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

  === Multi Layer Insulation (MLI) <ref name ==="STC">Spacecraft Thermal Control Handbook[[FileVolume I:MLI1Fundamental Technologies, David G.png|frame|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.

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 \>

[[File:MLI2.jpg|frame|left]][[File:MLI3.jpg|frame|center]]The simplest MLI construction , in its simplest form, is a layered blanket which is assembled from thin embossed Mylar sheets. Each sheet has, on one side, each with a vacuum-deposited aluminum finish on one side. As a result Because of the embossing, the sheets touch come in contact at only a few points, and thereby minimizing the conductive heat paths between layers are thus minimized. The layers are aluminized have an aluminum finish on one side only so that the Mylar can act somewhat as a somewhat low-conductivity spacer. Higher-A construction which gives higher performance construction is composed of Mylar film metalized (with aluminum or gold) on both surfaces with sides. These have silk or Dacron net as the low-conductance spacers. <br \ref name = "STC"/>Heat transfer through MLI is a combination of radiation, solid conduction, radiation and, under atmospheric conditions, gaseous conduction. These All of these forms of heat transfer are minimized in different ways. Radiative heat transfer is minimized by interposing Interposing as many enclosing reflective surfaces (metalized metallized sheets) as is practical practically possible between the object being insulated and its surroundingsminimizes radiative heat transfer. Solid-One can minimize solid conduction heat transfer is minimized by keeping minimizing the density of the low-conductance spacers between the reflective surfaces as low as possible and making the blanket "fluffy" in order to minimize contact between layers. <ref name = "STC"/> <br \>Because the The heat transfer mechanisms operate simultaneously and interact with each other. Therefore, a useful technique is to refer to derive either an apparent thermal conductivity, <math>K_{eff}</math>, or an effective emittance, ϵ<math>\varepsilon^*</math>, through the blanket. Both values We can be derived experimentally during derive both the values for steady-state heat transfer. <br \>In theoryTheoretically, for a highly evacuated MLI systemssystem, the emittance ϵ <math>\varepsilon</math> for a blanket comprising of N non-contacting layers of having emissivities ϵ1 <math>\varepsilon_1</math> and ϵ2 <math>\varepsilon_2</math> on opposite sides is computed as:<ref name = "STC"/>[[File:Equation55PTC Equn.pnggif|frame|center]]In satellite applications, the MLI will be is full of air at launch time. As the rocket ascendsthrough the atmosphere, this air must should be able to escape without damaging the blanket. This may require Therefore, holes or perforations are required in the layers, even though this reduces their has an associated reduction in effectiveness. <br \>MLI blankets are traditionally constructed with sewing technology. The layers are cut, then stacked on top of each other, and finally sewn together at the edges. Seams and gaps in the insulation are responsible for the cause of most of the heat leakage leakages through MLI blankets. A new method is being developed to use polyetheretherketone (PEEK) tag pins (similar to plastic hooks used to attach price tags to garments) to fix the film layers in place instead of sewing to improve the thermal performance. Traditionally, MLI blankets are sewn together; the multi-layered blanket being held together by stitches<ref>https://en. However, any kind of hole that punches through the layers tends to degrade the overall thermal performance of the blanketwikipedia. Another method, of using tagorg/wiki/Multi-pins - the small nylon "I" looking pins that are used to hook price tags to clothes in stores - to fix the layers in place, has been mentioned in the literature, see paper by R. Hatakenaka, here). That way you don't need to punch as many holes as when you are sewing, and tagging - a few inches between tags - is faster and less error-prone than sewing around the whole perimeter of the blanket. Moreover, the tag-pins allow you to fasten the layers together without compressing them, which reduces stress around the holes. Lastly, the blankets tend to contract in the direction of sewing which might lead them to be to small if not oversized properly. layer_insulation</ref> <br \>In some applications the insulating layers must be grounded, so that they cannot do not build up a charge and arc, causing which cause radio interference. Spacecraft Satellites may also may use MLI as a first line of defense protection against dust impacts. <br \>

The Space Station has A satellite or a space station can have a variety of surface finishes because of the variance difference in thermal requirements from location region to locationregion. Thermal coatings and paints must be compatible with the environment and . They must also be resistant to both radiation and atomic oxygen that they will be constantly exposed to. <br \>

The This proposed technology will is expected to enable efficient thermal control by maintaining a constant temperature heat sink or heat source for a wide range of electronic components in rapidly changing thermal environments. The PCM Phase Change Material panel is being designed as a lightweight and flexible component. Even then, yet having it will have high thermal capacity. Thus, requiring it will require less mass and volume than currently presently used carbon-fiber and aluminum honeycomb composite panels. <ref>https://techport.nasa.gov/view/89663</ref><br \>ExExample:<ref>http: PCM //www.puretemp.com/pcmatters/pcm-briefing: Managing heat in low-orbit satellites 110</ref> <br \>Roccor LLC of Longmont, Colo., is using making use of phase change material in a device that is designed to manage internal heat in low-orbit satellites. A product with containing paraffin wax inside within a flat structure i is used. When the spacecraft is hot, it dumps heat into that the paraffin wax acts as a heat sink and turns that into a liquid — basically a store of energy. The Later on, the heat is later released into the satellite to keep temperatures stable. <br \>

[[File:PCMPCM2.jpgpng|frame|center|Principle of Phase Change material. Image inspired from [https://www.slideshare.net/prashantchikya/phase-changing-material here]]]

== 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