed engineering solutions for the problem that were subsequently adopted in practice: first of all, strengthening the tile material itself, and secondly, toughening the base of the tile to provide stronger load paths from the tile bottom surface through the SIP to the surface of the vehicle.

"This effort required many hours of research and testing of alternative and then optimal systems," says Bollard. "All who participated, faculty and students alike, were highly motivated and worked steadily and well beyond normal hours to help solve this pressing national problem. The pressures were at times enormous but the real-world environment provided, in retrospect, a marvelous and rewarding period for all of us, especially the students. In fact, we were very proud that it was an undergraduate student, Richard Pfaff, from Forks, Washington, who, by a very simple but very inventive experiment, guided the program to the realization and proof of the initial causes of the tile bond failures."

The success of this "crisis" program at the UW resulted in special commendation from the Washington State Legislature and the Governor of Washington, from NASA, and from the UW. The success of the "crisis" committee of 12 was recognized by NASA with citations to its members and a Group Achievement Award to Mueller and Bollard.

At the first flight of the refurbished space shuttle Columbia on April 21, 1981, Mueller and Bollard were participants in the flight readiness decision-making process, right up to the time of lift-off. After that historic event, the UW ceramics design-group activity continued for several years—during the first five shuttle missions—to carry out research on improved tile materials and attachment systems, with NASA support. In addition to Mueller and Bollard, participating UW faculty included Raymond Taggart, Ashley Emery, Albert Kobayashi, and Howard Merchant, from mechanical engineering; Billy Hartz, from civil engineering; and William D. Scott, Alan D. Miller, and O. J. ("Whit") Whittemore from the ceramics division of material sciences.

  1. Information provided by Professor Bollard is gratefully acknowledged.

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