The GSLV Developmental-flight 6 launch by the Indian Space Research Organisation on August 27 was three things: the launch of the GSAT 6 satellite for the Indian military, the fifth successful launch of a GSLV rocket, and the second successful test-flight of the indigenous cryogenic upper-stage engine. The satellite is a two-tonne behemoth that’s too heavy for a PSLV rocket, whose maximum payload capacity to the geostationary transfer orbit is 1,410 kg, to heft – so the GSLV. And the cryogenic upper-stage enables the GSLV to lift a heavier payload: 2,500 kg to the geostationary transfer orbit.
But the most important takeaway lies in the big picture. This may be the fifth successful launch of the GSLV out of nine tries but it’s the second successive one. This may be the third successful flight with the cryogenic upper-stage but it’s the second successive one. And both accomplishments signify that ISRO’s scientists have been learning the right lessons from previous failures and that the GSLV is on the road to establishing reliability.
The previous successful test flight of the cryogenic engine was in January 2014, and the test before that in 2010 was a failure. While the PSLV rocket has four stages, of alternating solid and liquid ones, the GSLV Mk rockets that use the engine have three: solid, liquid and cryogenic stages. The solid stage is derived from the S139 booster tried and tested on board the PSLV, and the liquid stage, from the Viking 4 engine built for the Ariane 1 launcher. As a result of the extended legacies, it was easier for ISRO to adapt them for Indian rockets to use. However, the cryogenic engine had to be developed indigenously after the required tech. transfer from the Soviets fell apart in the 1980s due to political reasons.
With two successful flights in two years, the space agency now has reason to believe the engine could be finally past its teething troubles. And despite its intricate engineering, its success makes things simpler for ISRO. Before January 2014, ISRO was also considering a variety of Russian engines to power the GSLV Mk I’s and II’s upper-stage, all to no avail. Now it can focus on perfecting the cryogenic engine for the next big-picture milestone: at least two GSLV launches every year, signifying 4-5 tonnes equipment right there.
Note: This article was updated at 1.42 am on August 29, 2015, to say that the solid stage of the GSLV uses the S139 engine, not Nike-Apache, and that the liquid stage uses a modified Viking 4 engine, not the Vulcain.