The Indian Space Research Organisation (ISRO) successfully tested its indigenously developed Cryogenic Upper Stage (CUS) for GSLV MkIII on February 17, 2017. The cryogenic stage designated as C25 was tested for a flight duration of 640 seconds at ISRO Propulsion Complex (IPRC) in Mahendragiri. C25 Stage had earlier been tested successfully for 50 seconds on January 25, 2017 to validate all the systems.

Prior to Stage development hot tests, three CE20 engines were realised and two engines were subjected to qualification tests in sea level conditions. This included 800 seconds duration hot test and the third engine identified for flight was tested in high

Altitude conditions for a duration of 25 seconds.

This Stage test is a significant milestone as it is the last in series of engine and stage development hot tests before the first development flight of GSLV MkIII.

The C25 stage is the most powerful upper stage developed by ISRO and uses Liquid Oxygen (LOX) and Liquid Hydrogen (LH2) propellant combination. The stage carries 27.8 tons of propellants loaded in two independent tanks. Development of a cryogenic stage has unique design challenges, with liquid Hydrogen stored at -253 deg C and liquid Oxygen stored at -195 deg C in its tanks. To store these cryogenic fluids, special multi-layer insulation is provided for the tanks and other structures.

The development of the cryogenic stage was led by Liquid Propulsion Systems Centre (LPSC) with support from various System Development Agencies from other ISRO Centres of ISRO, viz., Vikram Sarabhai Space Centre (VSSC), ISRO Propulsion Complex (IPRC) and Sathish Dhawan Space Centre (SDSC), SHAR.

The Cryogenic Stage development tests were carried out in two phases. First, the Stage was subjected to fluid mock-up, wherein the stage preparation and servicing at launch complex in SDSC, SHAR, Shriharikota was completed to prove all the ground facilities established for servicing the cryogenic stage. This was followed by testing of the Stage at ISRO Propulsion Complex, Mahendragiri. From the Stage realisation to completion of testing was accomplished in four months.

The performance of the Stage during the hot test was as predicted. Successful hot test for flight duration qualifies the design of the stage and the robustness of the facilities conceived and established towards its development.

The flight cryogenic stage is in advanced stage of realisation, and forms the upper stage of GSLV MkIII the next generation launch vehicle of ISRO, capable of launching 4 ton class satellites into Geosynchronous Transfer Orbit (GTO). The vehicle consists of two solid strap-ons (S200) motors, one earth storable liquid core stage (L110) and the Cryogenic Upper Stage (C25).

The GSLV MkIII vehicle integration activities are in progress at SDSC, SHAR for its first development flight (GSLV MkIII-D1) targeted for April 2017.