GSLV-F02/INSAT-4C Mission

The launch of GSLV-F02, carrying the communication satellite, INSAT- 4C, took place from Satish Dhawan Space Centre (SDSC, SHAR) Sriharikota, on July 10, 2006 at 17.38 Hrs IST. This was the second operational flight of GSLV and the fourth in GSLV series. The ground checkout system after confirming the minimum threshold performance of the four liquid
strap-ons (L40s) issued the command for ignition of the S139 solid core booster at T0 and the vehicle lifted off from the launch pad. About 55 sec into the flight,GSLV-F02 started deviating significantly from its nominal flight path resulting in the vehicle breaking up at 62 sec after lift-off. The debris fell into Bay of Bengal.

The preliminary flight data indicated that out of the four liquid strap-on motors, the pressure
buildup for one of the strap-ons (S4) was above the nominal operating conditions and it showed sudden fall in pressure to zero immediately after vehicle lift-off. As a result, the vehicle was flying with only three liquid strap-ons operating along with core solid booster. This resulted in loss of thrust, reduction in control availability and vehicle carrying full inert mass of one of the strap-on motors. In spite of these adverse operating conditions, the vehicle was maintaining its orientation and trying to follow the desired trajectory to the extent possible.

When the vehicle reached about 9 km altitude, due to the higher atmospheric disturbances, proper vehicle orientation could not be maintained. This resulted in excessive angle of attack buildup and consequent breakup of the vehicle.

The 49 metre tall and 414 tonne GSLV comprises three stages. The first stage consists of a solid propellant motor and four liquid propellant strap-ons. The second stage is also a liquid propellant stage and the third is a cryogenic stage. GSLV-F02 launch was the fourth in the series. All the three earlier missions (GSLV-D1, D2 and F01) were completely successful. The mission objective of GSLV-F02 was to place INSAT-4C, an operational satellite into Geosynchronous Transfer Orbit.

The 15 member Failure Analysis Committee (FAC) chaired by Mr K Narayana, former Director of SDSC SHAR, with the participation of experts from academic and research institutions besides ISRO, has reviewed the performance of GSLV-F02 from lift-off to the end of flight. FAC had detailed deliberations for over 100 hours in several sittings and was assisted by eight specialist sub-committees examining the flight data of vehicle subsystems, manufacturing documents, inspection, calibration and test results, etc. Especially, the details related to the realisation of liquid propulsion stage of GSLV were closely scrutinised. Several tests simulating possible failure modes were also conducted to identify the exact cause.

FAC has concluded that the performance of all vehicle subsystems, except one strap-on stage was normal until 56.4 sec. The primary cause for failure was the sudden loss of thrust in one out of 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 attitude could be controlled till about 50 sec. At the same time, the vehicle reached the transonic regime of flight and the vehicle attitude errors built up to large values, resulting in aerodynamic loads exceeding the design limits thus leading to break up of the vehicle.

The thrust of the liquid engines used in the strap-on stages is precisely controlled by a set of regulators. Detailed analyses have indicated that in S4 engine, the thrust control was not effective. Instead of stabilising at 5.85 MPa (Mega Pascal) chamber pressure, it reached 7.11 MPa at 2.8 sec. This was much beyond the design limits and the engine failed at 0.2 sec after lift-off, that is, 5 sec after its ignition.

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. This regulator has functioned satisfactorily in all the previous 50 engines manufactured and tested so far.

The larger flow of propellant led to higher operating pressure in the gas generator (4.7 MPa against design specification of 3.6 MPa). Due to this higher operating pressure of the gas generator, the water flow rate into it got reduced. The combined effect of larger flow of propellants and reduced flow of water led to a very high gas temperature of 1823 K against the design specification of 900 K and pressure of 4.7 MPa against the design specification
of 3.6 MPa. The very high operating pressure and temperature resulted in the structural failure
of the gas generator. The consequent abrupt stopping of the turbopumps that feed propellants at very high pressures to the engines led to the loss of thrust of S4 engine. The water calibration tests conducted simulating the malfunction of the propellant regulator hardware could closely reproduce the flight phenomenon thereby confirming the larger flow area.

FAC has concluded that the design of GSLV is robust and recommended implementation of strict control on fabrication, inspection and acceptance procedures. Among others, FAC has recommended fabrication processes to be critically reviewed and updated. It has recommended for independent inspection of all critical dimensions of components and subassemblies by in-house agencies. Further, long duration hot test on one out of every 20 engines fabricated has been recommended to ensure that production process is under control. In addition, FAC has recommended strengthening the process of clearance of launch during Automatic Launch Sequence phase.

FAC conclusions and recommendations have been accepted and necessary action has been initiated to implement all of them.