The Indian Space Research Organization (ISRO) completed a flight duration test for the cryogenic upper stage (CUS) of its GSLV MkIII in mid-February. The move marked a significant milestone, according to ISRO, as the 640-second test was the last in series of engine and stage development hot tests before the launch vehicle’s first development flight.

ISRO carried out the CUS development tests in two phases. First, engineers subjected the CUS to fluid mock-up, ensuring the readiness of ground support preparation and servicing at the ISRO launch complex. Tests at the ISRO Propulsion Complex in Mahendragiri followed, including those for system validation and flight duration. The entire process—from realization to test completion—took four months, according to ISRO.

The agency says the new CUS, designated C25, is its most powerful. The unit uses 28 tons of propellant, a combination of liquid oxygen and liquid hydrogen stored inside two independent tanks at -195° and -253°C respectively. Engineers used a special multilayer insulation on both the tanks and other CSU structures to help maintain the temperatures.

ISRO’s Liquid Propulsion Systems Center led development efforts for the C25 with support from various system development agencies at other ISRO centers, including Vikram Sarabhai Space Center, ISRO Propulsion Complex and Sathish Dhawan Space Center.

The launch vehicle will be capable of launching four ton-class satellites into Geosynchronous Orbit. It consists of two solid strap-on motors, one earth storable liquid core stage and the C25 cryogenic upper stage. ISRO has scheduled the vehicle’s first development flight for April 2017.