Multi-layer insulation

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.

[1] For use in cryogenics, wrapped MLI can be installed inside the annulus of vacuum jacketed pipes.

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 300 K (27 °C; 80 °F), 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 1 cm (0.4 in) away from the plate, also with an emissivity of 1.

The blanket can be further improved by making the outside surfaces highly reflective to thermal radiation, which reduces both absorption and emission.

Clearly, increasing the number of layers and decreasing the emissivity both lower the heat transfer coefficient, which is equivalent to a higher insulation value.

The separation space only needs to be minute, which is the function of the extremely thin scrim or polyester 'bridal veil' as shown in the photo.

To reduce weight and blanket thickness, the internal layers are made very thin, but they must be opaque to thermal radiation.

[4] The layers may be embossed or crinkled, so they only touch at a few points, or held apart by a thin cloth mesh, or scrim, which can be seen in the picture above.

[7] Therefore, it is commonly used in liquefied gas tanks (e.g. LNG, LN2, LH2, LO2), cryostats, cryogenic pipelines and superconducting devices.

[9] Methods tend to vary between manufacturers with some MLI blankets being constructed primarily using sewing technology.

Other more recent methods include the use of Computer-aided design and Computer-aided manufacturing technology to weld a precise outline of the final blanket shape using Ultrasonic welding onto a "pack" (the final set of layers before the external "skin" is added by hand.)

In most applications the insulating layers must be grounded, so they cannot build up a charge and arc, causing radio interference.

Since the normal construction results in electrical as well as thermal insulation, these applications may include aluminium spacers as opposed to cloth scrim at the points where the blankets are sewn together.

The conventional sewing methods cause compressions along stitch lines in multilayer insulation blankets.

Hassan Saeed developed a new technology called Spacer Stitching during his research work at ITM, TU Dresden.

The patented technology can avoid compressions along stitch lines in multilayer insulation assemblies.