Re: Debug Question ?
They figured this one out from telemetry. There was an over pressurisation event in the second stage tank, which had counter intuitive causes. They used analysis (ie timing which sensor fired when) to find out where the pressure spike started (IIRC), which indicated a strut failure around the helium tank (COPV). The strut however were rated for 3x the expected load so should not have failed, so they then tested a LOT of struts (1000+) and found a couple that broke well below the certified load. Further analysis of the bolts (I think) showed bad grain structure inside the bolts which caused the failure.