Estimating Orion Heat Shield Failure due to Ablator Cracking

Before the successful Exploration Flight Test 1 (EFT-1) flight test in December 2014, the Orion EFT-1 Heat Shield suffered from two major certification challenges: First, the mechanical properties used in the design were not evident in the flight hardware, and, second, the flight article itself cracked in several locations during fabrication. These events motivated the Orion Program to pursue an engineering-level Loss of Mission Probabilistic Risk Assessment (PRA) as part of the Heat Shield certification rationale. The PRA provided likelihoods considering the probability of ablator cracks occurring during the mission and the likelihood of subsequent structure overtemperature. After a general discussion illustrating the relationship between typical stress design policy and reliability, the Orion-specific methods and input data for the PRA are presented along with a discussion of the test data used to anchor the results. The Orion program accepted an EFT-1 Loss of Vehicle risk of 1 in 160,000 due to in-mission Avcoat cracking based on the results of this analysis. Conservatisms in the result, along with future considerations for Exploration Missions, are also addressed.