Thermal In-Orbit Performance and Evaluation of Multi-Staged Passive Radiant Cooler with Sun-Shield Configuration Variation

Passive multi-staged Radiant coolers with appropriately configured sun-shield are an attractive choice to provide reliable cooling for electronic components onboard spacecrafts to cryogenic temperatures. If the tolerance on the temperature and allowable temperature fluctuations are not severe, these coolers and their variants like V-groove passive cooler offer a good choice, especially at higher orbits like Geosynchronous and Geostationary orbits. The current work focuses on the thermal performance evaluation of multi-staged IMAGER cooler with a rectangular sun-shield configuration, developed by ISRO for the IMAGER payload of INSAT-3D spacecraft placed in a Geostationary orbit around the Earth with a specified detector heat load of 18mW on patch. Evaluation of in-orbit thermal performance introduces complexity in view of handling distributed surface heat loads, diffuse-specular surface for radiant heat exchange, and non-homogeneous surface characteristics, which vary with the specific application requirements. In the current work, performance evaluation of a three-staged radiant cooler is carried out in commercial software SINDA/FLUINT 6.1 for thermal analysis with Thermal Desktop® as modeler and RadCAD® for surface radiation calculations. The geometry of the existing IMAGER cooler is modelled, and the in-orbit performance evaluation is benchmarked with reported data. The model boundary conditions are the solar heat load considering seasonal flipping of the cooler, planetary loads, and the spacecraft base temperature. We systematically present the influence of sun-shield angle variation on the thermal in-orbit performance, which helps the designer to choose an appropriate sun-shield opening angle based on the trade-off between patch temperature decrement and the associated mass penalties involved. The performance of a radiant cooler with an un-symmetrical sun-shield, which typically blocks all the planetary loads falling on the patch, is also presented. With curved sun-shield geometry posing implementational challenges in configuring a radiative surface with desired homogeneous characteristics, the current work presents an improved sun-shield design with a 12-sided sun-shield. In comparison with the original IMAGER cooler design, the do-decagon sun-shield is found to bring down the patch temperature and overall cooler mass by 7.51 and 9.17%, respectively. The projected surface area of the overall cooler is also reduced by 8.14%, making it more compact. The current work is an attempt toward performance assessment with a sun-shield configuration change, considering weight saving and improving the cooler's performance at cryogenic temperatures for the IR sensors as a part of satellites intended for high- altitude orbits.