Intermediate failure states in simulation-based launch vehicle risk study

A framework for representing intermediate failure states in an assessment of abort risk during the ascent phase of a crewed space launch vehicle mission is presented. The framework refines a previously established, simulation-based risk assessment approach (Ref. 1) by improving the characterization of vehicle failure prior to the onset of final, potentially catastrophic ¿loss of crew¿ (LOC) hazards through the introduction of system-level ¿loss of mission¿ (LOM) failure states, or ¿LOM environments.¿ The intermediate failure state framework is found to improve the risk analysis with respect to both risk model fidelity and risk model management. While LOM environments primarily provide an incremental point of reference for failure evolution modeling and refinement, they also serve to consolidate the risk analysis in late failure evolution and thereby reduce overall analysis effort. Ultimately, the logical boundary in failure event sequences formed by LOM environments is found to effectively delineate areas of failure analysis responsibility between the teams that provide inputs to the risk assessment, and furthermore, define the data interface between those teams. The merits of this framework are illustrated in a case study concerning the treatment of upper stage liquid fuel engine failures.