System reliability analyses usually begin with the definition of sub-system schematics that are provided by discipline experts. Sometimes these are quite detailed, but often they are just rough guesses at a potential configuration. For conceptual-level design studies, we define a reliability block diagram that identifies the major sub-systems and the critical events that must succeed to assure mission success. Then individual failure rates are ascribed to each event and these are rolled-up into a system-level failure probability – either via a formal fault tree analysis or an approximate, but rapid spreadsheet failure model.  

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The end result is usually displayed in a tabular form that summarizes the sub-system contributions to the overall system failure probability, along with the loss-of-mission (LOM) and loss-of-crew (LOC) probabilities.