Launch Vehicles and Propulsion

This note covers India's launch vehicles. The orbits they place satellites into, and what those orbits are used for, are covered in Satellite Orbits and Applications.

Three vehicles, three jobs, not one upgrade path

ISRO operates three active launch vehicles, and the natural but wrong assumption is that they form a simple ladder from smallest to largest. They are actually each purpose-built for a different kind of orbit, which is the fact worth holding rather than a ranking by size alone.

PSLV (Polar Satellite Launch Vehicle) was developed specifically to place Indian Remote Sensing satellites into Sun-Synchronous Polar Orbits, and it remains ISRO's most consistently reliable workhorse. It is a four-stage vehicle with alternating solid and liquid propellant stages, and it comes in several configurations distinguished by how many solid strap-on boosters are attached (commonly six, four, two, or a "core alone" version with none), chosen depending on the payload weight and the target orbit. In its standard configuration it can carry roughly 1,750 kg to a 600 km polar Sun-Synchronous Orbit.

GSLV (Geosynchronous Satellite Launch Vehicle) was built to place satellites into Geosynchronous Transfer Orbit, the intermediate orbit satellites destined for a final geostationary position need. It reuses PSLV's proven solid and liquid stages but adds a cryogenic upper stage, using super-cooled liquid hydrogen and liquid oxygen as propellants, which delivers considerably more thrust per unit of propellant mass than the stages below it, precisely what is needed to push a satellite the rest of the way toward geosynchronous altitude.

LVM3 (formerly known as GSLV Mk-III) was developed as the deliberate next step up in payload capability, built to place the roughly 4-tonne class of GSAT communication satellites, too heavy for GSLV, into Geosynchronous Transfer Orbit. It is also the vehicle rated to carry India's human spaceflight programme, since a crewed mission demands a proven vehicle with substantially more lift capacity margin than an uncrewed satellite launch requires.

The propellant staging pattern worth noticing

Across all three, a consistent pattern emerges: earlier stages tend to use solid propellant, valued for its simplicity, reliability, and ability to deliver a large amount of thrust immediately at liftoff, while later, upper stages increasingly use liquid or cryogenic propellant, valued for the finer control and greater efficiency it offers once the vehicle is already moving and heavy initial thrust is no longer the priority. This mirrors the same solid-versus-liquid trade-off already covered for missiles elsewhere on this site, applied to civilian launch vehicles rather than to weapons.

Quick revision points

• PSLV: built for Sun-Synchronous Polar Orbits, ISRO's workhorse for remote sensing satellites. Four stages, alternating solid and liquid, multiple strap-on-booster configurations, roughly 1,750 kg to a 600 km polar SSO.
• GSLV: built for Geosynchronous Transfer Orbit, reuses PSLV's lower stages plus a cryogenic upper stage (liquid hydrogen/liquid oxygen) for the extra thrust efficiency GTO requires.
• LVM3 (formerly GSLV Mk-III): the next step up, carries the ~4-tonne GSAT class to GTO, too heavy for GSLV, and is also the vehicle rated for India's human spaceflight programme.
• The three are not a simple size ladder; each is purpose-built for a distinct kind of orbit and payload class.
• Pattern across all three: solid propellant early stages for immediate high thrust at liftoff, liquid/cryogenic later stages for efficiency once moving, the same solid-versus-liquid trade-off covered for missiles elsewhere on this site.