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→Working of MLI
The simplest MLI construction is a layered blanket assembled from thin embossed Mylar sheets, each with a vacuum-deposited aluminum finish on one side. As a result of the embossing, the sheets touch at only a few points, and conductive heat paths between layers are thus minimized. The layers are aluminized on one side only so that the Mylar can act somewhat as a low-conductivity spacer. Higher-performance construction is composed of Mylar film metalized (with aluminum or gold) on both surfaces with silk or Dacron net as the low-conductance spacers. <br \>
Heat transfer through MLI is a combination of radiation, solid conduction, and, under atmospheric conditions, gaseous conduction. These forms of heat transfer are minimized in different ways. Radiative heat transfer is minimized by interposing as many enclosing reflective surfaces (metalized sheets) as is practical between the object being insulated and its surroundings. Solid-conduction heat transfer is minimized by keeping the density of the low-conductance spacers between the reflective surfaces as low as possible and making the blanket "fluffy" to minimize contact between layers. <br \>
Because the heat transfer mechanisms operate simultaneously and interact with each other, a useful technique is to refer to either an apparent thermal conductivity, <math>K_{eff}</math>, or an effective emittance, ϵ<math>\varepsilon^*</math>, through the blanket. Both values can be derived experimentally during steady-state heat transfer. <br \>In theory, for highly evacuated MLI systems, the emittance ϵ <math>\varepsilon</math> for a blanket of N non-contacting layers of emissivities ϵ1 <math>\varepsilon_1</math> and ϵ2 <math>\varepsilon_2</math> on opposite sides is computed as:
[[File:PTC Equn.gif|frame|center]]
In satellite applications, the MLI will be full of air at launch time. As the rocket ascends, this air must be able to escape without damaging the blanket. This may require holes or perforations in the layers, even though this reduces their effectiveness. <br \>