Thermals

Thermal control is a process of energy balance and management in which environmental heating plays a major role. The principal forms of environmental heating on orbit are:

• direct sunlight
• sunlight reflected off Earth (albedo)
• infrared (IR) energy emitted from earth

The thermal control is achieved by balancing the energy emitted by the satellite as IR radiation against the energy dissipated by its internal electrical components plus the energy absorbed from the environment; atmospheric convection is absent in space.
Establishing a thermal design for a satellite is usually a two-part process. The first step is to select a thermal design for the body, or basic enclosures, of the satellite that will serve as a thermal sink for all internal components. The second step is to select thermal designs for various components located both within and outside the satellite body.

Why is thermal control important?

Even though the thermal control system accounts for only about 2-5 % of the total satellite cost and mass, it is absolutely important for its efficient operation (because electronic component have an optimum operation range) and sustainability of the physical structure.
For example, certain payloads with infrared detectors will require an extremely low temperature for operation.
Many components have their lifetimes reduced by sustained high temperatures.
Large temperature differences within the satellite can lead to thermal expansion or contraction, potentially distorting its structure and thereby result in e.g misalignments of optical systems, thus making large temperature differences extremely undesirable.
It is a task of the thermal subsystem to manage the distribution of heat within the satellite so as to ensure that the temperature level is adequate for all phases of a mission (launch, transfer orbit, operation in orbit).The requirements for a thermal-control system usually exist at several levels. Top-level system requirements define allowable temperature margins, testing requirements, and environmental conditions the satellite will encounter. Possible environmental conditions are flux levels for direct solar, Earth-reflected solar, and Earth-emitting energy. Weight and cost restrictions are derived by the satellite project management. Component temperature limits are defined based upon information given by the supplier of the components.

There are two types of thermal control systems:

• The Active Thermal Control System (ATCS) uses a mechanically-pumped fluid to perform heat transfer. Although this approach is more complex, the ATCS is able to handle much greater heat loads and provide a degree of control over how heat loads are managed.
• The Passive Thermal Control System consists of insulation, coatings, heaters, and heat-pipe radiators. Its components generally have few operational requirements and require low maintenance. In addition, PTCS components are also less complex and are easier to implement.

In general LEO satellites use passive thermal control systems because of their low maintenance requirements and less complexity.

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