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K.A. Parrish (NASA GSFC), NGST ISIM Team
The thermal design and analyses of NASA's Next Generation Space Telescope (NGST) Yardstick Integrated Science Instrument Module (ISIM) is described. The NGST mission concept of a large aperture optical telescope passively cooled to less than 40 K and instrument detectors passively cooled to below 30 K is unique from any other mission flown to date. Taking advantage of the low temperature sink provided by the observatory's large sunshield, the ISIM utilizes a three-stage multi-radiator configuration to passively cool the near infrared (NIR) detectors. Two radiators dissipate mechansim power and intercept parasitics before they reach the third primary radiator that radiates the detector's power dissipation. Warm and high dissipating electronics are placed external to the ISIM with dedicated radiators. The primary ISIM structure is conductively isolated from the optical telescope assembly (OTA) and warm spacecraft bus. Passive cooling provides a robust, long-life, low-mass, and low-cost approach compared to the use of stored-cryogen or mechanical cooler configuration for the 30 K operational range. If NGST's sensitivity is extended to 12 microns, a mechanical cooler is baselined to cool the mid-infrared (MIR) detectors to approximately 6 K. The presented thermal models and analyses require a high level of detail and precision to accurately predict the thermal loads on the radiators and mechanical cooler. Passive cooling analyses at these low temperatures is extremely sensitive to modeling errors. An extensive effort has been made to capture in the thermal models all of the parasitic and dissipation heat loads that may render passive cooling unfeasible. Illustrated results indicate that the current yardstick ISIM configuration is compatible with passive cooling and has adequate cryogenic thermal design margin. Results are also presented which illustrate the extreme importance of the NGST sunshield's thermal performance to successful ISIM passive cooling.
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