Spaceflight Insider

India successfully launches GSAT-6 communications satellite

GSLV-D6 taking off.

GSLV-D6 taking off. Photo Credit: ISRO

The Indian Space Research Organisation (ISRO) successfully launched the country’s GSAT-6 (also known as INSAT-4E) communications satellite to orbit on Thursday, Aug. 27. The mission, designated GSLV-D6, started with a lift-off of a Geosynchronous Satellite Launch Vehicle (GSLV) at 4:52 p.m. local time (7:22 a.m. EDT) from the Second Launch Pad at the Satish Dhawan Space Centre SHAR (SDSC SHAR) located in Sriharikota.

Shortly after the lift-off, the rocket headed east, across the Indian Ocean. The launcher’s Strap-Ons shut down 2 minutes and 29 seconds after the launch. Two seconds later, the first stage of the rocket separated. Next, the payload fairing separated, approximately 3 minutes and 50 seconds into the flight. About one minute later, the second stage shut down and separated from the launcher.

The Cryogenic Upper Stage (CUS) continued to ascend for nearly 12 minutes until its burnout. Then, the GSAT-6 satellite was deployed. The CUS cryogenic stage was loaded with 12.8 metric tons of liquid oxygen and liquid hydrogen, consumed by a single engine that begins its burn in a high-thrust mode.

For this mission, the GSLV launch vehicle was configured with all of its three stages including the CUS. This arrangement is similar to the one that was successfully flown during the previous GSLV-D5 mission in January 2014, which had successfully placed the GSAT-14 satellite into GTO.

The metallic payload fairing of the GSLV-D6 has a diameter of 11.1 ft (3.4 m). The overall length of GSLV-D6 is 161 ft (49 m) with a lift-off mass of 416 metric tons.

The CUS stage that was used on the GSLV-D6 mission is designated as CUS-06. A Cryogenic rocket stage is considered by some to be more efficient and provides more thrust for every ton of propellant it burns compared to solid and Earth-storable liquid propellant rocket stages.

A view of the fully integrated GSLV-D6 from the top of Vehicle Assembly Building.

A view of the fully integrated GSLV-D6 from the top of Vehicle Assembly Building. (Click to enlarge.) Photo Credit: ISRO

The cryogenic stage is a complex system due to its use of propellants at extremely low temperatures and the associated thermal and structural challenges involved with sending it skyward. Oxygen liquefies at –183 degrees C and hydrogen at –253 degrees C. The propellants, at these low temperatures, are pumped using turbo pumps running at around 40,000 rpm.

The main engine and two smaller steering engines of the CUS together provide a nominal thrust of about 73.55 kN in a vacuum. During the flight, the CUS fires for a nominal duration of approximately 720 seconds.

Before the launch, the flight software was loaded into the vehicle’s computers. In the final minutes of the countdown, the payload was switched to internal power and the launch vehicle also transitions to battery power and propellant tank pressurization is started. Shortly before the lift-off, the flight computers assumed control of the countdown to guide the vehicle through the final crucial steps before the blast-off.

Preparations for the mission were initiated earlier in the year with the stacking of the GSLV launch vehicle starting with the assembly of the large Solid Rocket Motor. Next was the attachment of the four boosters which burn for two and a half minutes and actually carry the dead weight of the core stage with them until the entire unit separates as a whole.

Last week, the assembled launch vehicle was rolled to the launch pad. Then, the rocket was connected to the various ground systems for a final week of checkouts. On Thursday, just hours before the launch, final close-outs and hands-on work were performed to prepare the rocket and the launch pad for the blast-off.

The cuboid-shaped GSAT-6, weighing 2.1 metric tons, is an advanced multimedia communication satellite. It will offer a Satellite Digital Multimedia Broadcasting (S-DMB) service across several digital multimedia terminals or consoles that can be used to provide information services to vehicles on the fly and to mobile phones.

The spacecraft will also provide a platform for developing techniques and technologies which could be useful in future satellite-based mobile communications applications. These include demonstrating the use of large, unfurlable antenna on spacecraft, handheld ground terminals, and network management techniques.

The satellite will have five C × S transponders, each with a 9 MHz bandwidth, and five S × C transponders, each with 2.7 MHz bandwidth. It is anticipated that together they will cover the entire country. One of the advanced hardware of the GSAT-6 satellite is its 20 ft (6 m) long S-Band Antenna, the largest antenna ever built by ISRO. The satellite is planned to be operational for nine years.

GSAT-6 is the first Indian communications satellite to use a 70-volt main power bus employing a number of changes in its power and avionics system to create a modern satellite system with increased capabilities.

The satellite is based on the I-2K satellite platform that features two deployable solar arrays generating 3100W of power, batteries for power storage along with avionics and data handling equipment as well as a propulsion unit and navigation equipment.

The spacecraft is the country’s 25th geostationary communications satellite built by ISRO and the 12th in the GSAT series.

The GSLV is an expendable launch system developed to enable India to launch its satellites without dependence on foreign launch service providers. It uses major components that have already been proven by the Polar Satellite Launch Vehicle (PSLV) launchers in the form of the S125/S139 solid rocket booster and the liquid-fueled Vikas engine.

For Thursday’s launch, the GSLV Mk II variant was used. This version of the rocket uses an Indian cryogenic engine – the CE-7.5 – and it is capable of launching 2.5 metric tons into GTO. Previous GSLV vehicles (GSLV Mk I) have used Russian cryogenic engines.

GSLV-D6 is the ninth flight of the GSLV booster.

The first development flight of the GSLV Mk I (GSLV-D1) took place on April 18, 2001. That flight carried GSAT-1 failed to reach the correct orbit. Attempts to save GSAT-1 by using its own propulsion system to maneuver it into the proper orbit were also unsuccessful as it ran out of fuel several thousand miles below geosynchronous orbit.

GSLV-D6 taking off.

GSLV-D6 take-off sequence. (Click to enlarge.) Photo Credit: ISRO


Tomasz Nowakowski is the owner of Astro Watch, one of the premier astronomy and science-related blogs on the internet. Nowakowski reached out to SpaceFlight Insider in an effort to have the two space-related websites collaborate. Nowakowski's generous offer was gratefully received with the two organizations now working to better relay important developments as they pertain to space exploration.

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