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Thermal Design

by Sarah Varley

When designing anything that goes into space there is a lot of consideration into the thermal environment that the object will encounter. Depending on the mission, temperatures on board can reach exceedingly low levels, and these extreme environments can cause parts or even the whole satellite to shut down. Thermal engineers design and test the satellite in these extreme environments to see if they can work, and the easiest place to start testing is on earth itself.
Now testing the ability of satellite to survive the cold is far cheaper doing it on earth than actually sending one up there and hoping for the best. Also with so much debris in space at the moment it is not really favourable to do so. But scientists can mimic and model the space environment in 3 main ways: 1) computer modelling 2) using cryogenics chambers and 3) high altitude testing. But firstly you are going to need to understand the temperatures that the satellite will encounter on its mission.

The space environment

The actual thermal environment that a satellite can expect to encounter will vary depending on its mission. A satellite heading towards the outer regions of the solar system, like Voyager, will have to be protected against the cold more than a satellite going to Mercury. This is because the main supplier of this solar systems thermal energy is the sun, and the further you are away the colder it will get. It should be noted that in space, due to it being a vacuum, that thermal energy is only really transferred through radiation. The key factors that affect the amount of energy absorbed or radiated by a satellite are size (the bigger it is the more thermal energy it will radiate and absorb), material (different materials act as either insulators or radiators to thermal energy) and finally the time spent in either eclipse or sunlight.
So for this example we will discuss Project BLAST’s mission. If this satellite was to launch it would go into something called a Low Earth Orbit (LEO) which is anything between 160 and 2000km above the Earth’s surface. At this altitude the satellite would orbit the Earth in just a few hours, so the Earth itself would at times eclipse the satellite from the sun numerous times throughout the day. Using the differential equations of thermal radiation it is possible to work out the coldest temperature the satellite will experience. Knowing this temperature a thermal engineer is able to design the satellite with insulation or even radiators to make sure the satellite is able to function.

Computer modelling

From the data collected from understanding the space environment it is then possible to start inputting details of different material, thicknesses to optimise the parameters of the satellite. Mass is the most important thing to optimise on the satellite, lower mass means lower costs, the thermal engineer will optimise the choice of materials with the mass offset. Kapton heaters can sometimes be more beneficial than covering in Kapton foil, due to the mass trade off, however of course in this instance a power requirement is necessary, which leads to yet another trade off.
Computer modelling can be done very simply in an excel spread sheet, using equations to model the temperature the satellite would have both in sunlight and eclipse. Then using Excel solver to optimise the parameters to find the most appropriate mass for the satellite. Other programmes can be used, like solid works and other programmes used by ESA during thermal modelling stages.

High altitude/cryogenic testing

This is another stage that can be completed by satellite designers before is actually getting to see how the satellite perform in extreme environment on earth. The space environment, as stated before, relies mainly on radiation as the transfer mechanism of heat. However unless the satellite is placed into a vacuum this cannot be properly experienced. But there are methods of allowing the satellite to experience the actual low temperatures and this is by either cryogenics or high altitude testing.
In cryogenics chemicals like liquid nitrogen are used to induce a cold environment for the satellite to experience. The chemicals however can cause issues with mechanisms and other structural materials, also it is an expensive process with large satellites. In the case of large satellites mainly will be tested in parts and rely heavily on state of the art thermal modelling to gain accurate results.
The other method of high altitude testing, is the method being used being used by BLAST. This method is favourable for small satellites, mainly cubesats, as a way to allow the satellite experience the cold environment before actually launching. The atmosphere itself has many layers and experiences many different temperatures. The picture below demonstrates this:

Atmospheric temperatures
Graph of atmospheric temperature (http://www.aerospaceweb.org/question/atmosphere/q0112.shtml)

The extreme temperatures of the atmosphere are perfect for testing satellites’ functionality in the thermal environment similar to space. BLAST is expecting to launch into the mid stratosphere, and as evident by the graph will be potentially experiencing temperature of -56 degrees Celsius. This is far more extreme than predicted by the calculations for the satellite in space but with an amount of insulation around BLAST it will be possible to imitate the thermal environment in a cheap and efficient way. This insulation again can be optimised using computer modelling as before.
Without thermal design a satellite can fail without warning, the most vulnerable system normally being the batteries on board, but can encompass many other payloads depending on their purpose. A thermal engineer’s job is making sure the satellite can and will function in any temperature that the mission will encounter without failing due to a little cold.

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