The Columbia shuttle disaster may been triggered by a combination of a new external fuel tank design and ageing of the spacecraft, a Columbia Accident Investigation Board press conference was told on Tuesday.
Although some remaining alternatives remain to be ruled out, the CAIB鈥檚 focus is now on two crucial factors. First, it appears that a new fuel tank design led to foam falling off during liftoff. A one-kilogram block was seen striking Columbia鈥檚 left wing during launch.
Secondly, the damage this impact caused may have been made worse by an unexpectedly high degree of wear on wing components. The loss of a key wing component is thought to have allowed hot plasma to enter and destroy the shuttle鈥檚 wing during re-entry on 1 February.
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CAIB member Scott Hubbard did add a note of caution: 鈥淐urrently the data are not all pointing in the same direction.鈥� But CAIB hopes that a series of new tests on shuttle spare parts and models will resolve the questions that remain.
Shedding events
NASA鈥檚 problems with foam only began recently, when it shifted to the new 鈥渦ltra lightweight鈥� external tank used in the Columbia launch, said CAIB member Steve Turcotte.
Before the change, the foam had 鈥渧ery few stringer cracks鈥�, up to 15 centimetres long, he said. 鈥淲e don鈥檛 know if this caused the shedding events,鈥� Turcotte said, but the tanks had not shed foam in the 10 years before the new design was introduced.
New analysis of the launch film, also presented on Tuesday, showed the foam hit the leading edge of the wing about half a metre further out than had been thought, on panels 7 and 8.
Analysis of recovered debris shows severe heat damage and erosion to the reinforced carbon composite of panels 8 and 9. That could indicate that the T-shaped seal between the two panels failed after being hit by the foam, Hubbard said. Only two small bolts hold it in place.
Radar images from the second day of Columbia鈥檚 mission show an object falling off near this part of the wing. If the T-seal broke off, it would have left a gap about three centimetres wide for hot plasma to enter the wing on re-entry, CAIB chairman Hal Gehman said.
Cracks and pinholes
Perhaps most ominously for NASA, CAIB reported finding more wear than expected to the reinforced carbon composite on the remaining shuttles. Zinc contaminants from the shuttle launch platform have oxidised pinholes in the wing edges. Expansion and contraction are also causing tiny cracks.
Gehman says CAIB will urge NASA to replace current visual tests with thorough nondestructive testing of hidden as well as exposed structures before any future shuttle launches.
However, NASA Administrator Sean O鈥橩eefe recently told Associated Press: 鈥淭here is no technology right now to do effective nondestructive testing.鈥�
Finally, CAIB announced that it expects to end its search for shuttle debris in east Texas by the end of April. About 70,000 pieces of debris have already been found 鈥� 36 percent of the orbiter鈥檚 dry mass. However, investigators will continue hunting further to the west for pieces that fell from the shuttle earlier in its re-entry.
