Active Versus Standby Redundancy for Improved Cryocooler Reliability in Space

Long-life space cryocooler applications, such as NASA's Atmospheric Infrared Sounder (AIRS) instrument, require that the cryocooler system possess a very high level of reliability. This need for high reliability not only demands that high reliability coolers be used, but often requires that some form of redundancy be incorporated. One common implementation is standby redundancy; however, active redundancy is another equally viable choice. Recently, experience with both types of redundancy has been gained with the AIRS instrument. The AIRS cryocoolers were initially designed and launched as standby redundant units; they were then switched over to active redundancy after six months of in-space operation. This paper examines the performance trade for the two redundancy approaches with explicit treatment of the effect of operational level (off, versus low power, versus high power) on the reliability of the redundant and primary unit. This is accomplished through the derivation and use of a new reliability model that explicitly includes the probability of failure both prior to and after the time of a cooler failure. Also presented, is a discussion of the effect of the two redundancy approaches on the overall space-instrument system including input power level, robustness to transient single-event shutdowns, and robustness to in-space load increases—such as from in-space contamination of cryogenic surfaces. The active redundancy approach is shown to have advantages in terms of improved reliability as well as improved overall system performance.