Failure Analysis Committee Identifies Propellant Leak as Cause of GSLV Failure

GSLV EOS-3 booster on its way to the launch pad. (Credit: ISRO)

BENGALURU, India (ISRO PR) — The GSLV-F10/EOS-03 Mission lifted-off normally from Sriharikota on August 12,2021 at 0543 hrs after a smooth countdown of 26 hours. In the flight, the performance of the first stage (GS1), the strap-on stages (L40) and the second stage (GS2) were satisfactory and in accordance with the pre-flight predictions. However, the onboard computer aborted the mission at 307 seconds into the flight leading to mission failure.

Initial investigations with the post-flight data conducted immediately after the launch indicated that an anomaly in the Cryogenic Upper Stage led to the mission abort. A National level Failure Analysis Committee (FAC) consisting of experts from academia & ISRO, was constituted to identify the causes of the anomaly and to recommend corrective actions for future missions. 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.

The Failure Analysis Committee recommended a series of computer simulations & confirmatory ground tests to simulate the conditions very close to the GSLV-F10 flight scenario and validate the findings of the Committee. Subsequently, the respective teams have completed the simulations & ground tests and presented the results, based on which FAC has completed its deliberations and presented its findings and conclusion on March 24, 2022.

The time line of flight events and major observations on the Cryogenic Upper Stage in the GSLV-F10 mission along with all previous GSLV missions, were thoroughly discussed and deliberated. 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.

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.

Detailed studies indicate 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 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 & 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 committee 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 .