Subject : Satellite Communication and Networks

Topic : Space Segment

The thermal control subsystem is used to maintain the temperature of the various spacecraft subsystem within specified limits

The main sources of heat during the launch phase of the mission are

1. Aerodynamic friction
2. Earth’s albedo
3. Aerodynamic friction near perigee
4. Heating due to motor firing (Apogee Kick Motors [AKM] and Perigee Kick Motors [PKM]).
5. Thermal control can be achieved either by passive or active techniques. Passive techniques are simple and reliable and therefore preferred whenever feasible.
6. Passive control is based on the absorptive (α) and emissive (T) properties of surfaces.
7. Thermal blankets and shields may be used to provide insulation.
8. Radiation mirrors are often used to remove heat from the communication payload.

The active control method involves the use of some mechanical devices to alter or enhance the properties of satellites parts. Active control methods may consist mainly of heaters, movable shutters and heat pipes.

Electric heaters are generally used during eclipses. They are controlled either by heat sensors on board or activated by ground commands through TTC.

Heat pipes which transfer heat from hot points to radiators under small temperature difference by means of successive vaporization and condensation of a fluid at the two extremities of a tube.