ISRO achieved a major milestone on October 28, 2006 in the development of Cryogenic Upper Stage (CUS) for GSLV when the stage was hot tested for a duration of 50 seconds at the Liquid Propulsion Systems Centre (LPSC) test complex at Mahendragiri.

ISRO has taken up indigenous development of cryogenic stage with regenerative cooled engine, which produces a thrust of 69.5 kN in vacuum, to replace the existing procured stage from Russia currently used in GSLV flights. As part of this effort, the cryogenic engines have been realised and tested earlier for a cumulative duration of 6,000 sec.

In the stage level hot test, all stage elements like engine, insulated propellant tanks, booster pumps, fill and drain systems, pressurisation systems, gas bottles, igniters, cold gas orientation and stabilisation system, etc., work in unison as per flight standards.

The stage uses the cryogenic propellants namely liquid hydrogen at -250º C and liquid oxygen at -196º C. The turbo pumps used for drawing the propellant from the tanks operate at a very high speed of 42,000 rpm. The materials used to operate at cryogenic temperatures, chilling processes, interplay of various critical engine parameters and a host of other technical aspects make the development of cryogenic stage a very challenging task.

The hot test demonstrated the design adequacy and performance of the integrated flight system. The long duration hot test was attempted on January 19, 2007. However, certain anomalies were observed during the preparatory initial short duration test and hence the long duration test was postponed. After completion of all the qualification tests, the indigenous cryogenic stage is planned to be flight tested in GSLV-D3 flight planned during 2007-08.

Cryogenic Upper Stage test