Geosynchronous Satellite Launch Vehicle

The third stage was to be procured from Russian company Glavkosmos, including transfer of technology and design details of the engine based on an agreement signed in 1991.[9] Russia backed out of the deal after United States objected to the deal as in violation of the Missile Technology Control Regime (MTCR) in May 1992. As a result, ISRO initiated the Cryogenic Upper Stage Project in April 1994 and began developing its own cryogenic engine.[12] A new agreement was signed with Russia for 7 KVD-1 cryogenic stages and 1 ground mock-up stage with no technology transfer, instead of 5 cryogenic stages along with the technology and design as per the earlier agreement.[13] These engines were used for the initial flights and were named GSLV Mk I.[14]

The first GSLV flight, GSLV-D1 used the L40 stage. Subsequent flights of the GSLV used high pressure engines in the strap-on boosters called the L40H.[16] The GSLV uses four L40H liquid strap-on boosters derived from the L37.5 second stage, which are loaded with 42.6 tons of hypergolic propellants (UDMH and N₂O₄). The propellants are stored in tandem in two independent tanks 2.1 m (6 ft 11 in) diameter. The engine is pump-fed and generates 760 kN (170,000 lb_f) of thrust, with a burn time of 150 seconds.

GSLV-D1 used the S125 stage which contained 125 t (123 long tons; 138 short tons) of solid propellant and had a burn time of 100 seconds. All subsequent launches have used enhanced propellant loaded S139 stage.[16] The S139 stage is 2.8 m in diameter and has a nominal burn time of 100 seconds.[17][18]

Hoisting of the GSLV-F09 second stage during vehicle integration.

The GS2 stage is powered by the Vikas engine. It has a diameter of 2.8 m (9 ft 2 in).[17]

The third stage of the GSLV Mark II is propelled by the Indian CE-7.5 cryogenic rocket engine while the older defunct Mark I is propelled using a Russian made KVD-1. It uses liquid hydrogen (LH2) and liquid oxygen (LOX)[19] The Indian cryogenic engine was built at the Liquid Propulsion Systems Centre[20][21] The engine has a default thrust of 75 kN (17,000 lb_f) but is capable of a maximum thrust of 93.1 kN (20,900 lb_f).

Payload fairing with GSAT-6A being integrated.

The first developmental flight of GSLV Mark I had a 129 tonne (S125) first stage and was capable of launching around 1500 kg into geostationary transfer orbit. The second developmental flight replaced the S125 stage with S139. It used the same solid motor with 138 tonne propellant loading. The chamber pressure in all liquid engines were enhanced, enabling a higher propellant mass and burn time. These improvements allowed GSLV to carry an additional 300 kg of payload.[24][25] The fourth operational flight of GSLV Mark I, GSLV-F06, had a longer third stage called the C15 with 15 tonne propellant loading and also employed a 4 meter diameter payload fairing.[26][27]

Launch of GSLV F11 GSAT-7A from Second Launch Pad of Satish Dhawan Space Centre

This variant uses an Indian cryogenic engine, the CE-7.5, and is capable of launching 2500 kg into geostationary transfer orbit. Previous GSLV vehicles (GSLV Mark I) have used Russian cryogenic engines.[28]

For launches from 2018, a 6% increased thrust version of the Vikas engine was developed. It was demonstrated on 29 March 2018 in the GSAT-6A launch second stage. It was used for the four Vikas engines first stage boosters on future missions.[29]

A 4m diameter Ogive payload fairing was developed and deployed for the first time in the EOS-03 launch on 12 August 2021, although this launch was a failure due to technical anomalies with the Cryogenic Upper Stage. This will allow GSLV vehicles to accommodate larger payloads.[30]