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 Cryocooler System Design and Development
JPLs Atmospheric Infrared Sounder (AIRS) instrument is based on a cryogenically cooled infrared spectrometer that uses a pair of pulse tube cryocoolers operating at 55 K to cool the HgCdTe focal plane to 58 K; the instrument also includes cryoradiators at 150 K and 190 K to cool the overall optical bench to 150 K. The cryocooler system design is a key part of the instrument development and focuses heavily on integrating the cryocoolers so as to maximize the performance of the overall instrument. The cryocooler system development activity is a highly collaborative effort involving development contracts with industry and extensive cryocooler characterization testing at JPL. In the first phases of the effort, the overall cryocooler integration approach was developed by Lockheed Martin, and TRW was selected to develop and produce the flight coolers. The selected state-ofthe-art pulse tube cooler has excellent thermal performance, and has a number of attributes particularly light weightthat greatly improve instrument integration. This paper describes the AIRS instrument overall cryogenic system design and the results achieved to date with respect to integration of the TRW pulse tube cryocoolers into this demanding instrument. Results are presented detailing the cryogenic loads on the cooler, the overall cryocooler thermal performance margins achieved, and thermal heatsinking considerations. Mass properties of the cryocooler system, and thermal properties of the developed cold link assembly are also presented.
Contractor Failure: Analysis
The possibility exists for any construction contractor to fail to filfill the contract requirements associated with a facility. Contractor failure occurs when a contractor is unable to perform his/her contractual duties, thus requiring the facility owner to invoke the contract's nonperformance clause. This paper provides aggregate contractor business failure statistics and outlines potential causes for failure. A framework to identify and quantify the associated costs is also introduced. Several case studies are presented to examine the cause and cost of failure and the means by which the facilities were completed. Alternatives available for project completion when contractor failure occurs are discussed. Recommendations to identify and monitor contractors at high risk for failure are outlined as well. This paper provides facility owners with a greater awareness of the possibility of contractor failure, how to quantify costs of contractor failure, and avoidance techniques.