Composite Overwrapped Pressure Vessels (COPV): Flight Rationale for the Space Shuttle Program

3Cornell University, Ithaca, NY, 14850, USA Each Orbiter Vehicle (Space Shuttle Program) contains up to 24 Kevlar49/Epoxy Composite Overwrapped Pressure Vessels (COPV) for storage of pressurized gasses. In the wake of the Columbia accident and the ensuing Return To Flight (RTF) activities, Orbiter engineers reexamined COPV flight certification. The original COPV design calculations were updated to include recently declassified Kevla r COPV test data from Lawrence Livermore National Laboratory (LLNL) and to incorporate changes in how the Space Shuttle was operated as opposed to originally envis ioned. 2005 estimates for the probability of a catastrophic failure over the life of the prog ram (from STS-1 through STS-107) were one-in-five. To address this unacceptable risk, th e Orbiter Project Office (OPO) initiated a comprehensive investigation to understand and mitigate this risk. First, the team considered and eventually deemed unfeasible procuring and replacing all existing flight COPVs. OPO replaced the two vessels with the highest risk with existing flight spare units. Second, OPO instituted operational improvements in ground procedures to significantly reduce risk, without adversely affecting Shuttle capability. Th ird, OPO developed a comprehensive model to quantify the likelihood of occurrence. A fully-instrumented burst test (recording a lower burst pressure than expected) on a flight-cer tified vessel provided critical understanding of the behavior of Orbiter COPVs. A more accurate model was based on a newly-compiled comprehensive database of Kevlar data from LLNL and elsewhere. Considering hardware changes, operational improvements and reliability model refinements, the mean reliability was determined to be 0.998 for the remainder of the Shuttle Program (from 2007, for STS-118 thru STS-135). Since limited hardware resources precluded full model validation through multiple te sts, additional model confidence was sought through the first-ever Accelerated Stress Ru pture Test (ASRT) of a flown flight article. A Bayesian statistical approach was devel oped to interpret potential ASRT results. Since the lifetime observed in the ASRT exceeded initial estimates by one to two orders of magnitude, the Space Shuttle Program deemed there was significant conservatism in the model and accepted continued operation with existing flight hardware. Given the variability in tank-to-tank original proof-test response, a non -destructive evaluation (NDE) technique utilizing Raman Spectroscopy was developed to directly measure COPV residual stress state. Preliminary results showed that patterns of low fib er elastic strains over the outside vessel surface, together with measured permanent volume growth during proof, could be directly correlated to increased fiber stress ratios on the inside fibers adjacent to the liner, and thus reduced reliability. Associated with this volumetr ic response, thought tied to void compaction, was the discovery, though laser profilo metry inspection of the interior of several