Detailed Report: PSLV-C61 EOS-09 Mission Failure And Suspected Nozzle Anomaly
India’s PSLV-C61 mission, tasked with deploying the advanced EOS-09 Earth
observation satellite, ended in failure on the morning of May 18, 2025, due to
a suspected malfunction in the third-stage (PS3) flex nozzle system. This
incident marks only the third failure in 63 launches for the Polar Satellite
Launch Vehicle (PSLV), a workhorse of the Indian Space Research Organisation
(ISRO) since the 1990s.
The PSLV-C61 lifted off at 5:59 a.m. IST from the Satish Dhawan Space Centre,
with its initial flight phases proceeding nominally. The solid-fuelled first
stage (PS1) and the liquid-fuelled second stage (PS2) both performed as
expected, maintaining the planned trajectory and velocity.
However, approximately 203 seconds into the flight, during the PS3 solid motor
phase, telemetry data indicated a sudden and abnormal drop in chamber
pressure. This anomaly coincided with the period when the third stage’s flex
nozzle system is actively steering the vehicle during its 114-second burn.
ISRO Chairman V Narayanan confirmed the sequence of events, stating, “First
two stages performed as expected. In the third stage, we observed less chamber
pressure. The mission could not be accomplished”. The rocket subsequently
deviated from its intended path, and the EOS-09 satellite failed to reach its
planned 525 km sun-synchronous orbit.
The third-stage PS3 motor is equipped with a flexible bearing nozzle system,
or “flex nozzle,” which is critical for thrust vector control. Constructed
from layered elastomeric materials, the flex nozzle allows precise adjustment
of the rocket’s thrust direction without the need for complex hydraulic
actuators. This system steers the vehicle during the entire third-stage burn.
A failure in the flex nozzle-whether in its materials, actuation, or
control-can result in thrust misalignment. This misalignment can cause the
rocket to veer off course rapidly, as was observed in the PSLV-C61 mission.
The PS3 stage uses hydroxyl-terminated polybutadiene (HTPB) fuel, typically
generating 240 kN of thrust under optimal conditions. The sudden drop in
chamber pressure suggests a loss of effective thrust vectoring, likely due to
the suspected nozzle malfunction.
Variants of Vikas Engine
The following are the different variants of the Vikas Engine:
| Type | Fuel | Launcher Stages |
|---|---|---|
| Booster/First Stage | ||
| Vikas-2 | UDMH / N2O4 | GSLV Mk-I L40H Strap-On |
| Vikas-2B | UH 25 / N2O4 | GSLV Mk-II L40H Strap-On |
| Vikas-X | UH 25 / N2O4 | LVM3 L110 stage |
| Second Stage | ||
| Vikas-4 | UDMH / N2O4 | GSLV Mk-I GS2 stage, PSLV PS2 stage |
| Vikas-4B | UH 25 / N2O4 | GSLV Mk-II GS2 stage, PSLV PS2 stage |
The loss of the EOS-09 satellite is a significant setback for India’s
space-based surveillance and remote sensing capabilities. EOS-09, equipped
with a C-band Synthetic Aperture Radar, was intended to strengthen India’s
52-satellite constellation for all-weather, day-and-night Earth
observation-particularly for border and coastal monitoring.
ISRO has convened a failure analysis committee to investigate the root cause,
focusing on the manufacturing records and test protocols of the PS3 stage and
its flex nozzle assembly. While the precise cause is yet to be confirmed, this
incident underscores the critical importance of propulsion and
stage-separation reliability; globally, 74% of launch failures between 2018
and 2023 have been attributed to similar issues.
Despite this failure, the PSLV remains one of the world’s most reliable launch
vehicles, with only three failures in 63 launches since its inception. The
last failure occurred in 2017 (PSLV-C39), and before that, during the
inaugural flight in 1993. The PSLV’s track record has made it the backbone of
India’s space program, trusted for both domestic and international satellite
deployments.
The PSLV-C61 mission’s failure highlights the vulnerability of even mature
launch systems to component-level anomalies, particularly in critical systems
like the flex nozzle. The ongoing investigation by ISRO’s failure analysis
committee will be crucial in identifying the exact cause and implementing
corrective measures to maintain the PSLV’s reputation for reliability and to
restore momentum to India’s ambitious space program.
Agencies
