GRAHAM WARWICK / WASHINGTON DC
Loss of thermal protection panels may have led to break-up, but cause is still unknown
Aerodynamic analysis is leading investigators to suspect that loss or failure of thermal-protection panels on the leading edge of the left wing led to the breach that caused the Space Shuttle Columbia to break up during re-entry. But the cause of the breach has yet to be isolated.
Further analysis of a low-resolution image of the orbiter, taken during re-entry as it passed over the US Air Force's Starfire Optical Range in New Mexico, suggests some reinforced carbon-carbon (RCC) panels were missing from the left wing leading-edge. Only small pieces of the inboard RCC panels from the left wing have been recovered.
NASA windtunnel tests simulating one missing panel did not reproduce the aerodynamic forces measured during re-entry. "The moment co-efficients were [in the right direction], but the magnitude was about four times smaller than it needed to be," says Columbia accident investigation board member Sheila Widnall. "We'd be looking to take four or five panels out of the leading edge to reproduce the observed flight characteristics."
Early turbulent transition of the boundary layer over the left wing also does not generate the observed aerodynamic imbalance, although it could cause heating problems, Widnall says. "Asymmetric boundary-layer transition appears to be a factor of four too small to explain the moments on the vehicle."
Widnall believes the break-up was a thermal event followed by a "very rapidly developing aerodynamic event" as dynamic pressure increased during re-entry. The latest timeline shows unusual yaw forces appearing soon after re-entry, before the orbiter crossed the California coast, more than 8min before break-up and 2min before the first report of debris being shed by Columbia.
Widnall also says evidence shows hot gas entering the wing upstream of, and exiting from, the left wheel well. This has focused attention on the leading edge and possible pre-existing damage to the RCC panels. Analysis of the launch video shows insulation foam shed by the external tank hitting the leading edge. "It appears to hit RCC panels six, seven and eight - no closer than six, no further than eight," says board chairman Hal Gehman.
While healthy panels might not have sustained damage, there is concern that Columbia's age - the orbiter first flew in 1981- might be a factor. Voids in the carbon-carbon caused by pinholes leading to oxidation were previously found in a repaired RCC panel on the orbiter Atlantis, while a crack and void were found in a panel from Discovery.
The investigation is also looking at the fact that the external tank was demated and remated to the solid rocket motors (SRMs) last year, potentially damaging the foam; and that the launch saw one of the largest-ever transient stresses on an SRM as the Shuttle ascended through windshear, possibly placing unusual loads on the tank.
Source: Flight International
