Multi-layer insulation

 Multi-layer insulation, or MLI, is thermal insulation composed of multiple layers of thin sheets. It is mainly intended to reduce losses by thermal radiation. In its most common form, it does not appreciably insulate against other thermal losses such as heat conduction or convection. It is therefore most commonly used on satellites and other applications in vacuum where conduction and convection are not important and radiation dominates. MLI gives many satellites and other space probes the appearance of being covered with gold foil.

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The 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, at 300 K, with an emissivity of 1, facing away from the sun or other heat sources. From the Stefan-Boltzmann law, the this surface will radiate away 460 watts of power. Now imagine 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 layer will cool until it is radiating 230 watts from each side, at which point everything is in balance. The new layer receives 460 watts from the original plate. 230 watts is radiated back to the original plate, and 230 watts to space. The original surface still radiates 460 watts, but gets 230 back from the new layers, for a net loss of 230 watts. So overall, the radiation losses have been reduced by half by adding the additional layer.

More layers can be added to reduce the loss further. Ideally, a N layer blanket reduces the thermal radiation to 1/(N+1) of the original value. So a 9 layer blanket can reduce the thermal losses to just 1/10 of the value of the original surface. The blanket can be further improved by making the side(s) that face the warmer surface(s) highly reflective to thermal radiation.

The layers of MLI can be arbitrarily close to each other, as long as they are not in thermal contact. To reduce weight and blanket thickness, the internal layers are made very thin, but they must be opaque to thermal radiation. Since they don't need much structural strength, these internal layers are usually made of very thin plastic, often 1/4 mil (6 micrometres) thick plastic such as Mylar or Kapton, coated on one side with a thin layer of metal, typically silver. For compactness, the layers are spaced as close as possible, though without touching, since there should be little or no thermal conduction between the layers. 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. The outer layers must be stronger, and are often thicker and stronger plastic, re-enforced with a stronger scrim material such as fiberglass.

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.

MLI blankets are constructed with sewing technology. The layers are cut, stacked on top of each other, and sewn together at the edges. Seams and gaps in the insulation are responsible for most of the heat leakage through MLI blankets.

Additional properties

Spacecraft also may use MLI as a first line of defense against dust impacts. This normally means spacing it a cm or so away from the surface it is insulating. Also, one or more of the layers may be replaced by a mechanically strong material, such as beta cloth.

In some 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 aluminum spacers as opposed to cloth scrim at the points where the blankets are sewn together.

References

• Satellite Thermal Control Handbook, ed. David Gilmore. ISBN 1-884989-00-4. In particular, Chapter 4, Section 3, Multilayer Insulation and Barriers, by Martin Donabedian and David Gilmore.

• Tutorial on temperature control of spacecraft by JPL

• Press release about MLI on Cassini

• Typical specialist article on tests of Cassini's MLI