Evaluation of reliability, availability, maintainability and safety requirements for manned space vehicles with extended on-orbit stay time

Abstract

The recent manned space transportation vehicles studies performed by ESA and European Industry, investigate the possibility of extended on-orbit stay time. From the RAMS (Reliability, Availability, Maintainability, Safety) point of view, these vehicles will have to meet, in addition to the multi-phase mission safety and reliability constraints, stringent on-orbit availability levels which will be among the most important design and operations drivers. The objective of the paper is to derive the lessons learnt from the Assured Crew Return Vehicle (ACRV) feasibility phase study, in terms of RAMS requirements specification. The paper briefly assesses the deterministic RAMS requirements derived from ESA and NASA applicable standards and focuses on the probabilistic requirements which were the subject of numerous interpretations. Different approaches are presented together with their impacts on the design (maintainability, testability, on-orbit replaceable units concept,…) and on the operations (check-out frequency, logistics,…). The main uncertainties are also assessed. In conclusion, recommendations are made for the specification of RAMS requirements for manned space vehicles with extended on-orbit stay time.