The NASA Orion spacecraft's primary purpose is to provide crew transport to and from the International Space Station (1SS) and the Moon. In a departure from previous spacecraft programs, crewed or otherwise, reliability requirements are part of the basic Orion requirement set in the form of Loss of Crew (LOC) and Loss of Mission (LOM) probabilities. The post-Preliminary Design Review (PDR) spacecraft level LOC probabilities are 1 in 200 and 1 in 290 respectively for Lunar and ISS missions. NASA prescribed spacecraft reliability requirements and the Prime Contractor formally allocated them via apportionment to each of the Orion subsystems, requiring the corresponding verification effort. Post-PDR Orion Thermal Protection System (TPS) LOC allocations are 1 in 2,100 for Lunar missions and 1 in 5,600 for ISS missions. In advance of Orion PDR, Heat Shield reliability predictions were made as part of the Orion Advanced Development Project (ADP), The ADP approach identified several challenges in identifying failure modes, and generating a physics-based likelihood estimate for each mode. In some cases, use of limited input data distributions for a well-defined failure mode prevented failure probability distribution tails from being captured. In other cases, failure modes were characterized only in as much as their name was written down. Reliability verification requires resolution of the failure likelihood distribution tails. The ADP effort did result in a quantitative estimate of Heat Shield reliability, but this was far less valuable than the awareness of the challenges that resulted. Although the "Best Estimate" Heat Shield reliability met the allocated requirements, several simplifications of physical phenomena remain un-addressed and multiple failure modes remain un-characterized. Furthermore, a rigorous yet practical verification approach for the formal reliability requirements has not been established. The ADP process, results, and deficiencies are described along with plans for future work.
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