This paper addresses the importance of considering the initial reliability and reliability growth as opposed to only the mature risk estimate when making relative comparisons among developmental launch vehicle (LV) alternatives and introduces the current model used to perform this type of analysis.Probabilistic risk assessments (PRA) often focus on modeling the mature state of a system under consideration; however, in the aerospace field of LV design such an assessment can be dangerously misleading. Due to the low flight rate, a given LV may never reach maturity prior to retirement and will fly mostly in an immature state. The historical record of early LV flights suggests a risk posture well above the mature estimate predicted through the standard PRA approach. Thus, any decision based upon the mature estimates may be significantly different than a decision based upon the predicted risk during the bulk of its useful life while it is still maturing. In order to make an informed decision about the relative merits of competing LV architectures, decision makers must consider not only the mature system risk, but also the reliability growth for the system along the path to maturity.The current model described in this paper uses a reliability growth methodology, which has expanded the scope of risk influencing factors and has been able to provided Loss of Mission (LOM) and Loss of Crew (LOC) risk estimates for over 20 LVs in a period of less than two months. The advantage of employing such a methodology to conceptual LV designs is that it enables a more realistic estimate of campaign success during early flights without the need for detailed design information. This model captures the reality that element heritage and maturity are more important to early flight success than first order component reliability calculations while yielding valuable insights for designers of future vehicles.

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In order to achieve an optimal design of a complex space system that meets all constraints, the requirements placed upon the performance, mass, cost, and risk of the system must be considered, understood and traded against each other during the conceptual design of the system to avoid costly redesigns or project cancellation later in the development process [1]. A design process that follows this tenet of riskinformed design will need detailed insight into the relative risks facing the system, as well as quantitative estimates that can be produced through probabilistic risk assessment (PRA), in order to evaluate design decisions based upon the impact to all requirements on a co-equal basis [2].In this study, four types of methodologies used to produce risk estimates for spacecraft and satellites are examined. These include two traditional PRA methodologies [3,4], an innovative approach [5], and a top-down approach [6], all of which are explored by using the propulsion subsystem of the Lunar Reconnaissance Orbiter (LRO) as a comparative basis for the methodologies considered [7]. Similarities, differences, benefits, and drawbacks of various bottom-up, componentbased PRA approaches and the top-down approach are elucidated in terms of the process of modeling a system, the actionable information produced for the design team, and the overall quantitative risk evaluation of the system as compared to similar heritage space systems.Results of the various PRA methodologies are examined at the level of component failure rates, single-component failure probabilities, single-function failure probabilities where redundancy exists in the design, as well as the subsystem failure probability for the nominal LRO mission.Ultimately, all of the bottom-up, component-based PRA methods capture only the risk of a mature system and miss the risk contribution of design defects, which have been shown to be key drivers of reliability in single-use developmental systems [8,9]. Therefore, further steps must be taken to incorporate this contribution in future PRA methodologies.

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