Design-For-Reliability Considerations and Modeling for Liquid Propulsion Engines Starting from Conceptual Design

Reliability is often considered to be one of the most important launch vehicle requirements and design parameters. In many NASA and US Air Force launch vehicle studies, reliability assessment results become one of the necessary attributes to support configuration and design option down-selections. One of our early papers discussed a comprehensive set of reliability drivers related to launch vehicle liquid propulsion engine systems, and presented a reliability model that parametrically analyzes some of the key reliability parameters. This paper extends that reliability model by analyzing more reliability parameters including: engine-out switching reliability; engine component, subsystem and system inherent designed reliability; manufacturing, inspection and operation capability and human error effects on the system reliability. The paper focuses the discussion on design-for-reliability starting from vehicle and propulsion system feasibility study, concept exploration, and early design stages (conceptual and preliminary designs). For the inherent designed reliability modeling, this paper introduces a Complexity Index (CI) approach that correlates the inherent designed reliability to the engine complexity related to the engine configuration, structural and performance requirements. For the manufacturing, inspection, testing, operation capability and human error effects on the system reliability, an As-built/As-tested/As-used Effectiveness Index (AI) is defined to address the delta between the inherent reliability and real flight reliability. Both the CI and AI indexes evolve from an abstract level to a detailed design level as design and development progresses from feasibility study, conceptual exploration, architectural design, to conceptual design, detailed design, testing, and to flight certification. The use of these two indexes to guide design-for-reliability starting from architectural and conceptual design to the detailed design is discussed and illustrative examples are presented. With the enhanced reliability model and the design-for-reliability approach, a more reliable, safer and affordable liquid propulsion engine system can be developed.