On May 18, 2025, the Indian Space Research Organisation (ISRO) launched its 101st space mission—PSLV-C61—from Sriharikota. The goal was to place the EOS-09 Earth observation satellite into orbit. While the rocket’s first and second stages performed well, a problem in the third stage caused the mission to fail. ISRO later confirmed that the chamber pressure dropped in the third stage, which prevented the rocket from delivering the satellite to its planned orbit.
Let’s understand what may have happened—in simple, easy-to-follow language.
What Is The PSLV, And What Does The Third Stage Do?
The Polar Satellite Launch Vehicle (PSLV) is one of ISRO’s most trusted rockets. It has four stages that work one after another to carry a satellite into space.
The third stage, called PS3, runs on solid fuel—a packed chemical mix that burns to push the rocket further upward. The chamber pressure is the pressure inside the motor when the fuel burns. It must stay high to generate enough thrust (push). If this pressure drops, the motor can’t produce the power needed to keep going—and the mission fails.
What Went Wrong in PSLV-C61?
ISRO confirmed that the third stage failed because the chamber pressure dropped unexpectedly. This means the motor didn’t push hard enough to carry EOS-09 into its correct orbit. The exact cause is still under investigation, but here are six possible reasons, explained with simple examples:
Problem With Solid Fuel
If the fuel had cracks, air bubbles, or was packed unevenly, it might not have burned properly. Think of a Diwali rocket with bad gunpowder—it just fizzles out instead of flying. Leak in the Motor Case
The motor is like a strong container. If there’s a crack or hole, hot gases escape, dropping the pressure. It’s like a pressure cooker with a small hole—the steam leaks and the food won’t cook well.
Nozzle Malfunction
The nozzle directs the gases out of the motor. If it’s damaged or blocked, pressure builds up poorly. Imagine a water hose with a bad nozzle—the water doesn’t spray forcefully.
Ignition or Burning Fault
The fuel must burn evenly after ignition. If it doesn’t, pressure drops. Like trying to light wet firewood—it starts but doesn’t stay lit.
Vibration Or Structural Stress
Rockets shake a lot. Excess vibration could have caused tiny breaks in the motor or nozzle. Like a car tyre bursting after hitting too many potholes.
Missed Defects In Testing
If a problem wasn’t found during pre-launch tests, it might show up mid-flight. Just like a phone that looks fine but has a hidden fault not caught at the factory.
Why This Failure Matters
The third stage is critical—it gives the final push to get the satellite into orbit. If this stage fails, the rocket doesn’t reach the right height or speed. In this case, EOS-09, meant for Earth observation tasks like city planning, agriculture, and national security, was lost.
How Often Has PSLV Failed?
PSLV is very reliable, with only three failures in over 60 missions:
1993 – PSLV-D1: Software and stage separation issue.
2017 – PSLV-C39: Heat shield failed to open.
2025 – PSLV-C61: Pressure drop in third-stage motor.
Although ISRO’s GSLV-F10 in 2021 also failed due to pressure loss, that was in a liquid-fuel stage, not a solid stage like PSLV’s.
What Will ISRO Do Next?
ISRO has already formed an expert review team. They will:
Analyse telemetry data (rocket’s in-flight information).
Inspect the design, fuel, motor case, and nozzle.
Check all pre-launch test results.
ISRO has a strong track record of recovering from setbacks—whether it was Chandrayaan or Gaganyaan testing—and this time will be no different. They’ll identify the fault, fix it, and return stronger.
In Short
The PSLV-C61 mission failed because of a drop in chamber pressure in the third stage, which uses solid fuel. This could be due to fuel issues, leaks, nozzle faults, or missed defects. EOS-09 was lost, but ISRO is investigating and will fix the problem for future missions.
It’s like a car breaking down mid-journey—the mechanics (ISRO) are already checking the engine (third stage) so the next trip will go smoothly.
(Girish Linganna is an award-winning science writer and Defence, Aerospace & Geopolitical Analyst based in Bengaluru. He is also Director of ADD Engineering Components India Pvt. Ltd., a subsidiary of ADD Engineering GmbH, Germany.)
