Launch vehicle reliability growth

Abstract

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.

Small Satellite Launch to LEO: A Review of Current and Future Launch Systems

“…Vehicles based on existing and mature technologies and design elements should experience a much quicker growth in reliability as only the integration aspects of the system are unknown. However, launchers based on immature technologies and designs will experience slower growth in reliability and may present a greater risk during early flights even if the projected reliability at maturity is higher 21) .…”

Near-Earth phase risk comparison of human Mars campaign architectures

“…Ares V ascent failure probability was adapted from Ares I risk assessments. Commercial launcher failure risk was based on heritage data [6]. Delivery risks stemmed from complex, temporally sensitive mission operations such as rendezvous and docking, on-orbit fuel transfer, and solar array deployments, and are also heritagebased.…”

An exploration of PRA methodology used in spacecraft design

“…This can be explained by the fundamental difference in modeling approach between a bottom-up and top-down PRA methodology: bottom-up approaches contain only component-originated failures, while top-down approaches also contain failures stemming from unintended subsystem interactions with the environment, other subsystems, and within the overall subsystem. All of these interactions could be considered as design defects, as they are potentially knowable interactions; as processing defects, which are somewhat unavoidable; or as unknown-unknown type defects, which may be completely unavoidable [8]. It has been shown that these types of defect-originated failures are dominant in the risk of complex, single-use space systems [9].…”

“…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.…”

“…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.…”