ISRO's PSLV-C61 Mission Fails: Understanding the Third Stage Anomaly and Its Implications

ISRO's PSLV-C61 Mission Faces Setback: A Deep Dive into the Failure

The Indian Space Research Organisation (ISRO) experienced a setback with the failure of its PSLV-C61 mission. This mission aimed to place the EOS-09 Earth Observation Satellite into a 524-kilometer sun-synchronous polar orbit. Unfortunately, a technical snag occurred, marking a rare blemish on the record of ISRO's 101st launch mission. Let's break down what happened and why it matters.

Before we delve deeper, let's clarify some key technical terms:

• Polar Satellite Launch Vehicle (PSLV): ISRO's reliable workhorse, designed to launch satellites into Earth's orbit, particularly polar and sun-synchronous orbits (600-800 km). It’s a four-stage launch vehicle (solid → liquid → solid → liquid) used for remote sensing, communication, and scientific missions.
• Ground-Lit Strap-On Motors: Additional booster engines attached to the rocket that ignite on the ground at launch, providing initial thrust. These are crucial for giving rockets like the PSLV the power needed for lift-off.
• Onboard Instrumentation: Sensors and equipment on the rocket or satellite, such as accelerometers, gyroscopes, and inertial measurement units (IMU), that measure speed, direction, altitude, and other data in real-time. This data is sent to ground stations via telemetry to monitor the mission's status and trajectory.
• Telemetry: The process of transmitting data (e.g., speed, altitude, pressure, orientation) from the rocket or satellite to ground stations in real-time via radio signals. This data is vital for mission monitoring and analysis.
• Ground-Based Tracking: Using radar, antennas, and ground stations to track the rocket or satellite's position, speed, and direction in real-time. This is independent of onboard instrumentation data and helps in monitoring the mission.
• Closed Loop Guidance: An automatic control system where the rocket's onboard computer analyzes real-time data and coordinates trajectory, thrust, and orientation. This ensures the rocket reaches its intended orbit accurately, even without constant ground control intervention.

The Mission's Initial Stages: Smooth Sailing

The PSLV-C61 launch occurred at 5:59 AM from the Satish Dhawan Space Centre, Sriharikota. Initially, everything proceeded as planned. The countdown, first stage ignition, lift-off, and solid motor performance were all nominal. The four ground-lit strap-on motors and the central core functioned as expected. Subsequently, the air-lit strap-on motors ignited on time, and the rocket continued on its designated trajectory. The second stage also performed perfectly, utilizing the Vikas engine, which runs on liquid fuel. Throughout this phase, the onboard instrumentation and ground-based tracking data were in complete alignment, indicating that the rocket was gaining speed and altitude as intended.

The Critical Anomaly: Where Things Went Wrong

The problem arose when the rocket reached its third stage (PS3), which employs a solid motor. According to ISRO, the third stage ignition occurred normally at 262.9 seconds. Initial data suggested everything was fine. At this point, the rocket was at an altitude of 344.9 kilometers, traveling at 5.62 km/second, and had a range of 888.4 kilometers. However, at approximately 376.8 seconds, anomalies began to appear in the telemetry data.

ISRO's ground stations analyze two primary data streams. These data sources are represented graphically, with the green line indicating onboard instrumentation data and the yellow line representing tracking data. In the initial stages of the mission, these lines overlapped perfectly, confirming the rocket's proper performance. However, around 376.8 seconds during the third stage, the lines began to diverge. The green line, representing data from onboard sensors, displayed a zigzag pattern and deviation, while the yellow line, indicating tracking data, showed a different picture. This divergence was a clear sign that something was amiss.

Possible Causes: Thrust Irregularities or Data Transmission Issues

During the third stage, the rocket was in closed loop guidance mode, where the onboard computer makes independent decisions based on real-time data. In this mode, the Inertial Measurement Unit (IMU), Global Navigation Satellite System (GNSS), and other sensors control the rocket's orientation, thrust, and trajectory. However, in this mission, the data transmitted by the onboard system did not align with the tracking data.

Analysts suggest several potential causes for this deviation:

• Thrust Imbalance: Uneven combustion or structural issues within the solid motor could have led to an imbalance in thrust, causing the rocket to deviate from its intended path.
• Data Transmission Glitch: Although less likely, a temporary glitch in the telemetry system could have caused the discrepancy between onboard and ground-based data.
• Unexpected Aerodynamic Forces: Unforeseen atmospheric conditions or aerodynamic forces could have acted on the rocket, causing it to veer off course.

Following this deviation, the rocket began to adjust its trajectory incorrectly, preventing it from reaching its designated orbit. Although the fourth stage (PS4) ignition did occur, by that point, the mission's success was no longer possible. Ultimately, the mission had to be aborted mid-flight, resulting in the destruction of the rocket and the satellite.

ISRO Chairman's Statement: Chamber Pressure Drop in the Third Stage

ISRO Chairman V. Narayanan stated that a drop in chamber pressure within the motor case during the third stage was the cause of the mission's failure. He explained, "We have formed a Failure Analysis Committee to investigate the reasons for this failure. This committee will conduct a detailed analysis of telemetry data, onboard system logs, and ground tracking data. By studying millions of bits of data, we will identify the precise causes and implement improvements for future missions."

The Satellite's Intended Use: Agriculture, Forestry, and Disaster Management

The EOS-09 satellite, weighing 1,696.24 kilograms, was an integral part of ISRO's Earth Observation Program. Its purpose was to provide high-resolution imagery for applications in agriculture, forestry, and disaster management. The loss of this satellite is a setback for these crucial areas.

Why This Matters: The Bigger Picture

The failure of the PSLV-C61 mission, while disappointing, provides a valuable opportunity for ISRO to learn and improve. Space missions are inherently risky, and even the most reliable systems can experience unforeseen issues. ISRO's commitment to thoroughly investigating the failure and implementing corrective measures demonstrates its dedication to maintaining its high standards.

Behind the Scenes: What Happens Next?

The Failure Analysis Committee will likely spend weeks, if not months, poring over the data collected during the launch. This investigation will involve experts from various fields, including propulsion, avionics, and telemetry. The goal is not only to identify the immediate cause of the failure but also to uncover any underlying systemic issues that may need to be addressed.

The Reliability of the PSLV: A Question Mark?

The Polar Satellite Launch Vehicle (PSLV) has been ISRO's most dependable rocket, often referred to as the "workhorse." It boasts a success rate of approximately 96%. This rocket has been instrumental in launching satellites for both India and its global customers into orbit. However, this failure inevitably raises questions about its continued reliability.

ISRO has a history of learning from its failures. For example, after the unsuccessful EOS-03 mission in 2021, the issue of pressure in the cryogenic stage tank was identified and subsequently corrected. Similarly, this time, ISRO's team will conduct an in-depth analysis of all data logs from pre-launch preparations, the launch process, and onboard systems.

Looking Ahead: Ensuring Future Success

While this setback is significant, it's crucial to remember that setbacks are part of space exploration. What truly matters is how ISRO responds. By thoroughly investigating the cause of the PSLV-C61 failure and implementing necessary improvements, ISRO can reinforce the reliability of its launch vehicles and continue to play a leading role in the global space industry. The incident underscores the complexities of space travel and the importance of continuous improvement and rigorous testing.