Multilayer Insulation

Multi Layer Insulation ( MLI ) is a thermal insulation material (English insulation ), which is mainly used in space travel. It consists of a few to several tens of layers ( multi-layer Sheet ) of metallized plastic films. MLI films protect against radiant heat, but not against loss of heat by convection, which almost does not exist in the relative vacuum.

Principle

A warm body emits heat in the form of thermal radiation from a colder environment. An additional impermeable layer as a barrier reduces the heat transport by radiation according to the Stefan- Boltzmann law. In this case, it is warming up, and half of the radiation is reflected back towards the body in the thermal equilibrium. The other half it emits to the surroundings. At n layers, the radiation loss is reduced, in the ideal case ( planar geometry, and the same degree of emission ) by the factor n 1.

If the reflecting layers you get an additional reduction due to the low emissivity. Of the factor is only once this, not to the power of the number of layers.

Execution

In the simplest case, an MLI of metallised plastic films. The individual layers resemble a rescue blanket. The metallization consists of silver, which assumes a golden color at a thickness of a few atomic layers. For critical applications, it makes use of gold, which has a reflectance of more than 99% in the infrared.

The metal layer acts as a mirror, the resin layer as a mechanical support. The thermal insulation between the mirror layers improves significantly when one considers the films at a distance. A coarse mesh of silk or synthetic material between the films reduces thermal bridges on the few connecting bridges. In a bilayer can be reached values ​​from 0.1 to 0.05 for ε *. They reduce ten layers at 0.01. Even with more than 30 layers, the lower limit is in practice to 0.003.

Area of ​​application

Without special measures spacecraft are exposed to large temperature gradients. It lacks the atmosphere that could cause convection temperature compensation. The heat conduction is limited to the body, which are rigidly connected. In the vacuum of space heat is transmitted almost exclusively by radiation.

As electronic devices require an operating temperature of around 20 ° C, one winds spacecraft often in MLI slide. It protects the spacecraft by re-radiation from cooling and supports the even heat distribution. To the same extent it protects against heating by radiation from the outside.

• Insulation
• Aerospace Engineering