Isro will launch its Insat-3DR satellite on a GSLV rocket on Thursday evening.
The Indian space agency's workhorse launch vehicle (in layman's language, rocket) is the PSLV, or Polar Satellite Launch Vehicle, which has had 34 consecutive launch successes so far.
But the GSLV, Geo-synchronous Satellite Launch Vehicle, has been a difficult bet, thanks to the complexity of cryogenic engine stage.
India has had a mixed record of GSLV launches since the first one in April 2001.
Three more successful launches were accomplished with GSLV-D2, F-01 and F-04, using Russia-supplied cryogenic engines.
The first indigenously developed Cryo stage, on the GSLV-D3, suffered a turbo-pump failure and the mission failed.
GSLV-D5, launched in January 2014, was the first successful launch with an indigenous Cryo stage. It carried the 1,982 kg GSAT-14 satellite into a Geosynchronous Transfer Orbit (GTO).
GSLV-D6, carrying GSAT-6, in August 2015, was also a success.
GSLV-F05 will carry a 2,200 kg spacecraft, the INSAT-3DR meteorological satellite, into Geosynchronous orbit.
Isro plans to launch GSLV-F09 in May 2017, with the GSAT-9 satellite.
Chandrayaan-2, India's second unmanned foray to the moon, expected to be launched in 2018, will also be on a GSLV rocket.
1. The GSLV-F05, which is on the launch pad ready to be launched, weighs 416 tons, the solid, liquid and cryogenic fuel included, but all it can carry into space is a 2-ton class satellite.
2. It's a Mk II GSLV. To be able to launch heavier satellites, India needs a GSLV Mk III rocket, which is scheduled to launch for the first time by the end of the year.
3. India's planned manned space mission can also take place only once Isro has proven the Mk III rocket.
4. The Cryo stage fuel is liquid hydrogen, with liquid oxygen as oxidiser.
5. Liquid Oxygen is stored at -183 °C, while liquid Hydrogen is stored at -253 °C in two separate tanks. Imagine this temperature: The most extreme temperature at the top of Mt. Everest is about -60 °C.
6. The Cryogenic turbo-pump has to function precisely at these temperatures to allow liquid oxygen and liquid hydrogen to mix and combust.
7. This is what Isro, or any other space agency launching a rocket with cryogenic engine, has to deal with. That's the big deal about Thursday's GSLV-F05 launch.