“While a single root cause of the cracks has not been determined, it appears the stringers in question were made using an aluminum alloy from a batch produced in 2002 that has a mottled appearance and may not be as strong as engineers expected.”
No word here on when this was noticed. You would think they would have run something like a coupon of the batch through structural testing before using it for a critical system like the ET. What other compromises have been made in STS production and processing over the years?
They will try to put doublers in this tank and will “probably” get away with it a couple more times. Then they can put these things in museums and let historians argue whether it was all worth it.