posted August 23, 2012
During cryoloading operations prior to the first launch attempt of STS-133, a visible foam defect was identified at the junction between the LOX tank and intertank. Video and photographic data showed a 7-inch wide hemispherical crack in the foam had pushed out from the surface of the tank. Such a defect has good potential to liberate a large piece of foam debris as aerodynamic loads pressurize the crack during ascent. Given the mass and time of foam debris release, the debris had a high probability of inflicting critical damage to the Orbiter. Subsequent investigation identified large cracks at the end of the intertank stringer, which allowed the stringer to push away from the intertank and crack the foam. Although mitigation using doublers to strengthen the majority of stringers will likely be the primary path to flight rationale, a probabilistic risk assessment continues to be developed to evaluate foam debris risk in the event a stringer crack produces foam debris during ascent.
Aerospace performed a probabilistic risk assessment of foam debris damage for delayed STS-133. There were several key parts of the assessment, including sensitivity studies to assess Orbiter damage risk using our end-to-end foam debris damage simulation tool, and performing high-subsonic wind tunnel tests in the Aerospace laboratory to characterize the size and breakup characteristics of the foam.