India launched the third flight of its Geosynchronous Satellite Launch Vehicle Mk.III Wednesday, carrying the GSAT-29 satellite into orbit. Liftoff from the Second Launch Pad of the Satish Dhawan Space Centre occurred at 17:08 local time (11:38 UTC).



The Geosynchronous Satellite Launch Vehicle Mk.III, or GSLV Mk.III, is India’s newest and most powerful rocket. After making a suborbital demonstration launch in late 2014, the rocket made its first orbital mission last June when it deployed the GSAT-19 spacecraft.

Wednesday’s launch was designated D2, indicating that it was the rocket’s second developmental launch, however like last year’s flight its payload – GSAT-29 – is a fully operational satellite.

GSAT-29 is a high-throughput telecommunications satellite that will join the Indian National Satellite (INSAT) fleet in geostationary orbit.

The 3,423-kilogram (7,546-pound) spacecraft was built by the Indian Space Research Organisation (ISRO) and is based around the I-3K platform. ISRO will also operate the satellite and conducted Wednesday’s launch that placed it into orbit.

GSAT-29 will position itself in geostationary orbit over the equator, at a longitude of 55 degrees East. The satellite’s communications payload consists of Ku and Ka-band transponders – producing four spot beams in each band with an additional steerable Ka-band beam – which will be used to serve rural and remote parts of India.

In addition to its primary mission, GSAT-29 will demonstrate new technologies that could be incorporated into future satellites. These include Q-band and V-band payloads, operating at higher frequencies that are not widely used by current satellites, and an optical communications experiment, the Optical Communications Technology Demonstrator (OCT).

GSAT-29 also carries the GEO High Resolution Camera (GHRC), a high-resolution imaging payload equipped with a telescope to photograph the Earth from high above, which will investigate the value of such systems for future geostationary missions.

GSAT-29 is designed to operate for at least ten years.

Wednesday’s launch saw ISRO take another step towards becoming self-sufficient for launching its own satellites. Its Polar Satellite Launch Vehicle (PSLV) already provides India with reliable access to space for Earth observation and scientific missions requiring low orbits or smaller satellites, but the country’s ability to launch communications satellites to vital geostationary orbits has been constrained by the modest payload capacity and poor reliability of the Mk.I and Mk.II GSLV rockets.

Compared to the Mk.I and Mk.II, which were developed from the PSLV, the GSLV Mk.III is a completely new rocket. A three-stage vehicle, it consists of a solid-fuelled first stage comprised of two boosters attached on either side of a liquid-fuelled core, or second stage. A cryogenic third stage completes insertion of the payload into orbit.



On its maiden flight in December 2014, GSLV Mk.III successfully demonstrated its first and second stages. The launch, which included an inert third stage, boosted a prototype capsule on a suborbital test flight in support of India’s plans for future crewed space missions. Two and a half years later, in June 2017, the rocket’s next flight carried GSAT-19 to orbit.

On the GSAT-19 launch, GSLV underperformed slightly, delivering the satellite to an orbit whose apogee was about 1,000 kilometers (620 miles, 540 nautical miles) lower than had been planned – representing a shortfall in velocity of about 13 meters per second (43 feet per second). GSAT-19 was able to correct its orbit using a modified series of orbit-raising burns as it climbed to its final operational orbit. No significant impact on its service life was expected.

GSLV Mk.III launches use the Second Launch Pad (SLP) at the Satish Dhawan Space Centre (SDSC) in Sriharikota, a pad which it shares with PSLV and GSLV Mk.II rockets. GSLV Mk.III is integrated vertically atop a mobile launch platform, in the nearby Vehicle Assembly Building (VAB). Once integration is complete the rocket is rolled out to the launch pad – which took place a week ahead of Wednesday’s flight in order to give time for additional testing due to the mission’s developmental nature.

Wednesday’s launch itself lasted sixteen minutes and 43.5 seconds – beginning with ignition of the first stage at the zero-second mark in the countdown and the immediate liftoff of the vehicle. Targeting geosynchronous transfer orbit, Wednesday’s launch took GSLV East over the Bay of Bengal, on an azimuth of 107 degrees.

GSLV Mk.III’s first stage is comprised of two S200 solid rocket motors that burn hydroxyl-terminated polybutadiene (HTPB) propellant. The first stage powered GSLV’s ascent for the first two minutes and 19.16 seconds of flight, after which the two spent motors separated from either side of the rocket.

The L110 second stage is the core of the rocket, with the first stage motors fixed to either side. For Wednesday’s launch the second stage was powered by a pair of High Thrust Vikas Engines (HTVEs), an enhanced version of the Vikas-4B engine that was used for the previous two GSLV Mk.III launches.

This was ISRO’s second launch with HTVE engines – following the use of a single engine on the second stage of a GSLV Mk.II earlier this year. The Vikas series of engines, which are license-built derivatives of the French Viking engine, burn UH-25 propellant (a mixture of unsymmetrical dimethylhydrazine and hydrazine hydrate) oxidized by dinitrogen tetroxide.

The second stage ignited while the first stage was still burning, about 29 seconds before separation, and burned for three minutes and 25.52 seconds. About two minutes after second stage ignition GSLV’s payload fairing separated from around GSAT-29, exposing the satellite to space for the first time.

The fairing is designed to protect the satellite from Earth’s atmosphere during the early stages of ascent, while also maintaining the aerodynamic characteristics of the rocket, but is no longer needed once it reaches space. At fairing separation, GSLV was at an altitude of 116 kilometers (72 miles, 62 nautical miles).

The spent second stage was jettisoned 3.1 seconds after it shut down at the end of its burn. After a further 2.42 seconds the third stage ignited. For the GSLV Mk.III the third stage is a C25, powered by a CE20 engine. This consumes cryogenic propellant – liquid hydrogen and liquid oxygen – and made a single burn lasting eleven minutes and 7.26 seconds.

At third-stage burnout, GSLV was traveling at a velocity of 10.21 kilometers (6.34 miles) per second. Fifteen seconds later GSAT-29 separated from the rocket. The target orbit for spacecraft separation is 190 by 35,975 kilometers (118 by 22,354 miles, 103 by 19,425 nautical miles) inclined at 21.5 degrees to the equator. From this initial geosynchronous transfer orbit (GTO), the satellite will make a series of burns with its liquid apogee motor in order to raise itself into geostationary orbit.

Wednesday’s launch was India’s fifth of 2018 and the only GSLV Mk.III launch planned this year. ISRO has announced an ambitious schedule over the next few months, with a PSLV launch later this month expected to carry the HySIS imaging satellite and a cluster of secondary payloads to orbit.

In December a GSLV Mk.II is slated to launch the GSAT-7A communications satellite, before the next GSLV Mk.III lifts off in January with the Chandrayaan-2 Lunar probe.