Expected productivity-based risk analysis in conceptual design : with application to the Terrestrial Planet Finder Interferometer mission

During the design process, risk is mentioned often, but, due to the lack of a quantitative parameter that engineers can understand and trade, infrequently impacts major design decisions. The definition of risk includes two elements - probability and impact. As a result of heritage techniques used in the nuclear industry, risk assessment in the aerospace industry is usually purely reliability based, and is calculated as the probability of a failure occurring before the end of the design lifetime. While this definition of risk makes sense if all failures result in the same impact, for many non safety-critical systems, the impact of failures may vary, including variance by when a failure occurs. While current risk assessment techniques answer the question "What is the probability of failure?", the true question that needs to be answered for many missions is "How much return can be expected?" Depending on the question answered, the relative ranking of risk items may be different - leading to different risk mitigation investment decisions. Consequently, to complete an accurate risk assessment, it is important to combine system performance and reliability, and model the probabilistic nature of the expected value of the total system productivity.