GetWiki

{{Short description|Materials science product key to spacecraft thermal management and cryogenics}}File:MultiLayerInsulationCloseup.jpg|right|thumb|Closeup of Multi-layer insulation from a satellite. The metal coated plastic layers and the scrim separator are visible.]]Multi-layer insulation (MLI) is thermal insulation composed of multiple layers of thin sheets and is often used on spacecraft and cryogenics. Also referred to as superinsulation,WEB, Using MLI Blankets under poor vacuum conditions,weblink 2020-11-25, Meyer Tool & Mfg., en, MLI is one of the main items of the spacecraft thermal design, primarily intended to reduce heat loss by thermal radiation. In its basic form, it does not appreciably insulate against other thermal losses such as heat conduction or convection. It is therefore commonly used on satellites and other applications in vacuum where conduction and convection are much less significant and radiation dominates. MLI gives many satellites and other space probes the appearance of being covered with gold foil which is the effect of the amber-coloured Kapton layer deposited over the silver Aluminized mylar.For non-spacecraft applications, MLI works only as part of a vacuum insulation system. For use in cryogenics, wrapped MLI can be installed inside the annulus of vacuum jacketed pipes.WEB, Wrapped MLI {{!, Quest Thermal Group|url=https://www.questthermal.com/products/wrapped-mli|access-date=2020-11-25|website=www.questthermal.com}} MLI may also be combined with advanced vacuum insulation for use in high temperature applications.WEB, 2019-07-31, High temp MLI takes vacuum insulation performance to the next level,weblink 2020-11-25, Advanced Vacuum Insulation for Applications from -270°C to 1000°C, en-US,

Function and design

File:Mars Reconnaissance Orbiter fully assembled.jpg|thumb|The golden areas are MLI blankets on the Mars Reconnaissance OrbiterMars Reconnaissance OrbiterThe principle behind MLI is radiation balance. To see why it works, start with a concrete example - imagine a square meter of a surface in outer space, held at a fixed temperature of {{cvt|300|K}}, with an emissivity of 1, facing away from the sun or other heat sources. From the Stefan–Boltzmann law, this surface will radiate 460 W. Now imagine placing a thin (but opaque) layer {{cvt|1|cm|sigfig=1}} away from the plate, also with an emissivity of 1. This new layer will cool until it is radiating 230 W from each side, at which point everything is in balance. The new layer receives 460 W from the original plate. 230 W is radiated back to the original plate, and 230 W to space. The original surface still radiates 460 W, but gets 230 W back from the new layers, for a net loss of 230 W. So overall, the radiation losses from the surface have been reduced by half by adding the additional layer.(File: A_superconducting_Fault_Current_Limiter_by_Frako-Term.jpg|thumb|The superconducting Fault Current Limiter covered by a MLI blanket)File:Huygens thermal multilayer insulation.jpg|thumb|MLI covering the heat shield of the Huygens probeHuygens probeMore layers can be added to reduce the loss further. The blanket can be further improved by making the outside surfaces highly reflective to thermal radiation, which reduces both absorption and emission. The performance of a layer stack can be quantified in terms of its overall heat transfer coefficient U, which defines the radiative heat flow rate Q between two parallel surfaces with a temperature difference Delta T and area A as Theoretically, the heat transfer coefficient between two layers with emissivities epsilon_1 and epsilon_2, at absolute temperatures T_1 and T_2 under vacuum, is where sigma=5.7times10^{-8} Wm−2K−4 is the Stefan-Boltzmann Constant. If the temperature difference is not too large (|Delta T|