Development of sunshield panels for passive radiant cooler on board meteorological instruments of ISRO

Abstract

Radiant cooler is used to cool the infrared and water vapour detectors of meteorological payload. It consists of patch (cold stage) on which detector is mounted, intermediate cooling stage radiator, vacuum housing to enclose optics of Meteorological payload and sunshields as the third stage. These sunshields are mounted on Vacuum Housing. The thermal control surfaces of sunshield facing patch is chosen to have low solar absorptance (αs) and low emissivity (εIR) to minimize the load on patch. Also to minimize the reflected solar radiation from sun shield surfaces on the patch, specular reflecting surface in solar spectrum wave length range is needed. To achieve such a surface, a specialized process was developed by carrying out a sequence of operations in collaboration with NAL, IIA and LEOS. The precision machining on the aluminium sun shield panels was carried out to achieve good flatness and finish. The machined panels were plated with electroplated nickel and hear treated to relieve the stress. The panels after lapping were electroless nickel plated, polished to optical finish and coated with vacuum evaporated aluminium. The surface characteristics like surface finish, (40 to 60 Ao) solar absorptance (0.11) Emittance (0.03) and solar specularity (98%) were measured. The surface has met the requirement of IR detector cooling to 105 K in coolers onboard INSAT 2A/2B/2E, KALPANA-1, INSAT3A Satellites. The KALPANA-1 on board flight performance of cooler which shows a good match of predicted and observed thermal performance is discussed here. To achieve IR detectors cooling requirement of 95 K for advanced Imager and Sounder Meteorological Payloads, the improved process was developed. It comprises of Precision Machining (Surface finish: <0.1µm, flatness 10µm), Single Point Diamond Turning (surface finish: 100 Ao), Electroless Nickel Plating (100 µm thickness), Optical Polishing (finish: <20 Ao) and improved Aluminium coating (αs:0.078, εIR: 0.022, specularity >99%) The new process was implemented on INSAT 3D Imager, Sounder and Filter Wheel Cooler sunshield (ETM and flight Models). The integrated cooler assembly with sunshields was subjected to the acceptance tests (Electrical Insulation test, Vibration test, Thermal cycling, thermal balance test under simulated space environment and it passed the tests successfully. The satellite is expected to be launched in 2009.