PROPULSION AND THRUSTERS
The propulsion subsystem is the reaction subsystem used for control carried by the satellite in the geo stationary orbit, so as to generate focus on it whenever needed. The reaction control system has a supply of fuel. It is helping the satellite to move to its assigned position in orbit, to maintain it in that position to maintain the direction of spin axis attitude control.
Propulsion system is composed of three units namely
1. Low Thrust actuators (0.001 to 20N) devoted to attitude control correction that provides an annual velocity increment of the order 50m/s.
2. High Thrust motor (400 to 50,000N) provides the velocity increment required for the geostationary orbit infection at the transfer orbit apogee of the order of 1500m/s.
3. With Space Shuttle launched satellite eventually a perigee kick motor (PKN) which provides the velocity increment required to inject the satellite into the transfer orbit.
Out of the above three units the low thrust actuator are of much importance for Geo Stationary orbit. These are responsible for keeping the satellite in orbit with its perfect attitude till its life end. The low thrust actuators or simply the thrusters can be either chemical ones or electrical ones.
The chemical thrusters have a thrust level between 0.5N and few 1900N. On the other hand the electrical thruster produces thrust only in between 2 and 10mN and therefore chemical thrusters are being used in the communication satellite. It is essential for a thruster that it should provide the specific impulse with the smaller mass because weight on board satellite is limited.
In chemical thrusters glasses are generated at high temperature by chemical reaction of propellers which may be either solid or liquid. The electric thrusters provide thrust by accelerating ionized mass in an electromagnetic or electrostatic field.
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