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[i]External tank crack repairs are not unusual. Some 29 stringer cracks were found in 18 previous tanks, according to an official familiar with their history. Four have now been found in Discovery's tank, ET-137, and three were found in a tank scheduled for use by the shuttle Atlantis next summer, ET-138. Doublers were used in 23 repairs.
Engineers suspect the use of a lightweight aluminum-lithium alloy in the tanks may be contributing to the crack problem.
While crack repairs are not unusual, the cracks in Discovery's tank are the first to be found at the launch pad, where access is more difficult. An environmental enclosure has been erected around the known damage site to facilitate repairs and the eventual application of fresh foam insulation.
It's not yet known whether more defects remain to be discovered, but engineers are optimistic high-tech instruments capable of detecting damage beneath the foam can be used to find any additional problems.[/i]
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