he Indian Space Research Organisation has crossed a significant milestone with the successful developmental flight of the country’s heaviest Geosynchronous Satellite Launch Vehicle, the GSLV Mark-III This is the first time a satellite weighing over 3.1 tonnes has been launched from India to reach the geostationary orbit about 36,000 km from Earth. The Mk-III can launch satellites weighing up to four tonnes, which almost doubles India’s current launch capacity. With communication satellites becoming heavier (up to six tonnes), the capability for larger payloads is vital. This can be done by switching over to electric propulsion for orbit rising and to keep the satellite in the right position and orientation in the orbit through its lifetime (that is, station keeping). The switch-over would reduce the weight of the vehicle as it can do away with nearly two tonnes of propellants and carry heavier satellites. Towards this end, ISRO has started testing electric propulsion in a small way; the South Asia Satellite (GSAT-9) that was launched last month used electric propulsion for station keeping. On Monday, an indigenously developed lithium-ion battery was used for the first time to power the satellite. Another key achievement is the use of an indigenously developed cryogenic stage, which uses liquid oxygen and liquid hydrogen; the 2010 GSLV launch using an indigenous cryogenic stage ended in failure. It can now be said without hesitation that India belongs to the elite club of countries that have mastered cryogenic technology. In the December 2014 experimental flight of the GSLV Mk-III, a passive cryogenic stage was used. Though the cryogenic stage was not meant to be ignited, the launch provided invaluable data on aerodynamic behaviour of the vehicle.

The Mark-III will be operational with the success of one more developmental flight, which is set to take place within a year. This will make India self-reliant in launching heavier satellites, bringing down costs substantially. Till now, heavier communication satellites have been launched on Europe’s Ariane rockets; in fact, ISRO will soon be using Ariane rockets to launch two of its heavier satellites. But as has been the case with lighter satellites, it is likely that other countries will soon turn to ISRO for the launch of heavier satellites at a lower cost. With fewer propulsion stages and, therefore, control systems, the Mk-III is far more reliable than the GSLV and the PSLV. Combined with its ability to carry eight to 10 tonnes into a low Earth orbit, the Mk-III can be considered for human-rating certification (to transport humans) once some design changes are made. Compared with the two-member crew capacity of the GSLV, the Mk-III can carry three astronauts and have more space to carry out experiments. The next developmental flight, therefore, will be crucial.