EOS-09 satellite launch: Flex nozzle malfunction may have led to Isro’s failed mission. What is it?

A preliminary probe has revealed that India’s PSLV-C61 mission failed soon after its launch on Sunday morning (May 18) due to a suspected fault in the flex nozzle. The Indian Space Research Organisation (Isro) confirmed that the Polar Satellite Launch Vehicle veered off its trajectory during the critical third-stage propulsion system after it blasted off from Sriharikota’s Satish Dhawan Space Centre at 5:59 am IST.

The 44.5-metre-tall rocket functioned as expected during its first two stages, with the solid-fueled PS1 core and liquid-fueled PS2 stage performing flawlessly. However, 203 seconds into flight, during the PS3 solid motor phase, telemetry revealed an abrupt decline in chamber pressure.

“First two stages performed as expected. In the third stage, we observed less chamber pressure. The mission could not be accomplished,” said Isro Chairman V Narayanan.

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Why flex nozzle is critical?

The PS3 third-stage motor, utilising a flexible bearing nozzle system, steers the rocket during its 114-second burn. These flex nozzles, constructed from layered elastomeric materials, enable precise thrust vector control without complex hydraulic actuators. A malfunction could result in thrust misalignment, causing rapid trajectory deviation.

Isro’s PS3 stage, powered by hydroxyl-terminated polybutadiene (HTPB) fuel, produces 240 kN of thrust under ideal conditions. The EOS-09 satellite, equipped with C-band Synthetic Aperture Radar for all-weather surveillance, failed to reach its planned 525 km sun-synchronous orbit.

This marks the PSLV’s third failure in 63 launches and its first since 2017. Isro has formed a failure analysis committee to examine PS3 stage manufacturing records and test protocols. The incident delays India’s goal of expanding its 52-satellite surveillance constellation.

While the exact cause remains unconfirmed, historical data indicates that 74 per cent of global launch failures from 2018–2023 stemmed from propulsion or stage-separation issues.