Quantifying the Value of Risk-Mitigation Measures for Launch Vehicles
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The efficient development of a highly reliable system, such as a new crew launch vehicle, cannot afford to ignore the lessons of history. A number of interesting studies of launch vehicle failures provide very valuable, albeit qualitative “lessons learned” on measures that a risk-informed program should take. If schedule and funds were unlimited, a very intensive and exhaustive test program would be the course to follow before the first flight of a new launcher. But when a program is faced with stringent schedule and cost constraints, it needs to optimize its test planning so as to meet constraints without sacrificing safety. Making such trade-offs intelligently requires having a way to quantify the relationship between the initial unreliability of a system, and the array of riskmitigating measures on hand. This paper proposes several analysis steps beyond the existing studies of historical launch vehicle failures, which can form the basis for quantifying the lessons of history. Firstly, risk cannot be quantified accurately by summing all failures across history, because systems were not exposed to the same design deficiencies at each flight. Early failures typically represent sources of high risk, which are eliminated by corrective actions after the early flights, while late failures are often indicative of low-risk, design deficiencies that remain present for many flights. Thus failures occurring in the early launches of a system actually represent more risk than failures occurring later in history. Quantifying historical risk properly requires taking into account the reality of reliability growth.
Keywords: RAMS 2011 Proceedings - Risk Assessment - Safety and Risk Management - Reliability Growth