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1 hour, 30 minutes ago
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By PAUL RECER, AP Science Writer
SPACE CENTER, Houston - NASA (news - web sites) on Wednesday defended the thermal tile system that has come under suspicion in the space shuttle Columbia disaster, saying it has worked well for many flights.
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The space agency was warned in a technical report at least nine years ago that the heat-protection tiles on the undersides of the shuttle's wings were susceptible to damage that could destroy the spacecraft.
"We've had a lot of experience with that thermal protection system," said Mike Kostelnik, a top spaceflight official. "We've had good success with the tile system."
He acknowledged, however, that there are key sites on the underside of the wing, such as the covers of the landing gear compartment, that make the craft vulnerable during re-entry into Earth's atmosphere.
"They would be source of problems if they are not properly sealed," Kostelnik said.
A technical report published in 1994 warned the space shuttle could be destroyed if tiles protecting critical wing parts were damaged by debris during liftoff, but NASA engineers never found a complete solution for the safety soft spot.
Now the failure of the tiles is a leading theory for the catastrophic end of Columbia.
NASA struggled for years in trying to ensure that the tiles were firmly attached to the shuttle, Paul Fischbeck, an engineering professor at Carnegie Mellon University, said in his analysis.
He 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.
A 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.
"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.
Meanwhile, the search for bits and pieces of the shattered Columbia was expanded westward to California and Arizona, where teams were checking reports of debris. That material could provide clues to the earliest stages of Columbia's disintegration.
Investigators also were examining military photos taken from an Apache helicopter of Columbia's fiery final descent.
Fischbeck and his colleagues made an initial report to NASA in 1990 and published a follow-up paper in 1994.
They 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.
In the follow-up paper, Fischbeck said he studied debris strikes during the first 50 shuttle launches and concluded that about 25 thermal tiles per flight sustained at least one inch of damage.
The analysis found that the most vulnerable shuttle parts were the undersides of the wings close to the fuselage and right under the crew compartment.
NASA 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.
Fischbeck'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.
An 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.
To 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.
Investigators 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.
Kostelnik 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.
Fischbeck said NASA has improved the protection and maintenance of the tile system since his study. Foam insulation on the fuel tank was changed, and there are stricter limits on how much ice is allowed on the fuel tank before launch. Ice forms because of the supercold liquid hydrogen and oxygen propellants in the tank.
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