How ISRO's rockets work: PSLV, GSLV and LVM3 explained

Why does ISRO use different rockets to launch satellites, and what changes when a mission uses PSLV, GSLV or LVM3? The answer starts with the payload and the orbit it needs to reach. Each vehicle combines stages, engines and propellants differently. Looking at these parts together tells you more than the rocket's name or a single figure for how much it can carry.

Getting into orbit takes more than height

NASA explains that reaching orbit takes horizontal speed as well as height. As the launcher rises, it sheds stages and becomes lighter. The lighter upper stage can then keep accelerating. This is why most launch vehicles have several stages rather than staying as one complete rocket throughout the journey.

The payload fairing has a different job. It is the protective enclosure around the payload during atmospheric flight. ISRO says GSLV's fairing improves aerodynamic efficiency and protects the payload from mechanical damage. On a rocket diagram, look for this enclosure, the propulsion stages and the spacecraft inside it as separate parts.

PSLV: four stages and several configurations

The Polar Satellite Launch Vehicle was initially designed for Sun-synchronous and low Earth orbit missions. It can reach more than polar orbits: ISRO also describes missions involving multiple satellites and multiple orbits.

PSLV's four main stages, PS1 to PS4, follow a solid, liquid, solid, liquid sequence. The first and third stages use HTPB-based solid propellant, as do its strap-on boosters. HTPB stands for hydroxyl terminated polybutadiene. It is one ingredient in the propellant mixture.

The second stage, PS2, uses a Vikas liquid engine. Its propellant details depend on which source you read. ISRO's generic PSLV page lists UDMH and N2O4, while its PSLV-C30 mission table lists UH25 and N2O4. UH25 is UDMH blended with 25 per cent hydrazine hydrate; N2O4 is nitrogen tetroxide. UH25 therefore differs from pure UDMH. For a particular flight's exact specification, use its mission table.

The final stage, PS4, uses two liquid engines to place payloads into their intended orbits. Its propellants are monomethyl hydrazine, or MMH, and mixed oxides of nitrogen, or MON. The PSLV-C30 table specifies MON-3. Both engines belong to the same stage.

PSLV variants differ in their solid strap-on boosters: XL has six, QL four, DL two and CA none. CA means core alone and retains the four main stages. PSLV can also carry multiple payloads using adaptors inside the fairing. It launched Chandrayaan-1 in 2008 and the Mars Orbiter spacecraft in 2013; those spacecraft then travelled to the Moon and Mars respectively.

GSLV Mk II: a cryogenic third stage

GSLV here means Geosynchronous Satellite Launch Vehicle Mark II. ISRO developed it mainly to launch communication satellites into a geosynchronous transfer orbit, or GTO, using a cryogenic third stage.

GTO is an intermediate, highly elliptical orbit. ISRO's GSLV description explains that the spacecraft uses its own propulsion at the far end of this orbit to make it circular. The spacecraft therefore completes an important part of the orbital journey using its own propulsion.

GSLV Mk II has three main stages. Its first-stage solid core works with four liquid strap-on boosters. Each booster has a Vikas engine and uses UH25 with N2O4. Its liquid second stage, GS2, also uses one Vikas engine and the same propellant combination. GS2's design came from PSLV's PS2.

The third stage is the indigenous Cryogenic Upper Stage, or CUS. Its CE-7.5 engine uses liquid hydrogen and liquid oxygen. ISRO introduced the indigenous stage in January 2014, from GSLV-D5 onwards. Initially, ISRO used Russian-supplied cryogenic stages.

LVM3: solid boosters, liquid core, cryogenic upper stage

ISRO's LVM3 vehicle page also identifies it as Geosynchronous Satellite Launch Vehicle Mk III. Its heavy-lift configuration has three stages: two solid S200 strap-ons, a liquid L110 core and a cryogenic C25 upper stage. GSLV Mk II has a different arrangement, with a solid core and liquid boosters.

The two S200 boosters provide lift-off thrust and use HTPB-based solid propellant. L110 uses two Vikas engines; ISRO's outreach description gives its propellants as UH25 and N2O4. The C25 upper stage uses liquid hydrogen and liquid oxygen with the CE-20 engine. C25 names the stage, while CE-20 names its engine.

LVM3 also shows how stages can burn at the same time. Both S200 boosters ignite together for lift-off. The liquid core starts later, while the boosters are still firing. ISRO's generic flight profile puts that ignition at about 113 seconds. The timing can vary by mission.

What makes cryogenic propulsion different

For the cryogenic systems used here, liquid hydrogen is the fuel and liquid oxygen is the oxidiser. ISRO gives approximately minus 253°C for hydrogen becoming liquid and minus 183°C for oxygen. These temperatures describe the change to liquid. Combustion temperatures are different. Such extreme cold makes temperature control, structural design and handling difficult.

The benefit is efficiency. ISRO describes this efficiency using a measure called specific impulse. Hydrogen and oxygen cryogenic propulsion has a higher specific impulse than the solid and Earth-storable liquid propellants in ISRO's comparison. This helps it carry more payload. Specific impulse measures efficiency rather than thrust at a given moment: LVM3 still relies on its solid boosters for lift-off.

Read the payload number with its destination

To understand how much a rocket can carry, read the capacity figure alongside its orbit. ISRO's vehicle pages give these figures and conditions:

• PSLV: up to 1,750 kg to a Sun-synchronous polar orbit at 600 km altitude.
• GSLV Mk II: 2,250 kg to GTO, and up to 6,000 kg to low Earth orbit, or LEO.
• LVM3: a four-tonne class to GTO, and 8,000 kg to LEO on a page that describes delivery to low Earth orbits at 600 km altitude.

Compare figures for the same destination: PSLV's polar-orbit figure and a GTO figure describe different journeys. LVM3's four-tonne class can also be exceeded: ISRO reports that LVM3-M5 successfully launched CMS-03 on 2 November 2025, with the satellite weighing about 4,400 kg and destined for GTO.

ISRO's outreach material separately describes a ten-tonne LEO category for LVM3 without the same altitude condition. Read that statement separately from the vehicle page's 8,000 kg figure at 600 km. General specifications describe what a vehicle can do under stated conditions. Use the mission's own specifications for a particular launch.

How to use this when studying a launch

Start with the spacecraft's intended orbit, then trace the vehicle's stages and propellants. Do not count each strap-on as an extra serial stage. Check whether each name refers to a stage or an engine. Finally, note the destination alongside every payload figure. This turns a list of abbreviations into an explanation of what the launcher does and what the spacecraft must do afterwards.

For guided exploration of concepts, Learnacy Labs, currently in beta, includes projectile motion among its examples. Each lab follows an Understand, Explore and Prove flow.