TIM FURNISS / LONDON & GRAHAM WARWICK / WASHINGTON DC
Board yet to pinpoint conclusive evidence that Shuttle's left wing was damaged by tank insulation during launch
Investigators have a "working scenario" for events leading to the loss of Space Shuttle Columbia, but have yet to prove conclusively that the left wing was damaged by external-tank insulation foam shed during launch. While crucial tests continue, the scenario makes clear that the different lines of investigation are drawing together.
Columbia re-entered on 1 February with pre-existing damage in the area of leading-edge reinforced carbon-carbon (RCC) panels 8 or 9, or the T-seal between them, says Columbia accident investigation board chairman Hal Gehman. Radar cross-section testing has not conclusively identified the object that separated from Columbia in orbit, but has eliminated all but a piece of RCC panel or T-seal from consideration, he says.
Data recorder measurements indicate that hot gas flowed into the cavity behind the RCC in the 5min following entry interface at 8.44.09 EST. Recovered pieces of RCC from the panel 8/9 interface are "flame eroded" to a knife edge, and deposits of "flame-sprayed" metal from the wing are heaviest in the area of panels 8 and 9.
Within a further 3min (by 08.52), the hot gas had breached the wing leading-edge spar and penetrated the wing, almost immediately causing sensors to fail as wires burned through. Over the next 4min, more than 160 left-wing measurements ceased. Investigators believe the wing then began to deform, causing Columbia to shed thermal-protection tiles less than 10min after entry interface.
At about the same time (08.52-08.54), coincident with several "very bright" debris shedding events, the orbiter's flight control system began compensating for increased drag on the left wing. There was a significant aerodynamic change 4min later (08.58.29), accompanied by more debris shedding, followed 80s later - 3s before loss of signal at 08.59.32 - by an abrupt change that caused Columbia to compensate by firing all four right yaw jets. Break-up occurred at 09.00.23.
Gehman says there is "ample evidence" that several pieces of external-tank insulation foam broke off the left bipod ramp during launch on 16 January, one hitting the left wing in the area of RCC panels 7 and 8. But testing to determine whether the impact could have broken a panel or T-seal, which then separated in orbit, has yet to be accomplished.
Investigators have managed to identify six known bipod ramp foam losses in 113 Shuttle launches, and have statistically extrapolated at least three more unseen losses. Questions are focusing on how NASA came to tolerate the probability of ramp foam loss on one of every 12-13 flights.
Source: Flight International
