Closeup of Multi-layer insulation from a satellite. MLI gives many satellites and other thermal insulation handbook pdf probes the appearance of being covered with gold foil. The principle behind MLI is radiation balance.
1, facing away from the sun or other heat sources. 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 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. More layers can be added to reduce the loss further. 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. The sheets are perforated to allow air passage during launch. 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. For compactness, the layers are spaced as close to each other 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 outer layers must be stronger, and are often thicker and stronger plastic, reinforced 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. 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. 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. 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.