37 Common Cause Case Study — An Estimated Probability of Four Solid Rocket Booster Hold-Down Post Stud Hang-Ups (PSAM-0372)
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Until Solid Rocket Motor ignition, the Space Shuttle is mated to the Mobil Launch Platform (MLP) in part via eight (8) Solid Rocket Booster (SRB) hold-down bolts. The bolts are fractured using redundant pyrotechnics, and are designed to drop through a hold-down post on the MLP before the Space Shuttle begins movement. The Space Shuttle Program has experienced numerous failures where a bolt has “hung up.” That is, it did not clear the hold-down post before lift-off, and was “caught” by the SRJBs. This places an additional structural load on the vehicle that was not included in the original certification requirements.
The Space Shuttle is currently being certified to withstand the loads induced by up to three of eight SRB hold-down post studs experiencing a “hang-up.” The results of preliminary loads analyses performed for four stud hang-ups indicate that the internal vehicle loads exceed current structural certification limits at several locations. To determine the risk to the vehicle from four stud hang-ups, the likelihood of the scenario occurring must first be evaluated. Prior to the analysis discussed in this paper, the likelihood of occurrence had been estimated assuming that the stud hang-ups were completely independent events. That is, it was assumed that no common causes or factors existed between the individual stud hang-up events. A review of the data associated with the hang-up events, showed that a common factor (timing skew between redundant pyrotechnics) was present. This paper summarizes a revised likelihood evaluation performed for the four stud hang-ups case considering that there are common factors associated with the stud hang-ups. The results show that explicitly taking into account the common factor of timing skew results in an increase in the estimated likelihood of four stud hang-ups of an order of magnitude over the independent failure case.