Multi-layer insulation
Encyclopedia
Multi-layer insulation, or MLI, is thermal insulation
composed of multiple layers of thin sheets often used on spacecraft
. It one of the main items of the spacecraft thermal design, mainly 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 satellite
s 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.
of 1, facing away from the sun or other heat sources. From the Stefan-Boltzmann law, this surface will radiate 460 watts. 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. 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
and area A as
Theoretically, the heat transfer coefficient between two layers with emissivities
and 
, under vacuum, is
where T is the mean of the temperatures (in K) of the two layers and
Wm-2K-4 is the Stefan-Boltzmann Constant. If each layer has the same emissivity 
on both sides, then a stack of N layers placed between two high-emissivity surfaces will have an overall heat transfer coefficient
Clearly, increasing the number of layers and decreasing the emissivity both lower the heat transfer coefficient, which is equivalent to a higher insulation value. In this equation, it is assumed that the temperature difference is small compared to the absolute temperature. In space, where the apparent outside temperature could be 3 K (cosmic background radiation), the exact U value is different.
The layers of MLI can be arbitrarily close to each other, as long as they are not in thermal contact. 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. Since they don't need much structural strength, these internal layers are usually made of very thin plastic, about 6 micrometres (1/4 mil) thick, such as Mylar or Kapton
, coated on one side with a thin layer of metal on both sides, typically silver
or aluminum. 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. A typical insulation blanket has 40 or more 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.
.
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.
Thermal insulation
Thermal insulation is the reduction of the effects of the various processes of heat transfer between objects in thermal contact or in range of radiative influence. Heat transfer is the transfer of thermal energy between objects of differing temperature...
composed of multiple layers of thin sheets often used on spacecraft
Spacecraft
A spacecraft or spaceship is a craft or machine designed for spaceflight. Spacecraft are used for a variety of purposes, including communications, earth observation, meteorology, navigation, planetary exploration and transportation of humans and cargo....
. It one of the main items of the spacecraft thermal design, mainly intended to reduce heat loss by thermal radiation
Thermal radiation
Thermal radiation is electromagnetic radiation generated by the thermal motion of charged particles in matter. All matter with a temperature greater than absolute zero emits thermal radiation....
. In its basic form, it does not appreciably insulate against other thermal losses such as heat conduction
Heat conduction
In heat transfer, conduction is a mode of transfer of energy within and between bodies of matter, due to a temperature gradient. Conduction means collisional and diffusive transfer of kinetic energy of particles of ponderable matter . Conduction takes place in all forms of ponderable matter, viz....
or convection
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....
. It is therefore commonly used on satellite
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
s and other applications in vacuum
Vacuum
In everyday usage, vacuum is a volume of space that is essentially empty of matter, such that its gaseous pressure is much less than atmospheric pressure. The word comes from the Latin term for "empty". A perfect vacuum would be one with no particles in it at all, which is impossible to achieve in...
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.
Function and design
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 emissivityEmissivity
The emissivity of a material is the relative ability of its surface to emit energy by radiation. It is the ratio of energy radiated by a particular material to energy radiated by a black body at the same temperature...
of 1, facing away from the sun or other heat sources. From the Stefan-Boltzmann law, this surface will radiate 460 watts. 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.
Thermal radiation
Thermal radiation is electromagnetic radiation generated by the thermal motion of charged particles in matter. All matter with a temperature greater than absolute zero emits 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
and area A asTheoretically, the heat transfer coefficient between two layers with emissivities
and , under vacuum, iswhere T is the mean of the temperatures (in K) of the two layers and
Wm-2K-4 is the Stefan-Boltzmann Constant. If each layer has the same emissivity on both sides, then a stack of N layers placed between two high-emissivity surfaces will have an overall heat transfer coefficientClearly, increasing the number of layers and decreasing the emissivity both lower the heat transfer coefficient, which is equivalent to a higher insulation value. In this equation, it is assumed that the temperature difference is small compared to the absolute temperature. In space, where the apparent outside temperature could be 3 K (cosmic background radiation), the exact U value is different.
The layers of MLI can be arbitrarily close to each other, as long as they are not in thermal contact. 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. Since they don't need much structural strength, these internal layers are usually made of very thin plastic, about 6 micrometres (1/4 mil) thick, such as Mylar or Kapton
Kapton
Kapton is a polyimide film developed by DuPont which can remain stable in a wide range of temperatures, from -273 to +400 °C...
, coated on one side with a thin layer of metal on both sides, typically silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
or aluminum. 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. A typical insulation blanket has 40 or more 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 clothBeta cloth
Beta cloth is a type of fireproof silica fiber cloth used in the manufacture of Apollo/Skylab A7L space suits and in other specialized applications....
.
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.