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ISRO: Seal damage, lower pressure in fuel tank led to GSLV failure in 2021

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The Indian rocket, carrying the country’s first Geo Imaging Satellite (GISAT-1), failed in its mission owing to damage in the soft seal in a critical valve which resulted in lower pressure in the rocket’s liquid hydrogen (LH2) tank, the Indian space agency said on Friday.

According to the Indian Space Research Organisation’s (ISRO) Failure Analysis Committee (FAC), the failure happened when the Geosynchronous Satellite Launch Vehicle (GSLV) rocket’s cryogenic engine was to kick in to take the rocket forward.

The GSLV-F10 is a three-stage/engine rocket. The core of the first stage is fired with solid fuel and the four strap-on motors by liquid fuel. The second is the liquid fuel and the third is the cryogenic engine.

“The FAC concluded that the lower LH2 tank pressure at the time of CUS (cryogenic upper stage) engine ignition, caused by the leakage of Vent and 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 ISRO said.

“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,” it added.

On August 12, 2021, the GSLV-F10 lifted off normally from Sriharikota rocket port at 5.43 a.m 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.

The space agency said the FAC observed that a deviation in performance of the cryogenic engine was observed at 297.3 seconds after lift-off due to which the onboard computer aborted the mission at 307 seconds.

According to it, the ground servicing of the cryogenic stage/engine was normal and the required lift-off conditions were achieved.

However, subsequent to lift-off, the FAC observed that the build-up of pressure in the propellant (liquid hydrogen or LH2) tank during the flight was not normally leading to a lower tank pressure at the time of ignition of the engine, the ISRO said.

“This resulted in anomalous operation of the Fuel Booster Turbo Pump 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,” the ISRO said.

According to the ISRO, detailed studies indicate that the most likely reason for the observed reduction in LH2 tank pressure is a leak in the respective 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 space agency said.

According to the ISRO, the FAC has submitted comprehensive recommendations to enhance the robustness of the cryogenic engine for future GSLV missions.

The recommendations include 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 VRV 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.

Incidentally, in April 2010, the ISRO flew a GSLV with its own cryogenic engine. The mission failed due to the problem in the cryogenic engine’s fuel booster turbopump.

It was then said a component with a higher dimension than what was designed was fixed and the rocket failed.


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