GRAHAM WARWICK / WASHINGTON DC
Further firings planned to reinforce wing panel findings
More firings are planned after an initial foam impact test on a section of Space Shuttle orbiter leading edge resulted in "significant and visible damage" to reinforced carbon-carbon (RCC) panels.
The test appears to confirm the Columbia Accident Investigation Board's theory that external-tank insulation foam shed just after lift-off damaged the left wing leading edge, resulting in a thermal breach on re-entry that caused the orbiter to disintegrate.
The 6 June test at the Southwest Research Institute, in San Antonio, Texas, was the first to involve actual RCC panels, and the T-seals between panels. Previous tests had used glassfibre panels. In the test, a pressurised nitrogen gun fired a 0.76kg (1.68lb) section of foam at 768ft/s (234m/s), striking the lower side of left-wing RCC panel 6 at an angle of 20¼. The panel flew 30 missions on the orbiter Discovery.
The impact resulted in a 75mm (3in) crack extending from a visible 20mm damage area on the outside through the panel to the RCC rib inside the leading edge. The T-seal between panels 6 and 7 was also cracked, and the strike caused RCC panel 6 and the T-seals either side to shift. A carrier, or close-out, panel on the wing's upper side was chipped. "We don't know the structural or thermal implications yet," said CAIB member Scott Hubbard after the test, "but if a 3in crack had been found on inspection, you would not fly with it." More tests were expected last week, possibly beginning with further firings at glassfibre panels. The CAIB is expected to issue its report in late July.
NASA administrator Sean O'Keefe has said that a breach in the leading-edge thermal protection system the length and width of a pen could have been enough to let superheated air enter the wing during re-entry, eroding the RCC and leading to structural failure of the wing and disintegration of the orbiter.
Source: Flight International
