GSLV (Geosynchronous Satellite Launch Vehicle) Programme

The Geosynchronous Satellite Launch Vehicle (GSLV) is an Indian rocket built by ISRO to carry heavier satellites, about 2 to 2.5 tonnes, into high orbits. Its main job is to place communication, weather and navigation satellites into a Geosynchronous Transfer Orbit (GTO), from where they climb to about 36,000 km. Its most important feature is its third stage, which burns liquid hydrogen and liquid oxygen, called a cryogenic stage. The current version is called GSLV Mk II.

Why was it needed?

India's workhorse rocket, the PSLV, is excellent for lighter satellites in lower orbits but cannot lift heavy satellites to high orbits. For years, India paid foreign rockets (mainly Europe's Ariane) to launch its big communication satellites. That cost a lot of money and meant depending on others. GSLV was built so India could launch these satellites on its own.

Where did it come from?

• In 1991, ISRO signed a deal with Russia's Glavkosmos for cryogenic engines and the technology to make them.
• The USA objected, saying the deal broke the Missile Technology Control Regime (MTCR), and imposed sanctions in 1992. Russia then agreed to sell only ready-made cryogenic stages (seven), without the technology.
• ISRO decided in 1993 to build its own cryogenic engine. This took about two decades.
• The first GSLV flight, GSLV-D1, flew on 18 April 2001 with a Russian cryogenic stage.
• The first flight with an Indian cryogenic stage (GSLV-D3, April 2010) failed. The first successful flight with the Indian cryogenic stage came on 5 January 2014 (GSLV-D5, carrying GSAT-14). This made India one of the few countries with its own cryogenic engine technology.
• Later missions include NVS-01 (2023), INSAT-3DS (2024), NVS-02 (2025, India's 100th launch from Sriharikota) and the India-US NISAR earth-observation satellite (30 July 2025, GSLV-F16).

How does it work? The three stages

• First stage: A large solid rocket motor (S139, carrying about 138 tonnes of solid fuel) in the centre, with four liquid-fuel strap-on boosters around it, each using one Vikas engine. All of these fire together at liftoff to give the big push.
• Second stage (GS2): A liquid stage with one Vikas engine.
• Third stage (CUS, Cryogenic Upper Stage): Uses the Indian CE-7.5 engine, burning liquid hydrogen and liquid oxygen, with about 75 kN of thrust.

A simple way to picture it: a rocket is like a relay race. Each stage runs its part and then drops off, making the rocket lighter so the next stage can go faster.

Key numbers

• Height: about 51.7 m; liftoff mass: about 420 tonnes
• Payload to GTO: about 2,250 kg (ISRO figure)
• Launch site: Satish Dhawan Space Centre, Sriharikota (Andhra Pradesh)
• Flight names: "D" for development flights, "F" for operational flights (for example, GSLV-F15, GSLV-F16)

Why is the cryogenic stage special?

Hydrogen and oxygen give the most energy per kilogram of fuel, so a cryogenic stage can push a heavy satellite much higher. But liquid hydrogen must be kept at about minus 253°C and liquid oxygen at about minus 183°C. Handling such cold fuels, stopping leaks and starting the engine in space are very hard. That is why only a few countries have mastered it.

India's launch vehicle family

• PSLV: Four stages (solid and liquid, alternating); best for satellites up to about 1.5 tonnes in polar and Sun-synchronous orbits.
• GSLV Mk II: Three stages with a cryogenic top stage; about 2.25 tonnes to GTO.
• LVM3 (earlier called GSLV Mk III): ISRO's heaviest rocket; about 4 tonnes to GTO. It uses two large S200 solid boosters and the C25 cryogenic stage. It launched Chandrayaan-2, Chandrayaan-3 and will carry Gaganyaan.
• SSLV: Small Satellite Launch Vehicle, for satellites up to about 500 kg in low orbit.

Commonly confused concepts

• GSLV vs LVM3: Despite the old name "GSLV Mk III", LVM3 is a completely different rocket, with different stages and engines. GSLV Mk II uses the CE-7.5 engine; LVM3 uses the more powerful CE-20.
• GTO vs GEO: A rocket usually drops a satellite into a GTO, an oval-shaped orbit. The satellite then uses its own engine to raise itself to the round GEO at 36,000 km. NVS-02 failed at this step, after a good launch.
• Cryogenic vs semi-cryogenic: Cryogenic engines use liquid hydrogen with liquid oxygen. Semi-cryogenic engines use refined kerosene with liquid oxygen; ISRO is developing one for future rockets.

Issues, criticism and the way forward

• Mixed track record: GSLV has had several failures, including GSLV-D3 (2010), GSLV-F06 (2010) and GSLV-F10 (August 2021), when the cryogenic stage did not ignite and the EOS-03 satellite was lost. Because of this, it was once nicknamed the "naughty boy" of ISRO.
• Limited capacity: At about 2.25 tonnes to GTO, GSLV Mk II cannot launch today's heavy communication satellites (4 to 6 tonnes). These go on LVM3 or foreign rockets.
• Low launch rate: GSLV flies only about once or twice a year, which slows down programmes like NavIC that need several satellites.
• Way forward: More reliable cryogenic stages, faster production with Indian industry, and new rockets such as the reusable Next Generation Launch Vehicle (NGLV).

Concepts to Know

• Cryogenic: Related to extremely low temperatures. Cryogenic fuels are gases cooled until they become liquid.
• Strap-on booster: An extra rocket attached to the side of the main rocket to give more push at liftoff; it falls off after its fuel is used.
• Thrust: The pushing force produced by a rocket engine. It is measured in newtons (kN = 1,000 newtons).
• Geosynchronous Transfer Orbit (GTO): An oval orbit whose farthest point is near 36,000 km, used as a "stepping stone" to reach geostationary orbit.
• Missile Technology Control Regime (MTCR): A group of countries that agree to control the export of missile and rocket technology. India joined it in 2016.
• Payload: The satellite or cargo a rocket carries into space.