ISRO’s PSLV-C61 Rocket Fails Due to Third Stage Anomaly, EOS-09 Satellite Lost
On May 18, 2025, the Indian Space Research Organisation (ISRO) attempted the launch of the PSLV-C61 mission from the Satish Dhawan Space Centre (SDSC). The primary objective was the orbital insertion of the EOS-09 (RISAT-1B) satellite into a 525 km Sun-Synchronous Orbit (SSO). While the first two stages performed within nominal parameters, a critical anomaly occurred during the third-stage (PS3) burn, resulting in a loss of mission. This report examines the technical divergence and the specific failure of the flex nozzle control system.
Mission Profile and Launch Sequence
The PSLV-C61, utilizing the XL configuration (six solid strap-on boosters), lifted off at 05:59 IST. The ascent trajectory was nominal through the following phases:
- PS1 (First Stage): Successful burn and separation.
- PS2 (Second Stage): Liquid-propellant Vikas engine functioned as expected, providing the necessary altitude gain and velocity transition.
Technical Anomaly: The PS3 Failure
The anomaly manifested at T+203 seconds during the ignition of the third stage (PS3), a solid motor utilizing Hydroxyl-Terminated Polybutadiene (HTPB). Telemetry indicated two primary physical symptoms:
- Chamber Pressure Drop: A significant reduction in internal combustion pressure, preventing the stage from generating the required 240 kN of thrust.
- Trajectory Divergence: High-rate gyro data confirmed the vehicle strayed from its planned flight corridor, indicative of a Thrust Vector Control (TVC) failure.
Root Cause Investigation: Flex Nozzle Malfunction
Central to the failure analysis is the Flex Nozzle Control (FNC) system. Unlike rigid nozzles, the PS3 uses a submerged flex nozzle to provide pitch and yaw control.
| Component | Function | Potential Failure Mode |
| Elastomeric Bearing | Provides the flexible pivot for the nozzle. | Structural breach leading to pressure leakage (“Burn-through”). |
| Electro-hydraulic Actuators | Physically tilts the nozzle for steering. | Mechanical seizure or loss of hydraulic pressure. |
| Ablative Liners | Protects the nozzle structure from 3000 K exhaust. | Premature erosion leading to asymmetric thrust or structural collapse. |
Current telemetry suggests a “burn-through” event where extreme thermal loads breached the nozzle interface, causing the observed pressure drop and making the rocket aerodynamically unstable.
Payload Impact: Loss of EOS-09 (RISAT-1B)
The failure resulted in the re-entry and destruction of EOS-09. As a C-band Synthetic Aperture Radar (SAR) satellite, its loss significantly impacts India’s Earth observation capabilities:
- All-Weather Imaging: The loss of SAR capacity hinders disaster monitoring during monsoon seasons.
- National Security: Delays in high-resolution border surveillance.
- Agricultural Intelligence: Interruption of time-series data for crop yield estimation.
Conclusion and Corrective Actions
The PSLV-C61 failure is a rare anomaly in the vehicle’s 30-year history (60/63 success rate). ISRO’s Failure Analysis Committee (FAC) is currently reviewing manufacturing batches of the elastomeric joints used in the flex nozzle. Future missions may see the implementation of Carbon-Carbon (C-C) composite nozzles—a material upgrade designed to withstand higher thermal stresses and prevent the leakage suspected in this mission.
Would you like me to dive deeper into the material science of Carbon-Carbon composites or provide a comparison of the PSLV’s failure rates against other global heavy-lifters?
PSLV C61 Mission Failure Analysis
This video provides a detailed breakdown of the third-stage anomaly and explains how the loss of control led to the mission’s failure.
