<title>Performance of the Spirit III cryogenic system</title>

The spatial infrared and imaging telescope (SPIRIT III) was flown aboard the Mid-Course Space Experiment (MSX), a Ballistic Missile Defense Organization sponsored program. SPIRIT III is a long-wave infrared instrument package consisting of a high off-axis rejection telescope, a five- color radiometer, and a six-channel interferometer. End-of- life temperature requirements for the radiometer, interferometer, telescope, and baffle are less than 12 K, less than 11.5 K, less than 20 K, and less than 70 K, respectively. Cooling for the instrument is provided by a single-stage, 9 K solid-hydrogen-filled cryostat. In November 1994, just prior to launch, the cryostat suffered a loss of vacuum in the insulation space, which resulted in extensive damage to it and the instrument. After months of investigation and analysis, it was determined that the primary cause of the vacuum loss was a stress corrosion failure in the cryogenic plumbing. A decision was made to rebuild portions of the cryostat and instrument, but because of time constraints and the way parts of the cryostat parts were bonded, many sections of multilayered (MLI) insulation were not replaced during the rebuild. To measure what impact, if any, the degraded insulation might have on thermal performance, a test using liquid helium was carried out prior to the MSX satellites' launch in April 1996. Once on orbit, temperatures inside the cryostat equilibriated, but the instrument temperatures ran warmer than expected despite the cryogen being near the predicted 9 Kelvin. The cryostat eventually ran out of cryogen one month short of the initial, on-orbit predictions. This paper describes the cryogenic system, illustrates the pre- launch thermal performance predictions, and discuses the on- orbit thermal performance.