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Keeping the heat outside the shuttle
By Gareth Cook, Globe Staff, 2/4/2003
Even before Saturday's accident, NASA has been looking for better designs, including using new types of tiles developed since the program began. NASA and other researchers also are developing other new materials and concepts, which could make the shuttle program safer, and perhaps help engineers in designing a new generation of space planes. ''[Current] tile materials are not as high tech as what can be done nowadays,'' said Doug Perovic, chairman of the department of materials science and engineering at the University of Toronto.
When the crew of the space shuttle Columbia turned their craft toward the Earth, leaving orbit at 25 times the speed of sound for the searing bite of the Earth's atmosphere, it was an act of faith. There is no way to return from space without the burn of reentry. A system of roughly 24,000 tiles, designed to take the heat without melting the aluminum body beneath, stood between them and the fire. ''It is one of the most spectacular light shows I have ever seen,'' said Jeffrey A. Hoffman, who flew on five shuttle missions and is now on leave from NASA as a professor of aeronautics and astronautics at MIT. ''You are basically protected by this eggshell.'' Called the ''thermal protection system,'' the Columbia's eggshell appears to have cracked. Possibly damaged on liftoff, the left wing started dragging through the atmosphere during the hottest portion of reentry -- indicating a breach -- as a web of sensors in the wing indicated heating problems before ground controllers lost contact. Speculation has centered on the tiles, and how they would perform if damaged. The tile's engineers have to make trade-offs between toughness and the ability to resist heating -- all while keeping the weight down. The tiles are made of a silica, the same substance used to make glass, but manufactured as a network of tiny fibers with many tiny air pockets, making them very light. A person could break the material in his or her hands, but it does very well under heating. Instead of melting, the tiles glow, radiating the heat back into the environment. And they are very good insulators. Even if a tile were red hot in the center, a person could hold it in his or her bare hands, according to NASA. The tiles are coated with a harder, glassy substance that protects them from impacts. Then they are glued to an insulating felt that is bonded to the shuttle's body. While it is technically a ceramic, this tile wouldn't fracture the way a bathroom tile does if dropped. Instead, scientists say, an impact would scrape the tile or possibly remove it entirely. Small pieces of damage to the tiles had become a routine part of shuttle flights. But if the damage were large enough, scientists say a vicious cycle could have ensued. As the hot plasma of reentry skimmed over the wing, a rough, damaged area would have been heated up more. That could have caused more damage, and the problem escalates. ''It can be a self-propagating type of thing,'' said Graham Candler, who does reentry research for NASA and is a professor of aerospace engineering and mechanics at the University of Minnesota. NASA has tried to make the tiles tougher by coating them with a harder glassy substance. And their engineers have developed a new kind of tile, called fibrous refractor composite insulation, or FRCI, tiles, which add another fiber to the silica to make it lighter, without losing its strength and heat resistance. At the NASA Ames Research Center, engineers have been working on even more advanced tiles that incorporate other materials. They also have been experimenting with a class of materials called ''ultrahigh temperature ceramics,'' which can withstand substantially higher temperatures but are generally too heavy to cover an entire craft. These substances move further away from the basic idea of a silica-based ceramic by adding in other elements. At another NASA facility, the Langley Research Center in Hampton, Va., researchers have built prototypes of metallic thermal protection systems, a system of metal panels sandwiched between other insulators. Other researchers are working on improving manufacturing techniques so that carbon composites, forms of which are used in the leading edge of the wings of the shuttle, can be used on more of the craft. But the tiles in use today are only just good enough to handle the heat, engineers said. The entire craft is designed to minimize the temperatures on the tiles and this, they said, is why the shuttle flies more like a brick with stabilizing wings than an airplane. During hypersonic flight, a searing shock wave forms in front of a craft, Candler said. With a blunt nose and design, a craft is less aerodynamic, but it keeps this shock wave farther in front of it, reducing the amount of heat transferred to the craft. More temperature-resistant materials would allow engineers to design a more aerodynamic space plane, Candler said. This could make liftoffs safer, too, because the plane would not cause as much drag on the way up, and, in case of trouble, the plane could separate from the boosters and fly back. The future also might hold another possibility: Place sensors in the heat-resistant tiles themselves. Some bridges are built now with sensors that can report structural problems, said Subra Suresh, the Ford Professor of Engineering and head of the department of materials science and engineering at MIT. The same thing, in theory, could be done with the tiles, so that if any are damaged, the pilot would know at once. ''It would be much better to know immediately when it happens,'' Suresh said, ''and not have to wait until reentry.'' Gareth Cook can be reached at cook@globe.com.
This story ran on page C4 of the Boston Globe on 2/4/2003.
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© Copyright 2003 New York Times Company |
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s more details emerge in the shuttle disaster, scientists say they expect NASA will thoroughly reconsider the way it protects its shuttle fleet from heat.
