A 1:5 scaled down model of polar satellite launch vehicle(PSLV) designed and developed by Indian Space research Organisation( ISRO), weighing about 295 tonnes and as tall as a 15 storey building, it has successfully placed in space several Indian satellites as well as satellites belonging to Korea, Belgium and Germany. A highpoint of PSLV is placing of Indian remote sensing satellites in polar orbits at about 600-900 km above the Earth.
Polar satellites go around the Earth in the direction of the North pole to South pole about fourteen to sixteen times a day. During the journey, they collect accurate data about our resources such as agriculture, forestry, underground water potential and mineral deposits.
PSLV is the most reliable workhorse launch vehicle of India and has given consecutive successes in launching many satellites (both Indian and foreign) ever since its first successful launch in 1994. Apart from satellites, the vehicle has successfully launched spacecrafts Chandrayaan-1 and Mars Orbiter spacecraft.
It is 44m in height and 2.8m in diameter. It is the first Indian launch vehicle to have liquid stages. It has four stages that are alternately liquid and solid stages. The uppermost stage is the fourth stage called PS4. This comprises of two earth storable liquid engines.
The third stage PS3 has a solid rocket motor. This provides a larger thrust to the upper stage after the atmospheric phase of the launch.
The second stage PS2 has again an earth storable liquid rocket engine and the first stage PS1 has solid rocket motor with six solid strap on boosters.
The launch vehicle has to carry the payload and place it in some particular orbit. For the payload to orbit successfully, it should be injected with a certain velocity.
Initially the lift off mass will be high. Therefore the first stage should have a solid propellent to provide the required high thrust.
A precise injection is required at the last stage. A solid fuel continues to burn once lit but a liquid fuel flow can be controlled. Hence the last stage has to be a liquid stage. So, the first on e should be a solid stage and the fourth one a liquid stage.
In many missions the cut off of the second stage is based on the target velocity. Hence the second stage too has to be a liquid stage in order to meet this requirement.