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=== Multi Layer Insulation (MLI)<refname = "STC">Spacecraft Thermal Control Handbook
Volume I: Fundamental Technologies, David G. Gilmore
</ref> ===
Imagine a 1 <math>mm^2</math> surface in outer space, at 400 K. Assume that its emissivity 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. Using the Stefan–Boltzmann law, we can show that this surface will radiate 1452 watts. Now 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, then this layer will cool until it is radiating 726 watts from both its sides. Once this point is reached, everything will be in balance. The new layer gets 1452 watts from the plate, out of which 726 watts is radiated back, and 726 watts is radiated to space. The original plate still radiates 1452 watts, but gets 726 back from the new layer, which makes the net loss to be 726 watts. So overall, the radiation losses have been halved by adding the 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 \>
The MLI construction, in its simplest form, is a layered blanket which is assembled from thin embossed Mylar sheets. Each sheet has, on one side, a vacuum-deposited aluminum finish. Because of the embossing, the sheets come in contact at only a few points, thereby minimizing the conductive heat paths between layers. The layers have an aluminum finish on one side only so that the Mylar can act as a somewhat low-conductivity spacer. A construction which gives higher performance is composed of Mylar film metalized (with aluminum or gold) on both sides. These have silk or Dacron net as the low-conductance spacers. <ref name = "ref4STC">https://books.google.co.in/books?isbn=188498911X</ref> <br \>Heat transfer through MLI is a combination of solid conduction, radiation and, under atmospheric conditions, gaseous conduction. All of these forms are minimized in different ways. Interposing as many enclosing reflective surfaces (metallized sheets) as is practically possible between the object being insulated and its surroundings minimizes radiative heat transfer. One can minimize solid conduction heat transfer by minimizing the density of the low-conductance spacers between the reflective surfaces and making the blanket "fluffy" in order to minimize contact between layers. <ref name = "ref4STC"/> <br \>
The heat transfer mechanisms operate simultaneously and interact with each other. Therefore, a useful technique is to derive either an apparent thermal conductivity, <math>K_{eff}</math>, or an effective emittance, <math>\varepsilon^*</math>, through the blanket. We can experimentally derive both the values for steady state heat transfer. <br \>
Theoretically, for a highly evacuated MLI system, the emittance <math>\varepsilon</math> for a blanket comprising of N non-contacting layers having emissivities <math>\varepsilon_1</math> and <math>\varepsilon_2</math> on opposite sides is computed as:<ref name = "STC"/>
[[File:PTC Equn.gif|frame|center]]
In satellite applications, the MLI is full of air at launch time. As the rocket ascends through the atmosphere, this air should be able to escape without damaging the blanket. Therefore, holes or perforations are required in the layers, even though this has an associated reduction in effectiveness. <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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== 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