Investigators searching for the source of the thermal breach that brought down the Space Shuttle Columbia have shifted their focus further outboard on the left wing. Additional trajectory analysis suggests foam shed by the external tank during launch hit the leading edge at reinforced carbon-carbon (RCC) panels 7 and 8, further out than previously thought. This ties in more closely with sensor data and debris evidence.

At the same time, a T-seal installed between the RCC panels has emerged as the likely object observed separating from Columbia during its second day in orbit after the further analysis eliminated a carrier panel from consideration. Loss of a piece of bow-shaped T-seal could have allowed a jet of superheated air to enter and "play like a hosepipe" across the inside of the wing, says Columbia accident investigation board member Scott Hubbard. Little of the aluminium substructure of the left wing has been recovered.

Radar cross-section tests suggested a carrier panel - an access panel that fits between the leading-edge RCC panels and underwing tiles - was the prime candidate for the mystery object, but it did not fit the re-entry ballistics observed. "We are leaning towards a T-seal," says board member Stephen Turcotte.

Evidence supporting a leading-edge breach further out than first thought includes data from the recovered OEX orbiter experiments recorder, which shows temperature spikes near RCC panel 9 just 480s after re-entry began. Recovered debris also shows significant damage at the interface between RCC panels 8 and 9. RCC rib and lug fragments have been eroded "to a knife edge", says Hubbard, which suggests long exposure to extreme heating.

The analyses will guide foam impact testing, scheduled for mid-May. Investigation continues into defects in the insulation foam covering the bipod area where struts attach the nose of the orbiter to the external tank. In super-lightweight tanks like that used on the Columbia flight, cracks were found in stringer valleys close to the bipod area. These may have allowed cryogenic fluid to creep under the insulation to expand explosively on ascent and "liberate" a large piece of foam.

Source: Flight International