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.
AIRS Pulse Tube Coolers Performance Update-Six Years in Space
The Atmospheric Infrared Sounder (AIRS) instrument pulse tube cryocoolers began operation 39 days after launch of the NASA EOS/AQUA on May 4, 2002. Designed with redundant cryocoolers (a primary and a backup), the instrument began operation using a single cooler to bear the load of both the detector and the non-operating, backup cooler. During the early months of the mission, contamination of the cryogenic surfaces led to increased cryocooler loads and the need for periodic decontamination cycles. A change in operating strategy was made in November 2002 to run both coolers simultaneously to both overcome the increased cryogenic contamination load and to allow operation at a much reduced compressor stroke level. This change led to the successful continuous operation of the coolers since November 2002 and the non-interruption of science data collection from the AIRS instrument. After a brief review of the AIRS instrument cryogenic design, this paper presents detailed data on the highly successful continuous operation of the AIRS pulse tube cryocoolers and instrument thermal design over the past six years since the original turn-on in 2002. The data show that the cryogenic contamination reached an equilibrium level after a year of space operation and the cooler stroke required for constant-temperature operation has only increased by 2% since that time. This high level of operational stability not only indicates that the cooler contamination load has not increased, but also that the cryocoolers have maintained near-constant efficiency and that the instrument’s thermal design has presented a near-constant heat rejection and parasitic-load environment. At this time AIRS maintains continuous operation in space providing important scientific data on Earth’s atmospheric parameters.
Performance of the AIRS Pulse Tube Engineering Model Cryocooler
TRW has designed, analyzed, built and tested an engineering model (EM) pulse tube cryocooler for the NASA Atmospheric Infrared Sounder (AIRS) instrument. The cooler provides 1.63 W cooling at 55 K while rejecting to 300 K for a mission life of 50,000 hours in space. In the AIRS instrument, two of these split configuration pulse tube coolers are integrated into a structure that supports the cold head vacuum vessel, the compressors, and the heat rejection system to provide a standby redundant cooler system. The key components of the EM cryocooler are a dual-opposed balanced compressor, an efficient pulse tube cold head, and efficient high-reliability drive electronics. This paper presents the cooler performance test data, delineates system weight, and describes input power, envelope, and vibration and temperature control.