Thermals
Spacecraft thermal control is a process of energy management in which environmental heating plays a major role. The principal forms of environmental heating in orbit are:
- direct sunlight
- sunlight reflected off earth (albedo)
- infrared (IR) energy emitted from earth
The overall thermal control of a satellite in orbit is usually achieved by balancing the energy emitted by the spacecraft 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 spacecraft is usually a two-part process. The
first step is to select a thermal design for the body, or basic enclosures, of the
spacecraft 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 spacecraft body.
Why is thermal control important?
Even though the thermal control system accounts for only about 2-5 % of the total spacecraft cost and mass, it is absolutely essential for both the physical integrity of the satellite and for its efficient operation because electronic equipment have their optimum performance within a certain temperature range.
The satellite's payload will dictate its operating range. Some instruments with infrared detectors for example require extremely low temperatures for cooling.
Many components have their lifetimes reduced by sustained high temperatures.
Large temperature differences within the satellite are also undesirable because they can lead to thermal expansion or contraction, potentially distorting its structure and thereby result in e.g misalignments of optical systems.
It is a task of the thermal engineer 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 spacecraft 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 spacecraft 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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