IT SEEMS like such a simple problem. Yet a quarter of a century after the shuttle鈥檚 first launch, NASA still cannot guarantee that the foam insulating the launcher鈥檚 fuel tanks will stay attached during lift-off. It was a chunk of foam falling off Columbia in 2003 that punched a hole in the wing and sealed its fate.
Over the past couple of weeks, NASA has been explaining with unusual frankness why this problem is so intractable. Its account highlights a fact we all too easily forget: that much of manned space flight takes place at the limits of our technical knowledge. 鈥淭he shuttle is and always will be an experimental aircraft,鈥� says Paul Fischbeck, a risk analyst at Carnegie Mellon University in Pittsburgh.
In the latest edition of the document entitled NASA鈥檚 Implementation Plan for Space Shuttle Return to Flight and Beyond, issued last month, shuttle engineers outline progress on a number of fronts, among them the foam issue. What NASA lacks is a way of inspecting the sprayed-on layers of polyurethane and polyisocyanate foam to reveal where voids appeared when it was applied. Air trapped in these voids undergoes large temperature changes as the tanks fill with cryogenic fuel. When the air later warms and expands, it can push the foam away from the tank and cause chunks to be shed.
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鈥淣ASA has conducted comprehensive searches for non-destructive evaluation techniques and has made repeated attempts to develop a method of inspecting the foam for correct application,鈥� the report says. 鈥淲e continue these efforts.鈥� So with the second post-Columbia launch pencilled in for May, the agency still doesn鈥檛 know if the foam will stay put. In an attempt to cut down the risk of voids forming it has redesigned protruding parts of the tank, such as fuel-line plumbing, and is implementing strict controls on how the foam is applied. Similar redesign failed to stop 16 pieces of foam coming off Discovery鈥檚 fuel tank last year in the first shuttle mission since Columbia, however.
鈥淔oam will still come off the tank after we have taken all the mitigation efforts,鈥� shuttle programme manager Wayne Hale told a press conference last month. 鈥淲e believe the pieces that come off will be small.鈥� Whether they will be small enough to avoid serious harm remains to be seen.
