GRAHAM WARWICK / WASHINGTON DC
Size of hole raises doubt over in-orbit repair, but NASA plans to remove debris risk
Investigators believe impact testing has proved that Space Shuttle Columbia's wing leading-edge was damaged after lift-off by insulation foam shed from the external tank.
In a test on 7 July, a 0.76kg (1.67lb) piece of foam punched a 400 x 400mm (16 x 16in) hole in a reinforced carbon-carbon (RCC) leading-edge panel, prompting Columbia Accident Investigation Board (CAIB) member Scott Hubbard to declare the test had established the "direct cause" of the orbiter's disintegration on re-entry.
Testers were surprised by the damage, which was more extreme than in a previous test that resulted in a crack in an RCC panel. The chunk of foam was fired at 777ft/s (237m/s) at the underside of RCC panel 8, where investigators believe Columbia's left wing was breached. Test conditions were described as moderate, but the orientation of the impact was changed so that the flat side of foam block hit the panel. In the previous test only a corner of the block hit the panel.
The RCC panel used was taken from the orbiter Atlantis and had flown 27 times, while Columbia was on its 28th flight. Investigators believe the hole produced in the test is larger than would have occurred in flight, but still within the estimated range of sizes that would have led to a thermal breach of the leading edge, allowing superheated air to enter the left wing during re-entry. Hubbard believes a jagged piece of broken-off RCC panel, similar to that produced in the latest test, is the mystery object observed by radar separating from Columbia during its second day in orbit.
The hole size has raised questions about whether it will be possible to develop a capability to repair such extensive damage in orbit, as recommended by the CAIB, but NASA says the goal of its Shuttle return-to-flight effort is to prevent debris hitting the orbiter. This will include redesigning the tank to replace insulation foam with heater blankets. NASA is continuing impact tests to better understand the behaviour of the RCC panels, which vary widely in their impact resistance.
Source: Flight International
