CHENNAI: All eyes at the Sriharikota spaceport will be on the indigenous cryogenic engine which forms the third and upper stage of the GSLV-D6 rocket that will lift off at 4.52pm on Thursday with GSAT-6 , a 2,117kg communication satellite.

This will be the ninth flight of the Geostationary Satellite Launch Vehicle and the third development flight using a cryogenic engine. The success-failure score of the development flights has been 1-1.

Indian Space Research Organisation (Isro) had used up six of the seven cryogenic engines it got decades ago, and now have to master the technology for future launches of heavy satellites. Isro is building a more powerful cryogenic engine to be used in its GSLV-MIII vehicle to carry four-tonne satellites. This is also the designated carrier for Isro’s proposed manned missions.

After the mission readiness review committee and the launch authorization board cleared the launch, the 29-hour countdown for the launch started at 11.52am on Wednesday.

GSLV-D6 vehicle was configured with all its three stages including the CUS similar to the ones successfully flown during the previous GSLV-D5 mission in January 2014, an Isro statement said. GSLV-D5 had successfully placed GSAT-14 satellite carried on-board in the intended GTO very accurately.

The metallic payload fairing of GSLV-D6 has a diameter of 3.4 m. The overall length of GSLV-D6 is 49.1 m with a lift-off mass of 416 t.

The cryogenic upper stage (CUS) being flown in GSLV-D6 is designated as CUS-06. A cryogenic rocket stage is more efficient and provides more thrust for every kilogram of propellant it burns compared to solid and earth-storable liquid propellant rocket stages.

The cryogenic stage is technically a very complex system compared to solid or earth-storable liquid propellant stages due to its use of propellants at extremely low temperatures and the associated thermal and structural challenges. Oxygen liquefies at -183 degree Celsius and Hydrogen at -253 degree Celsius. The propellants, at these low temperatures, are to be pumped using turbo pumps running at around 40,000 rpm.

The main engine and two smaller steering engines of CUS together develop a nominal thrust of 73.55 kN in vacuum. During the flight, CUS fires for a nominal duration of 720 seconds. S-band telemetry and C-band transponders enable GSLV-D6 performance monitoring, tracking, range safety/ flight safety and preliminary orbit determination (POD).

