Saturday’s failure of the GSLV is the third unsuccessful mission of the total seven of this indigenously developed space rocket.
On April 15 this year, the third developmental flight of Geosynchronous Satellite Launch Vehicle (GSLV-D3) primarily for the flight testing of indigenously developed Cryogenic Upper Stage (CUS) could not accomplish the mission objectives.
Like the GSLV (carrying INSAT-4C on board) which failed on July 10, 2006, Saturday’s rocket was also fitted with the Russian cryogenic engine.
Unlike the April 15 mission when ISRO had faced anxious moments before the launch as it was the first time that the rocket was powered by the indigenous cryogenic engine, today’s mission was considered "routine" and ISRO never expected trouble.
The Failure Analysis Committee comprising multi-disciplinary experts constituted by ISRO to look into the April 15 failure concluded that ignition of the CUS Main Engine and two Steering Engines have been confirmed as normal, as observed from the vehicle acceleration and different parameters of CUS measured during the flight.
Vehicle acceleration was comparable with that of earlier GSLV flights up to 2.2 seconds from start of CUS. However, the thrust build-up did not progress as expected due to non-availability of liquid hydrogen (LH2) supply to the thrust chamber of the Main Engine.
This failure is attributed to the anomalous stopping of Fuel Booster Turbo Pump (FBTP). The start-up of FBTP was normal. It reached a maximum speed of 34,800 rpm and continued to function as predicted after the start of CUS. However, the speed of FBTP started dipping after 0.9 seconds and it stopped within the next 0.6 seconds.
Two plausible scenarios have been identified for the failure of FBTP: (a) gripping at one of the seal locations and seizure of rotor and (b) rupture of turbine casing caused probably due to excessive pressure rise and thermal stresses.
The Failure Analysis Committee set up by ISRO after the July 10, 2006, unsuccessful mission, had concluded that the primary cause for the failure was the sudden loss of thrust in one out of the four liquid propellant strap-on stages (S4) immediately after lift-off at 0.2 sec. With only three strap-on stages working, there was significant reduction in the control capability.
However, the vehicle altitude could be controlled till about 50 secs. At the same time, the vehicle reached the transonic regime of flight and the vehicle altitude errors built up to large values, resulting in aerodynamic loads exceeding the design limits, thus leading to break-up of the vehicle.
Simulations and analyses of flight data and verification through calibration tests have led to the conclusion that the propellant regulator in the failed engine had much higher discharge coefficient in its closed condition.
The reason for this could be an inadvertent error in manufacturing, which escaped the subsequent inspection, and acceptance test procedures.