NASA report warned debris could damage tiles

SPACE CENTER, Houston -- A technical report warned at least nine years ago that space shuttles could be destroyed if tiles protecting critical wing parts were damaged by debris, but NASA engineers never found a complete solution for the safety soft spot.\nNow the failure of the tiles is a leading theory for the catastrophic end of Columbia.\nNASA struggled for years to ensure that the tiles were firmly attached to the shuttle, Paul Fischbeck, an engineering professor at Carnegie Mellon University, said in his analysis.\nHe said Tuesday that NASA engineers "took a lot of our advice to heart" and made changes to lower the risk of debris hitting the tiles during launch. But the problems were never completely solved, he said.\nA patch of foam insulation breaking off from the shuttle's external fuel tank during launch and striking tiles on the underside of the left wing is being studied as the possible cause of Columbia's destruction Saturday, which left all seven astronauts dead.\n"There are very important tiles under there. If you lose the tiles on those stretches ... it can cause the shuttle to be lost," Fischbeck said.\nA NASA spokesman said Tuesday that nobody was available to comment on the report.\nMeanwhile, the search for bits and pieces of the shattered Columbia was expanded westward to California and Arizona, where teams are checking reports of debris. That material could provide clues to the earliest stages of Columbia's disintegration.\nInvestigators also are examining military photos taken from an Apache helicopter of Columbia's final fiery descent.\nFischbeck and his colleagues made an initial report to NASA on their findings in 1990, and they later published follow-up papers on the research.\nThey conducted a risk analysis of the shuttle's thermal protection tile system and found that the spacecraft was highly vulnerable to tiles being knocked off or broken by insulation falling from the fuel tank and from other debris.\nIn a follow-up to the report, Fischbeck said he studied debris strikes during the first 50 shuttle launches and concluded that about 25 thermal tiles per flight sustained damage of at least one inch.\nThe analysis found that the most vulnerable parts of the shuttle were the undersides of the wings close to the fuselage and right under the crew compartment.\nNASA experts said that data from Columbia shows a sudden temperature rise -- a marker for failed tiles -- in the left wheel well, an area Fischbeck's report said was a critical risk.\nFischbeck's report said that a key problem faced by NASA was training technicians to glue tiles on the hull of the space shuttle and then test the strength of the bonding.\nAn adhesive used for the tiles hardened more quickly if it was wet, and the report said NASA found one technician helping the process along by spitting into the glue. The wetting, however, compromised the bond.\nTo find loose tiles, workers conducted a pull test, using a special machine, but the study found this technique missed some problems. The best method was a "wiggle test" that only experienced technicians learned to do, the study found.\nLoose tiles are more easily knocked off by falling debris.\nInvestigators searching for clues to Columbia's loss are focusing on a 2 1/2-pound, 20-inch chunk of foam insulation that fell from the shuttle's external tank moments after liftoff and stuck the underside of the wing, possibly damaging the tiles. The shuttle was traveling at 2 1/2 times the speed of sound at the time, or just over 1,900 mph.\nMichael Kostelnik, NASA's deputy associate administrator for spaceflight, said that foam insulation has peeled off during earlier launches, but none was the size of the chunk that went sailing off Columbia's fuel tank.\nFischbeck said NASA has made improvements in protecting and maintaining the tile system since his study. Foam insulation on the fuel tank has been changed and there are stricter limits on the amount of ice allowed on the fuel tank before launch. Ice forms from the chill of the liquid hydrogen and oxygen propellants stored in the tank.