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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 \>
PTCS includes the following components:
==Thermal Insulations ==
=== 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
these processes.
=== Multi Layer Insulation (MLI) <ref name ==="STC">Spacecraft Thermal Control Handbook[[FileVolume I:MLI1Fundamental Technologies, David G.png|thumb|200px]]Gilmore</ref> ===Multilayer Insulation (MLI) is used to minimize temperature gradients throughout the spacecraft satellite and to control heat transfer rates. MLI blankets work in both ways: they prevent both excessive heat loss from a satellite component and also prevent excessive heating from the external environmental fluxes, rocket plumes, and other sources. Most spacecraft flown satellites today are covered with MLI blankets, with cutouts . Cutouts are provided at places for areas where radiators to reject internally generated waste heat. Single-If lesser thermal insulation is required, single layer radiation barriers are sometimes may be used , in place of MLI where less thermal insulation is required, since they are lighter and cheap to manufactureand also lighter. <br \>In practice, simply just by increasing the number of layers past beyond a certain value will not improve , performancecannot be improved. As the no. number of layers increases, radiative heat transfer becomes small compared with to conductive “shorts” between layers and other losses. Taking all these factors into account, about About 25 layers are usually suffice sufficient to obtain a minimum overall conductance value.
==== Working of MLI ====
[[File:MLI4.jpg|thumb|300px| Multi Layer Insulation Close-Up view. Image reproduced from [https://commons.wikimedia.org/wiki/File:MultiLayerInsulationCloseup.jpg here]]]
Lets understand this with an example.
Imagine a square meter of a 1 <math>mm^2</math> surface in outer space, at 400 K, with an . Assume that its emissivity of is 1, . Also assume that it is facing away from the sun (i.e. in the direction opposite to the sun) or other heat sources. From Using the Stefan–Boltzmann law, we can show that this surface will radiate 1452 watts. Now imagine placing if we place a thin (but opaque) layer 1 cm away from the plate, (thermally insulated from it), and also with an emissivity of 1. This new , then this layer will cool until it is radiating 726 watts from each sideboth its sides. Once this point is reached, at which point everything is will be in balance. The new layer receives gets 1452 watts from the original plate. , out of which 726 watts is radiated back to the original plate, and 726 watts is radiated to space. The original surface plate still radiates 1452 watts, but gets 726 back from the new layerslayer, for a which makes the net loss of to be 726 watts. So overall, the radiation losses have been reduced by half halved by adding the additional new layer. <ref>https://www.revolvy.com/main/index.php?s=Multi-layer+insulation&uid=1575</ref> <br \>
MLI is composed of multiple layers of low-emittance films. <br \>
== Surface Coatings and Paints ==
Different types of finishes are used to provide various degrees of thermal control for on-board equipment. Using coatings and paints with different emissivity and absorptivity with different characteristics allows the region or component to either be “warmed” or “cooled” as required. An example of this are radiators that use high emissivity and low absorptivity coatings to help radiate excess heat to space. <br \>
== Phase Change Materials ==
PCM is one of the developing areas in efficient passive thermal control systems which acts as both a heat sink or a heat source based on the demand of the system.
[[File:PCMPCM2.jpgpng|frame|center|Principle of Phase Change material. Image inspired from [https://www.slideshare.net/prashantchikya/phase-changing-material here]]]
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If you are done reading this page, you can go back to [[Mechanical Subsystem]]
== References==* http://pages.erau.edu/~ericksol/projects/issa/thermal.html* https://en.wikipedia.org/wiki/Spacecraft_thermal_control* Spacecraft Thermal Control Handbook Volume I: Fundamental Technologies, David G. Gilmore* http://www.esa.int/Our_Activities/Space_Engineering_Technology/Thermal_Control