The thermal control subsystem is essential to maintain the satellite platform within its operating temperature limits for the type of equipment on board the satellite. It also ensures the desirable temperature distribution throughout the satellite structure, which is essential to retain dimensional stability and maintain the alignment of certain critical equipments.
Different pieces of equipment may have different normal operating temperature ranges. As an illustration, the majority of electronic equipment has an operating temperature range of −20oC to +55o C, the batteries used on board the satellite usually have an operating temperature range of 00C to +30− 400C, the solar cells have a relatively much wider permissible operating temperature range of −1900C to +600C and energy dissipating components such as power amplifiers have an operational temperature limits of −100Cto +800C. The thermal control system maintains the satellite platform within its operating temperature limits for the type of equipment on board. The thermal control subsystem also ensures the desired temperature distribution throughout the satellite structure, which is essential to retain dimensional stability and to maintain the alignment of certain critical equipment.
Types of Thermal Control
Thermal control systems are either passive or active.
Passive techniques include having multilayer insulation surfaces, which either absorb or reflect radiation that is produced internally orgenerated by an external source.
They have no moving parts or electrical power input. These techniques include a good layout plan for the equipment, careful selection of materials for structure, use of thermal blankets, coatings, reflectors, insulators, heat sinks and so on.
The external conditions are widely different when the satelliteis facing the sun than they would be during the eclipse periods. Also, some satellites will always be in an orbit where one side of their body is always facing the sun and the otherside is facing the colder side of space. In order to achieve thermal regulation, the satellite is shaded as much as possible from changes of radiation from the sun by using highly reflective coatings and other forms of thermal insulation called thermal blanketing.
Thermal blankets are usually golden in colour (gold is a good IR reflector) and are used to shade the satellite from excessive heating due to sunlight or to retain internal satellite heat to prevent too much cooling.
Active techniques are usually employed to cope with sudden changes in temperature of relatively larger magnitude such as those encountered during an eclipse when the temperature falls considerably. Active systems include remote heat pipes, controlled heaters and mechanical refrigerators. The heaters and refrigerators are controlled either by sensors on board or activated by ground commands.
Heat pipes are highly effective in transferring heat from one location to another. A typical heat pipe of space craft quality has an effective thermal conductivity several thousand times that of copper. Wherever possible, heat producing components such as power amplifiers are mounted on the inner side of the outside wall. The excess heat is transferred to the outside through the thermally conducting heat pipes.
A heat pipe consists of a hermetically sealed tube whose inner surface has a wicking profile.The tube can be of any size or length and can be made to go around corners and bends. The tube is filled with a liquid with a relatively low boiling point. Ammonia is the commonly used liquid. Heat enters the heat pipe at the hot end from where the heat is to be transferred.
This heat causes the liquid at that end to boil. The resulting vapours expand into the pipe carrying the heat. On reaching the cold end, they condense back to the liquid form, releasing heat in the process. This heat then flows out of the pipe to warm that part of the satellite. The condensed fluid then travels back to the starting point of the pipe along the wick. The cycle of vaporization and condensation is repeated. It may be mentioned here that operation of the heat pipe is passively driven only by the heat that is transferred. This continuous cycle of vaporization and condensation transfers large quantities of heat with very low thermal gradients.
Principle of operation of a heat pipe
Thermal designs of spin-stabilized and three-axis stabilized satellites are different from each other. In a spin-stabilized satellite, the main equipments are mounted within the rotating drum. The rotation of the drum enables every equipment to receive some solar energy and to maintain the temperature at around 20–250C for most of the time, except during eclipse periods. To minimize the effect of the eclipse, the drum is isolated from the equipment by heat blankets. The temperature fluctuation is more in the case of three-axis stabilized satellites, as their orientation remains fixed with respect to Earth. Therefore, an insulation blanket is placed around the satellite to maintain the temperature of the equipment inside within desirable limits.
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