The Application of Probabilistic Risk Assessment to Habitable Payloads: Utilization of Risk-Based and Traditional Rule-Based Methodologies: A “First” for NASA

Abstract

The safety of the public, the astronaut crew, Agency assets, other payloads, and the environment are NASA’s priorities when assessing the adequacy of space flight designs. While Probabilistic Risk Assessment (PRA) has been successfully applied to Space Shuttle and Space Station vehicle risk decision-making, the mandated use of a non-probabilistic rule-based approach is unique to the safety certification of NASA’s habitable payloads. A 1997 survey of historical safety policies with NASA’s Payload Safety Review Panel (PSRP) revealed that the non-probabilistic approach for habitable payloads was not arbitrary but founded on informed risk decisions from 20 years ago by then NASA Headquarters policy makers. Based on a sound payload safety track record, there has been no compelling reason, until recently, to consider expanding from the present NSTS 1700.7B rule-based approach to include risk-based PRA as a viable alternative. However, with the Agency’s increased focus on structured risk management, the establishment of a Risk Assessment Program at NASA Headquarters, and refined PRA guidelines and techniques, PRA is now formally recognized as an essential method for evaluating complex and high risk systems. The PSRP recognizes a growing need and an opportunity for evaluating the efficacy of risk-based PRA methods for application to increasingly complex next generation payload technologies. Therefore, it is timely to revisit the potential application of PRA to habitable payloads. This paper discusses PRA as a risk-based method that, when properly implemented, will result in equivalent or improed safety compared with the rule-based failure tolerance requirements for achieving the Agency’s “Safety First” core value. The benefits and cautions associated with infusing PRA methodology into the PSRP safety certification process are also discussed, as well as a proposed deployment strategy of how PRA might be prudently tailored and applied to habitable payloads. The use of PRA for assessing payload reliability is unrestricted at NASA but this is beyond the scope of the present discussion of payload safety applications.