THE EFFECT OF MAINTAINABILITY ON THE MISSION RELIABILITY OF TWO-ELEMENT REDUNDANT SPACECRAFT SUBSYSTEMS

摘要

In-flight maintainability of spacecraft subsystems will have a ' great impact on the probability of mission success, especially for space missions of long duration. This Memorandum presents a technique that will assist the designer in assessing the effect of maintenance i on the reliability of spacecraft systems with two-element redundancy, i.e., where the prime system element is backed up by a redundant i secondary. Two types of mission reliability are considered. The first applies particularly to "critical" systems, such as the spacecraft life support system, and here a failure is defined as the event where both the prime s and secondary element are inoperable, even though one or both are still repairable. The second type of reliability applies to the case where temporary inoperability of both elements is allowed, and failure is defined as the event where both prime and secondary elements are inoper-able and nonrepairable.nA key notion in this Memorandum is that, in the event of breakdown, repair is not necessarily assured. An index of repairability, k, is developed, which defines the probability that a randomly occurring breakdown can be repaired. This index is a function of (1) the train¬ing of the astronaut, (2) the availability of spares, (3) the effi¬ciency of the fault-isolation equipment and repair tools, and (4) the maintainability design features of the equipment itself.nThe general structure of the two-element redundant repairable system is described, and the parameters that affect mission reliability, such as failure rate, repair time, and repair probability are defined. This system is general in that the parameter values associated with the primary may be different from the secondary. Four special versions of the general system are also described for which the exact mission relia¬bilities are obtained. It is found that the error from the simplifying assumption of neglecting repair time usually is negligible for system parameters associated with spacecraft systems, and by applying this assumption, a very workable formula for the approximate mission reliability of the general system is derived.' This formula is then applied to four illustrative cases.nDerivations of the general and four special systems for reliabil¬ities of the "critical" type are appended, as are tabulations of mission reliabilities for a broad range of parameter values, and a source list-ing.