NASA engineers have been working since November to figure out why cracks were emerging on the 22-foot-long U-shaped aluminum brackets, called stringers, on the shuttle's external fuel tank.
According to shuttle program manager John Shannon, the same problem may have existed on the fuel tank, ET-136, that flew in May 2010 with the shuttle Atlantis, a mission that ended without incident.
"It is likely that we flew ET-136 with some of these cracks," Shannon said, adding that NASA was not entirely certain but "we might have had a crack or two in those stringers."
Shannon said engineers performed exhaustive tests and found that the complex problem was not solely attributable to material quality or flaws that took root during assembly, but some combination of both.
He described the dilemma as "low risk" but at the same time, "hard to quantify."
In the end, engineers agreed that installing small metal strips, called radius blocks, on to the stringers would reinforce their strength.
Space Shuttle Discovery Feb 24, NASA Space Shuttle
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