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Using multi-layer insulation materials for space vehicle design

Posted: 18 Jul 2012     Print Version  Bookmark and Share

Keywords:Multi-Layer Insulation  Other Services 

Engineers on a constant quest to reduce the weight of space vehicles will look to next-generation materials, replacing metals with fabrics that provide the same thermal and radiation protection while giving the spacecraft a fighting chance of survival outside the Earth's atmosphere. This article will address current materials used, as well as a new generation of high-performing materials.

Most satellites and space vehicles such as the International Space Station (ISS) and the Space Shuttle travel in what is known as Low Earth Orbit (LEO), which is considered to be an orbital band between 100 – 1240 miles above sea level. The most common orbits, however, are between 200 – 250 miles because less energy is required to launch and communications distances are more favourable.

The LEO area is filled with elements we don't find on earth such as atomic oxygen. Atomic oxygen is highly unstable, particularly in this environment, and is a chief source of the degradation of exposed surfaces in space vehicles. At this altitude there is also a high concentration of debris and particles traveling at extremely high speeds relative to that of the vehicle. Even a particle the size of a marble can cause significant damage to a space vehicle.

In addition to the above affects, a spacecraft is also subject to extreme temperature fluctuations as it orbits the Earth. Different parts of the spacecraft may see very hot or cold temperatures during some or all of its "day," which can list minutes or hours depending on its orbit. The sun's radiation combined with the Earth's magnetic fields create a hostile environment for spacecraft systems and instruments, which must still perform flawlessly for the spacecraft's working life.

Thermal, radiant energy and TPS
The sun emits energy in the form of radiation that we on Earth feel as warmth or thermal energy. Our atmosphere protects us from receiving the sun's radiation while retaining enough of it to keep us warm. Spacecrafts operating outside of the atmosphere don't have that protection. They rely on specialised materials referred to as its Thermal Protection System (TPS) to perform this function.

In addition, thermal energy is generated by internal components of a satellite such as batteries, transmitters, computers and other devices. This thermal energy must be emitted so it does not damage the components. Thermal management control systems are an integral part of the design of a satellite. Techniques include passive and active systems. Some active techniques include the use of heaters or refrigerators. Multi-Layer Insulation (MLI) and radiators are other passive techniques.

MLI is a type of thermal protection system (TPS) used on spacecraft, launch vehicles, The Space Shuttle and the International Space Station. MLI insulation is typically found in blanket form and consists of films and fabric constructions ranging from five to forty layers as well as highly specialised tapes used in edge binding and cable wrapping.

The goal of the TPS is to maintain equipment temperatures in very specific ranges during the mission life. Keeping this temperature range allows all electronic equipment, instruments and systems to function in their optimal operating conditions. The most notable example may be the ISS, which has been continuously inhabited since November 2, 2000.

How MLI works
MLI Blankets consist of multiple layers of highly reflective, low emittance or "E" materials. Similar to "Low E" windows in your house, emittance measures a material's ability to reflect solar energy. A material that has low emittance properties will have highly reflective properties like a mirror and would deflect heat and specifically in a space environment, solar radiation.

For optimum insulation performance, successive metalized layers are separated by materials with low-thermal conductivity like Nomex or polyester netting. One layer of the MLI structure may reflect as much as 97 per cent of incident radiation and adding additional layers decreases absorptance between layers exponentially until it is completely dissipated by the time the energy would reach the protected component.

The simplest way is to support and separate layers using crinkled or embossed film. This ensures that adjacent layers are connected at only a few points. To avoid metal-to-metal contacts of high thermal conductivity, the film must be metalized on only one side.

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