Quantifying the value of risk-mitigation measures for launch vehicles

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 risk-mitigating 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. Secondly, knowing what failed in the past does not provide direct guidance as to how to reduce the risk of a new design. Of utmost relevance are the kinds of measures that could have prevented the failures in the first place. Simplistically put, knowing that the majority of launch vehicle failures originated in propulsion systems is of limited use to designers and managers, who already pay tremendous attention to that central subsystem. By contrast, a quantification of the potential risk reduction possible by submitting an engine to stress testing, for example, could be valuable in supporting the cost and schedule trade-offs that decision makers are unavoidably faced with. This paper proposes a method for re-considering the failures of historical launchers in that new light and illustrates its application to two historical examples, the Ariane and Centaur systems. The results provide an approximate quantification of the risk reduction potentially offered by improvements in areas such as: sufficient flight-like testing at the system level; definition of, and testing for, margins that consider all phases of flight, including not only steady-state but also transient conditions; stress testing and testing for variability at the component and engine levels; analysis of the results of every single flight with an eye towards uncovering design defects: “post-success investigations” re-examination of the margins of all components and systems (including software) and re-qualification after every single change in design, configuration, or mission profile; and maintenance of very rigorous levels of electrical and cabling parts control, quality assurance and contamination control in all phases of manufacturing, assembly and launch operations. The authors hope that the techniques and insights presented in this paper can be of use to the aerospace industry as it embarks on the flight certification program for the next-generation crewed launcher