Reliability and probabilistic risk assessment — How they play together

Since the Space Shuttle Challenger accident in 1986, NASA and aerospace industry has extensively used Probabilistic Risk Assessment (PRA) methods to assess, understand, and communicate the risk of space launch vehicles, especially manned space flight missions. Another area that was given a lot of emphasis at NASA is reliability engineering. Both PRA and reliability are probabilistic in nature; however; the reliability engineering is a broad design discipline that deals with loss of function, while PRA is a system scenario based risk assessment process that deals with Loss of Mission (LOM), Loss of Vehicle (LOV), and Loss of Crew (LOC). This paper discusses the PRA process and the reliability engineering discipline in details. It discusses their differences and similarities and how they are used as complementary analyses to support design and flight decisions. In summary: 1) Reliability Engineering is a discipline that involves the application of engineering principles to the design and processing of products; both hardware and software intended to minimize the loss of functions. 2) PRA at NASA is a process that deals with system risk focusing on understanding the system risk scenarios that could lead to LOM, LOV, and LOC. 3) PRA and reliability engineering are two different areas serving different functions in supporting the design and operation of launch vehicles. However, PRA as a risk assessment, and reliability as a metric could play together in a complementary manner in assessing the risk and reliability of launch vehicles. 4) In general, reliability analyses should be used as a critical data source for PRA.