How does ISRO place satellites into space and what launch vehicles does it use?

On February 14, the Indian Space Research Organisation (ISRO) launched the year’s first space mission — an earth observation satellite, EOS-04, along with two other smaller satellites. The space research agency used a PSLV (Polar Satellite Launch Vehicle) rocket to launch the satellites. This was the PSLV rocket’s 54th flight.

SATELLITES AND LAUNCH VEHICLES

Satellites don’t reach space on their own, but have to be carried by launch vehicles, or rockets, such as the PSLV. The rockets use powerful propulsion systems generating huge amounts of energy that overcome the Earth’s gravitational pull to lift heavy objects such as satellites into space.

The payloads, or satellites, sit inside the rocket before being ejected once they reach near the intended orbit. Most satellites have small propulsion systems to carry small amounts of fuel as they encounter little drag, or force, in outer space.

Satellites and rockets are both spacecraft — a generic term used for any object sent into space.

Satellites carry instruments for scientific work in space. Their operational life can extend up to decades. The rockets, or launch vehicles, on the other hand, become useless after launch. Their sole job is to take the satellites to their intended orbit.

Rockets have several energy-providing but detachable parts. These parts burn different kinds of fuels and power the rocket. Once the fuel is exhausted, the parts detach from the rocket. Only a small part of the original rocket goes to the satellite’s intended destination. Once the satellite is ejected, this last part either becomes part of space debris or burns off after falling into the atmosphere.

LAUNCH VEHICLE TYPES

PSLVs are not the only launch vehicle ISRO uses. The difference between the various kinds of launch vehicles is mainly in the power generated, the weight carrying capacity, and the distance they can travel into space. Everything boils down to the energy generated, which is a function of engine and fuel efficiency.

Several satellites need to be deposited in the lower-Earth orbits, beginning from around 180 km from the surface and extending up to 2,000 km. Most communication satellites, Earth-observation satellites, and even the International Space Station — a full-fledged space that hosts astronauts — function in this space.

Smaller, less powerful rockets that expend smaller amount of energy take the satellites to low-Earth orbits.

However, some satellites are required to go deeper into space. For example, geostationary satellites have to be deposited in orbits about 36,000 km from the Earth’s surface. Planetary exploration missions also need rockets to leave them deeper in space. For such missions, more powerful rockets are used.

In general, the trade-off is between the satellite’s weight and the distance that needs to be covered. The same rocket can deposit a smaller satellite deeper into space compared to a heavier satellite.

ISRO LAUNCH VEHICLES

ISRO uses two launch vehicles at present – the PSLV and the GLSV (Geosynchronous Satellite Launch Vehicle). But these also have several variants. The PSLV is ISRO’s most reliable rocket, with 52 successful launches out of 54 attempts. The GSLVs are much more powerful rocket and meant to carry heavier satellites deeper into space. Till date, ISRO has used 18 GSLV rockets for missions — four of these ended in failure.

ISRO’s first rocket was simply called SLV, or satellite launch vehicle. It followed it up with Augmented Satellite Launch Vehicle or ASLV. These two rockets could carry small satellites that weighed up to 150 kg to lower-Earth orbits. ISRO used the ASLV till the early 1990s before the PSLV, which was first launched in 1994 and has since become the space research agency’s main rocket. Today’s PSLV is vastly improved and more powerful than the ones used in the 1990s.

ISRO used the GSLV for the Chandrayaan-2 mission, and plans to use it for the Gaganyaan manned space mission. Its Mk-III version can carry satellites that weigh up to 4,000 kg to the geosynchronous transfer orbit, nearly 36,000 km from the Earth’s surface. It can also take 10,000-kg satellites to lower-Earth orbit. The Mk-III version has made ISRO self-sufficient for satellite launches. Prior to this, it had to depend on the European Arianne launch vehicle to transport heavier satellites into space.

The Indian space research agency has also developed a launch vehicle particularly for small and micro-satellites — Small Satellite Launch Vehicle, or SSLV — aimed at rising global demand for such satellite launches. Through the SSLVs, ISRO aims to offer cost-effective launch services for satellites weighing up to 500 kg. ISRO’s first SSLV launch is scheduled for March, carrying the indigenous Earth observation satellite EOS-03 into space.

REUSABLE ROCKETS

Rockets in the future are meant to become reusable. A small part of the rocket will be destroyed during the mission, with the bulk of it re-entering the atmosphere and landing like an airplane. These can then be used for future missions. Reusable rockets will reduce cost and energy. It will also cut down space debris, which is becoming a problem due to the large number of launches.

While fully-reusable rockets are yet to be developed, partially-reusable launch vehicles are in use already. ISRO has developed a reusable rocket, RLV-TD (Reusable Launch Vehicle Technology Demonstrator), which was successfully tested in 2016.