Passive thermal control systems involve no mechanical moving parts or fluids. There is no power consumption which ensures its low mass and cost. It is highly reliable due to its design simplicity, implementation and testing. The only drawback of PTCS is its low heat transport capability except for heat pipes when compared with ATCS. <ref>https://en.wikipedia.org/wiki/Spacecraft_thermal_control</ref> <br \>

=== Introduction <ref>https://books.google.co.in/books?isbn=0470501960</ref> ===Thermal insulations consist comprise of low thermal conductivity materialswhich are combined to achieve in a manner such that the system has an even lower system thermal conductivity. In conventionalConventionally, infiber-type, powder-, type and flake-type insulations, throughout an air space, the solid material is finely dispersedthroughout an air space. Such To characterize such systems are characterized by , an effective thermal conductivityis used,which depends on both the thermal conductivity and surface radiative propertiesof the solid material, as well as and also the nature characteristics and volumetric fraction of the void or air or voidspace. A special parameter of the system is its bulk Bulk density (solid mass/total volume)is a special parameter of the system,which depends strongly is very highly dependent on the manner in which of packing of the material is packedmaterials.If small voids or hollow spaces or voids are formed by as a result of bonding or fusing portions of thesolid material, a rigid matrix is will be created. When these spaces are sealed from eachother, the A system is referred to as a cellular insulation. Examples of insulations if such rigid insulationsspaces are foamed sealed from each other. Foamed systems, particularly those made from plastic and glass materialsin particular, are an example of such rigid insulations. Reflectiveinsulations are composed comprise of parallel, multilayered, parallel, thin sheets or foils of which have a highreflectivity, which . These sheets or foils are spaced to reflect radiant energy back to its source. The foil spacingbetween the foils is designed to restrict the air motion of air, and it is evacuated in high-performanceinsulations, the space is evacuated. In all types of insulation, evacuation of the The air inthe void space will reduce reduces the effective thermal conductivity of the systemin all kinds of insulations.It is important to recognize Please note that heat transfer through any of these insulation systemssystemmay include consist of several modes: conduction through the solid materials; conduction or convection through the air in the void spaces; and radiation exchange between thesurfaces of the solid matrix. The effective Effective thermal conductivity accounts for all of