Thermal design of cubesats with FEM
The goal of a cubesat thermal design is to ensure that all of its components remain within the operating temperature range for as long as possible, and ideally all the time. Heat transfer across the different satellite elements and with the surrounding space is complex and usually, worst-case calculations are used to dimension the thermal elements. FEM (Finite Element Method) is a powerful tool to numerically solve physical problems in complex domains. FEM models bring more accurate results and allow an optimized design in terms of SWaP (Size, Weight and Power).
Heat is transported mainly by conduction within the satellite and by radiation to the outside. There are three sources of heat incoming to a satellite: the Sun, the Sunlight reflected by the Earth (albedo) and the heat emitted by the Earth. All of these sources are directional and depending on the satellite attitude may affect even a single face assuming the satellite has a box-like body.
The satellite attitude is linked to its mission, for example if Earth observation, one side will constantly point downwards and will make a rotation per orbit. For a star or planet observation the attitude may be constant. Finally, if attitude control is lost, the satellite may be tumbling on one or more axes. This has an effect on the heat received and the temperatures found inside.
Consequences of a bad thermal design
Any subsystem out of its operational temperature range may underperform, fail or be partly or completely damaged. Electronic chips will typically work between -40 C and 80 to 125 C (die temperature). Solar sensors have a wider range like -100 to 150 C and more sensitive equipment like cameras with electromechanical parts can have a reduced range like -20 to +40 C.
Most electronic boards will have redundant temperature sensors and can be switched off in case of out’of’limits temperatures. That prevents damage by excessive temperature or malfunctioning at very low temperatures. But even though, excessive temperatures may damage plastic materials, either mechanically or chemically, or even metallic assemblies with thermal stress.
The least of the problems is that the satellite won´t be usable in certain conditions. That will reduce the amount of work that will do during its life in orbit and if, for example, a task is to lapse for a number of orbits, cyclical forced shutdowns may just make it impossible.
But there is more to that. An excessive temperature variation will be cyclical along the orbit and will produce thermal stress in all materials, especially joints of materials with different thermal coefficients. Particularly sensitive are optical instruments if they are to keep tight mechanical tolerances.
High temperatures will also accelerate plastics degradation in an already challenging environment. Outgassing, atomic oxygen and UV degradation accelerate with temperature. Very low temperatures will first affect batteries that are not in general tolerant of cold. Most batteries have self’heating systems, but that means less power will be available for the instruments to run the mission.
In short, an insufficient thermal design of a satellite will shorten its life, reduce its operability, increase the risk of failure and optical misalignments.
Cubesats are actually more challenging than big ones
While cubesats are sometimes overlooked when compared with “serious” satellites in the hundreds of kg and kW of power, the reality is that the power density for cubesats is much higher than for the big ones. This creates the problem of how to efficiently radiate the excess heat with a limited surface. Large satellites employ active cooling systems that simply cannot be used in cubesats for size, cost and power reasons.
Even the traditional MLI (multi layer insulation) is much less effective for cubesats because of the seam effect. The effective emissivity increases (gets worse) as the discontinuity density increases.
The current trend with cubesats is to pack more instruments for more complex missions that will also draw more power. While the first ones used up to 5 W, nowadays there are projects aiming for 100 W.
FEM in cubesat thermal design
There are many FEM software packages available, many of them free and even open source.
