ISRO’s launch vehicles – UPSC Prelims

What are Launch Vehicles?

The rockets have powerful propulsion systems that generate huge amount of energy required to lift heavy objects like satellites into space, overcoming the gravitational pull of the earth.
Normal rockets, or launch vehicles, become useless after the launch. Rockets’ only job is to take the satellites to their intended orbits.
Rockets have several detachable energy-providing parts.
They burn different kinds of fuels to power the rocket. Once their fuel is exhausted, they detach from the rocket and fall off, often burning off in the atmosphere due to air-friction, and getting destroyed.
Only a small part of the original rocket goes till the intended destination of the satellite. Once the satellite is finally ejected, this last part of the rocket either becomes part of space debris, or once again burns off after falling into the atmosphere.

Earth Orbits and choice of launch vehicles:

For Lower Earth Orbits:
- Several satellites need to be deposited only in the lower earth orbits, which starts from about 180 km from earth’s surface and extends up to 2,000 km.
- Most of the earth-observation satellites, communication satellites, and even the International Space Station, a full-fledged laboratory in space that hosts astronauts, function in this space.
- It takes a smaller amount of energy to take the satellites to low-earth orbits, and accordingly smaller, less powerful, rockets are used for this purpose.
For Higher Orbits:
- There are other satellites which need to go much deeper in space.
- Geostationary satellites, for example, have to be deposited in orbits that are about 36,000 km from earths’ surface.
- The planetary exploration missions also need their rockets to leave them much deeper in space.
- For such space missions, much more powerful rockets are used.
- In general, there is a trade-off between the weight of the satellite that needs to be launched, and the distance it needs to be taken to. The same rocket can take smaller satellites much deeper into space compared to a heavier satellite.

Launch Vehicles of ISRO:

Satellite Launch Vehicle (SLV)	The first rocket developed by ISRO was simply called SLV, or Satellite Launch Vehicle. It was followed by the Augmented Satellite Launch Vehicle or ASLV.
Augmented Satellite Launch Vehicle (ASLV)	SLV and ASLV both could carry small satellites, weighing up to 150 kg, to lower earth orbits. ASLV operated till the early 1990s before PSLV came on the scene.
Polar Satellite Launch Vehicle (PSLV)	PSLV’s first launch was in 1994, and it has been ISRO’s main rocket ever since. Today’s PSLV is vastly improved and several times more powerful than the ones used in the 1990s. It is the first Indian launch vehicle to be equipped with liquid stages. PSLV is the most reliable rocket used by ISRO till date, with 52 of its 54 flights being successful. It successfully launched two spacecraft – Chandrayaan-1 in 2008 and Mars Orbiter Spacecraft in 2013 – that later travelled to Moon and Mars respectively. ISRO currently uses two launch vehicles – PSLV and GSLV (Geosynchronous Satellite Launch Vehicle), but there are lots of different variants of these.
Geosynchronous Satellite Launch Vehicle (GSLV)	GSLV is a much more powerful rocket, meant to carry heavier satellites much deeper into space. Till date, GSLV rockets have carried out 18 missions, of which four ended in failure. It can take 10,000-kg satellites to lower earth orbits. The indigenously developed Cryogenic Upper Stage (CUS), forms the third stage of GSLV Mk II. Mk-III versions have made ISRO entirely self-sufficient for launching its satellites. Before this, it used to depend on the European Arianne launch vehicle to take its heavier satellites into space.
Small Satellite Launch Vehicle (SSLV)	SSLV is targeted at rising global demand for the launch of small and micro-satellites. SSLV is meant to offer cost-effective launch services for satellites up to 500 kg. It is supposed to carry an indigenous earth observation satellite EOS-03 into space.
Reusable Rockets/ Future Rockets	The future rockets are meant to be reusable. Only a small part of the rocket would be destroyed during the mission. The bulk of it would re-enter the earth’s atmosphere and land very much like an airplane, and can be used in future missions. Reusable rockets would cut down on costs and energy, and also reduce space debris, which is becoming a serious problem because of the large number of launches. Fully-reusable rockets are still to be developed, but partially-reusable launch vehicles are already in use. ISRO has also developed a reusable rocket, called RLV-TD (Reusable Launch Vehicle Technology Demonstrator) which has had a successful test flight in 2016.