Why did GSLV-F10 Mission fail? ISRO's Failure Analysis Committee has come up with the answers

The Mission, it was said, was destined to succeed. But the GSLV-F10 Mission, ISRO's 14th GSLV flight that was launched after six consecutive triumphs, had to be aborted midway on August 12, 2021.

The Mission lifted-off normally from Sriharikota at 5.43 a.m. It was a dream start. The first, strap-on and second stages behaved true to pre-flight predictions. But at the 307th second the on-board computer called off the Mission.

The objective of the launch was to place the Earth Observation Satellite EOS-3 in a geosynchronous orbit in space. Had it been deposited in space, EOS-3 EOS-3 would have provided real-time imaging of large areas of the Earth, monitored natural disasters and would have alerted us to disasters like cyclones.

Initial investigations with the post-flight data conducted immediately after the launch indicated "an anomaly in the Cryogenic Upper Stage".

Now, the Failure Analysis Committee (FAC) consisting of experts from academia and ISRO has submitted its report. The FAC has thoroughly reviewed the flight data along with all data related to the activities ranging from the stage preparation at the launch complex, countdown to lift-off.

FAC observed that a deviation in performance of the Cryogenic Upper Stage (CUS) was observed at 297.3 sec after lift-off due to which the onboard computer aborted the mission at 307 sec. In fact, the ground servicing of the Cryogenic Stage was normal and the required lift-off conditions were achieved.

However, subsequent to lift-off, the Committee observed that "the build-up of pressure in the propellant (Liquid Hydrogen or LH2) tank during the flight was not normal leading to a lower tank pressure at the time of ignition of the engine". This resulted in anomalous operation of the Fuel Booster Turbo Pump (FBTP) mounted inside the LH2 tank which feeds the main turbopump of the engine resulting in insufficient flow of Liquid Hydrogen into the engine thrust chamber.

In 2010 April, during the first flight test of the Cryogenic Upper Stage (CUS) designed and built by ISRO, the FBTP of the cryogenic upper stage had malfunctioned.

An official statement by ISRO said that detailed studies indicated that the most likely reason for the observed reduction in LH2 tank pressure is a leak in the respective Vent and Relief Valve (VRV), which is used for relieving the excess tank pressure during flight. "Computer simulations as well as multiple confirmatory ground tests, closely simulating the conditions in the GSLV-F10 flight, validated the analysis by the FAC," the release said.

The FAC concluded that the lower LH2 tank pressure at the time of CUS engine ignition, caused by the leakage of Vent & Relief Valve (VRV) resulted in the malfunctioning of the Fuel Booster Turbo Pump (FBTP) leading to mission abort command and subsequent failure of the mission.

"The most probable reason for the leakage of VRV valve is attributed to the damage in the soft seal that could have occurred during the valve operations or due to contamination and valve mounting stresses induced under cryogenic temperature conditions," the ISRO release said.

The FAC has submitted comprehensive recommendations to enhance the robustness of the Cryogenic Upper Stage for future GSLV missions, which includes an active LH2 tank pressurization system to be incorporated to ensure sufficient pressure in the LH2 tank at the appropriate time before engine start command, strengthening of Vent & Relief Valve and associated fluid circuits to avoid the possibility of leakage along with the automatic monitoring of additional cryogenic stage parameters for giving lift-off clearance.