Quantitative Reliability Analysis Of The STS Electric Auxiliary Power Unit Prototypes

摘要

This paper discusses the quantitative reliability analysis (QRA) of two Space Transportation System (STS) Electric Auxiliary Power Unit (EAPU) prototypes. The analysis was performed for the United Space Alliance (USA) Space Shuttle Upgrades Development Office. The QRA was performed using fault tree analysis combined with uncertainty analysis and common cause failure analysis. The Space Shuttle Program (SSP) has imposed the following reliability criteria on the EAPU upgrade: "The overall EAPU system reliability for successful mission completion of a single mission shall be no less than 0.99999 at the mean and no less than 0.9999 at the 95% confidence level" (ref. 1). The EAPU QRA showed that the architectures of both EAPU prototype designs meet the reliability criteria set forth by the SSP. The analysis further showed that one prototype design provides a higher reliability with less uncertainty when compared to the other. The results of this quantitative analysis have shown that the overall failure probabilities for both prototypes are overwhelmingly driven by the common cause failures of major hardware components (i.e., battery, controller, motor, and pump). Paying attention to these types of failures during the design, manufacturing, installation, and operations processes will further increase the EAPU system reliability.