IT CERTAINLY wasn鈥檛 the cause of the Columbia accident. But the ongoing investigation is increasingly homing in on the crucial role of the space shuttle鈥檚 aluminium airframe, which may have played a pivotal part in the disaster, perhaps turning a mishap into a calamity.
A line of questioning being persistently pursued by members of the Columbia Accident Investigation Board, especially Sheila Widnall, an aerospace engineer from MIT, suggests it wasn鈥檛 simply falling pieces of tile, reinforced carbon composite or even wheel-well covers that ultimately led to the shuttle鈥檚 demise. Rather, it may have been the secondary effect of a failure of the parts of the structure made from aluminium, such as the wing spars, after they had been subjected to a searing plasma. This could have entered a hole punched in the shuttle鈥檚 skin by lift-off debris.
Although aluminium is usually protected by a layer of aluminium oxide, the metal can burst into an intense and virtually unquenchable flame if that layer is breached 鈥� or melted. That鈥檚 why aluminium is used in high-surface-area powdered form as the fuel in the shuttle鈥檚 own solid rocket boosters.
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The metal鈥檚 propensity to burn up fast is well known. Aluminium spacecraft parts rarely survive an uncontrolled re-entry into the atmosphere, said William Ailor, director of the Center for Orbital and Reentry Debris Studies at the space research firm Aerospace Corporation, in his testimony to the CAIB last week.
Ailor said aluminium parts only reach Earth intact if they are shielded from heat. He told the board that in the past, re-entry debris has been found in which aluminium has melted and splashed back onto other metal surfaces. Aluminium 鈥渃an actually oxidise or burn, and the heat released by that can melt holes鈥�, said Ailor.
This might explain one of the mysteries in the Columbia disaster: despite the fact that fragments of the craft 鈥� tiles, perhaps 鈥� had already begun falling off, the craft appeared to fly stably for minutes. Then it suddenly seemed to undergo far more drastic stresses that led to break-up. Perhaps the secondary effect of burning aluminium weakened the wing enough for it to break.
Meanwhile, the board may just have had a major break in unravelling details of Columbia鈥檚 last moments. Lab tests have shown that most of the information can be recovered from the flight data recorder found intact last week. The briefcase-sized box contains readings from about 800 sensors all over the shuttle, recording temperatures, stresses, sound, and acceleration. Though no cause for the accident has yet been determined, the signs are that it will turn out to be a chain of failures 鈥� with the aluminum airframe last in line.
Because of the lightweight metal鈥檚 incendiary tendencies and the intense heat of re-entry, said Widnall, 鈥渘obody would ever build a re-entry vehicle out of aluminium鈥�. And yet once stripped of their thermal tiles, as parts of Columbia鈥檚 wing may have been, that鈥檚 exactly what the shuttles are. The investigation continues.