PSLV and GSLV (some basic facts)


Full form — PSLV stands for Polar Satellite Launch Vehicle. GSLV stands for Geo-stationary Launch Vehicle.


Weight and applications — Both are meant to carry satellites to space. The weight of a satellite may vary from 0.1kg to 4500 kg. Those with weights 0.1 kg to 1.0 kg are called nano satellites. Like their weights the applications of the satellites may vary greatly. It can be designed to conduct small scientific experiments, observe the ocean or the land surfaces, monitor weather conditions, spy on enemy troop movements, measure forest covers, offer high end communication services like telephone or TV transmissions, carry human beings, goods etc.. So the aim of a satellite can be purely scientific, civilian or military.


How high are the satellites kept in the sky —- Depending on their application they may be kept at distances of 6 to 7 kilometers to very large distances like 35000 kilometers above the earth. (A passenger aircraft like the Boeing 747 flies at a height of 10000 meters above the earth.)


How are the satellites lifted to such heights-— Rockets carry the satellites to the desired heights. The larger the payload (actual load of the satellite) and higher the distance, the stronger is the rocket’s power. The rocket does not travel very long in the sky. Its journey time is just about a few minutes. So within this small frame it has to generate a huge quantity of energy (thrust) by burning fuel at a extremely fast rate. This is the main challenge for the rocket engineers.


What are challenges for the engine designers? .. The main problem is to get a fuel which can release huge amount of energy in a short time. Scientists use solid and liquid propellants. The fuel is burnt in different stages. It can be two or three or even four depending upon the type of rocket. For the last stage of firing cryogenic engines (engines having fuel cooled to extremely low temperatures) are used. It is very difficult to fabricate such engines.

We were importing these engines from Russia till such imports were stopped at the behest of America because we did not sign the CTBT (Comprehensive Test Ban Treaty).With great effort Indian engineers of ISRO have designed and fabricated a cryogenic engine which was tested in the test bed in a laboratory. In the forthcoming GSLV launch scheduled for December the first Indian designed and Indian-built cryogenic engine will be used to fire the fourth and last stage of the rocket.


What is the main difference between the PSLV and GSLV rockets? In lifting power PSLV is less powerful than the GSLV rockets. A PSLV rocket can lift upto 1000kg of load. A GSLV rocket can lift something between 2500 to 4500 kg of load. All communication satellites like the Insat series ones are heavy. They weigh more than 2500 kg. Since India’s own GSLV rockets have not been tried and perfected yet, we get our communication satellites launched by the French Ariane rockets paying a hefty fee.


A PSLV rocket puts the satellite in a polar orbit. It means that the satellite circles the earth in exactly 24 hours. But its path passes the North and the South poles. A person observing the satellite in Bangalore will get to see it only for a few minutes in a day when the satellite crosses the Bangalore sky. As it moves away in its orbit it will become invisible to the observer.


A satellite sent by a GSLV rocket, however, travels in an orbit at a speed same as the rotational speed of the earth. So the position of the satellite, moving with considerable speed, does not change in respect of a particular position or region on earth. Thus an India-oriented satellite will always be looking down at India offering its communication services twenty four hours a day, 365 days a year. Such satellites are mainly used for our TV and telephonic applications.


How was the recent PSLV launch different? Although the satellite was a PSLV one, it put all the seven satellites in a geosynchronous transfer orbit (GTO). It means that the seven satellites will revolve around the earth exactly in 24 hours, and will remain fixed at a particular point above the earth. For doing this the rocket had to lift its load to a height of 35000 kilometers.

The heaviest of the seven satellites is named Metsat. It weighs about 964 kg. It will monitor the water temperature of the Indian Ocean and the Bay of Bengal. Such temperature monitoring will be analyzed to give valuable data to the fishermen.
By putting the Metsat in the PSLV vehicle, the scientists at ISRO have been able to free that much of space (and weight) in the next communication satellite. So the next communication satellite of India will offer more capability through higher transponder availability.


Why is the next GSLV launch so critical for India? The next GSLV rocket will be fitted with the first Indian designed, Indian manufactured cryogenic engine. It has taken long 11 years to build this engine indigenously.


What commercial benefits a successful Indian GSLV launch can bring? ..  If this launch is successful, India will not only launch its own communication satellites but also offer the same services to other countries in commercial basis. So it will be a competitor to NASA, Ariane, the Russian and the Chinese counterparts. Remember we pay 15 lakh rupees for each kilo weight of the satellite we get launched by Ariane.

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