Reliability of space-shuttle pressure vessels with random batch effects

In this article we revisit the problem of estimating the joint reliability against failure by stress rupture of a group of fiber-wrapped pressure vess els used on Space-Shuttle missions. The available test data were obtained f rom an experiment conducted at the U.S. Department of Energy Lawrence Liver more Laboratory (LLL) in which scaled-down vessels were subjected to life t esting at four accelerated levels of pressure. We estimate the reliability assuming that both the Shuttle and LLL vessels were chosen at random in a t wo-stage process from an infinite population with spools of fiber as the pr imary sampling unit. Two main objectives of this work are (1) to obtain pra ctical estimates of reliability taking into account random spool effects an d (2) to obtain a realistic assessment of estimation accuracy under the ran dom model. Here, reliability is calculated in terms of a "system" of 22 fib er-wrapped pressure vessels, taking into account typical pressures and expo sure times experienced by Shuttle vessels. Comparisons are made with previo us studies. The main conclusion of this study is that, although point estim ates of reliability are still in the "comfort zone" it is advisable to plan for replacement of the pressure vessels well before the expected lifetime of 100 missions per Shuttle Orbiter. Under a random-spool model, there is s imply not enough information in the LLL data to provide reasonable assuranc e that such replacement would not be necessary.