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Wear may have made shuttle's wing weak

17 March 2003

Columbia accident investigators are focusing more attention on the possibility that the protective leading edge of the spaceship's left wing may have been weakened by wear before the flight, making it more vulnerable when debris hit the wing on takeoff or possibly in space.

The scrutiny is on the tough, heat-resistant material that protects the edge from the searing heat of re-entry. Columbia broke apart Feb. 1, killing all seven astronauts. The left wing has been the focal point since then because a piece of foam insulation broke off the external fuel tank and bashed into the wing during takeoff Jan. 16.

Most public attention has swirled around possible damage to the tiles that form the heat shield on much of the wing. But questions have turned to the leading edge:

- A new analysis shows that the insulation struck the leading edge on the underside of the wing.
- Ongoing tests in a NASA wind tunnel show that the forces pulling the shuttle's nose to the left during re-entry were the result of a major distortion of the left wing, such as the loss of four or more of the 22 panels that make up the leading edge.
- Tiny pinholes can form in the material that protects the leading edge. Those pinholes can lead to small cavities hidden beneath the surface.
- Members of the Columbia Accident Investigation Board are examining whether pre-flight inspections of the panels are sensitive enough to detect hidden damage.

The leading edge is one of the parts of the shuttle that heats up the most when the spacecraft re-enters the Earth's atmosphere. To protect it, engineers needed something more resilient than the tile that protects much of the shuttle.

They chose reinforced carbon-carbon. This mixture of carbon fibers and carbon filler can heat to more than 2,800 degrees without melting.

Just behind the panels are the wing spars. They are aluminum, which melts at about 1,200 degrees. A failure of the panels could be catastrophic.

The carbon-carbon material can burn fiercely if exposed to oxygen during the heat of re-entry. To prevent that, a coating is baked on to it.

After numerous re-entries, tiny pinholes can form in the coating. Just enough oxygen flows through the pinholes to burn or melt away bits of the carbon-carbon. Tiny cavities are formed. The cavities themselves are probably no danger, says Robert Rapp a retired Ohio State University researcher who studies high-temperature materials and helped NASA scientists with pinhole research. But if something heavy hit a panel speckled with pinholes, ''that could be a different story,'' Rapp says. It ''could cause a fracture in the coating. If you lost a small amount of coating, you'd be in serious trouble.''

Besides the pinholes, more significant ''dings'' have been recorded in the leading edge six times in the past 12 years, according to the accident investigation board. Some of those dings came from collisions with micrometeoroids, tiny rocks in space.

Studies show that a hit from even a tiny bit of space debris could create an inch-wide hole. Board chairman Harold Gehman has said a pinhole alone would be ''highly unlikely'' to lead to the loss of a panel.

But, he said, debris striking panels that have pinholes and dings could ''do some damage.'' The carbon-carbon panels are tough, so NASA engineers are shocked that the investigation board is looking at them as a potential culprit.

But outside engineers say that if something hard struck, a panel could be shaken loose. ''If you hit it with a hammer, you certainly have the potential to break it,'' says a reinforced carbon-carbon specialist who asked not to be named. ''The foam (could have) hit that at high speed. That could be a problem, too.''