TWO days before the space shuttle Columbia was destroyed as it re-entered the atmosphere, a NASA engineer warned colleagues of the potentially catastrophic consequences if the spacecraft鈥檚 undercarriage bay had been damaged on lift-off. The fierce heat of re-entry could then enter the bay, he said, and cause the craft鈥檚 tyres to explode, with devastating damage to the wing.
The warning came in an internal memo released by NASA last week after reporters got wind of its existence and started asking questions. Sent by Robert Daugherty, an engineer at NASA鈥檚 Langley Research Center in Virginia, to colleagues at the Johnson Space Center in Houston, the memo vividly describes some of the possible consequences of damage to the wheel wells.
The message was a response to a query about what might happen if heat shielding tiles had been damaged in the vulnerable wheel-well area. It was asked as part of a post-launch analysis of the effects of a piece of fuel tank insulation falling off and striking the orbiter shortly after lift-off.
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Although NASA has said its initial post-launch analysis showed that tile damage posed no danger, the 30 January memo makes it clear that the issue was still being actively studied as a possible risk shortly before the shuttle鈥檚 return from orbit on 1 February.
If damage to the tiles were to lead to excessive heating in the wheel well as the shuttle passed through the 1500 掳C plasma during re-entry, Daugherty wrote, pressure could build and 鈥渁t some point the wheel could fail and send debris everywhere鈥�. That could set off a chain reaction: 鈥淚f both tyres blew up in the wheel well鈥he resulting loads on the gear door would almost certainly blow the door off the hinges or at least send it out into the slipstream.鈥� The consequences to the shuttle would be 鈥渃atastrophic鈥�, he wrote.
One of the landing-gear doors is among the debris that has been recovered. If it is from the left wing, where unusually high temperatures and excessive drag were recorded on re-entry, it may help to reconstruct what happened.
Last week, NASA鈥檚 Columbia Accident Investigation Board reported that its analysis has shown that the searing plasma must indeed have penetrated the wheel well during re-entry. Nothing else would explain the temperature rises detected by sensors there, the board said.
鈥淗eat transfer through the structure as from a missing tile would not be sufficient to cause the temperature indications seen in the last minutes of flight,鈥� the board reported. But damage to the tiles, either from insulation falling from the fuel tank or another impact, perhaps with space debris (快猫短视频, 15 February, p 8), may have allowed the plasma to burn through the carbon and aluminium skin under the tiles. Alternatively, it might have partially opened the undercarriage doors and allowed plasma to penetrate.
NASA says the memo raises 鈥渘o new issues鈥� that were not part of its original analysis, and so was not forwarded up the chain of command at the time.
The memo said that it would be 鈥渋rresponsible鈥� for NASA managers not to be prepared for the possibility of 鈥渁 gut-wrenching decision鈥� if sensors should indicate failures in the wheel well during the re-entry. Such a decision might involve opting for a belly landing without undercarriage, or having the crew bail out after reentry and ditch the shuttle.
The board鈥檚 investigation is still in its early stages, and it may be months before the cause of the accident is determined. In the meantime, the engineer鈥檚 memo provides a haunting account of at least one possibility that seems consistent with the evidence collected so far.