Understanding Redundancy Requirements in the Design of Non-Serviceable Systems

Abstract

Non-serviceable systems are often constrained by three requirements that address the need to reliably fulfill its mission: A lifetime requirement, a reliability at end-of-life requirement, and a requirement to avoid single point failures, generally through redundancy. From a requirements engineering perspective, a significant question is whether the requirements to use redundant solutions are necessary for space system design when a reliability target has been specified. Does a requirement to use redundancy impose a specific solution with no other effect on need satisfaction? If so, can it be deleted with no adverse effect on mission success? To answer these questions, the impact of different redundancy configurations on mission success are studied, using a space system as a test case. Mission success is modeled as a function that accrues value over the satellite’s operational life. The different redundancy configurations are then compared for different rates of value accumulation over the space system’s operational life, which encompass reasonable value accumulation rates observed in most real-life space missions. The numerical results reveal that when all design alternatives provide the same reliability at end of life at the same cost, systems that employ redundancy have a higher expected value than a system without redundancy.