Thrust of the matter: Why ISRO’s workhorse PSLV trips at Stage 3

(After PSLV-62 failure)

In back-to-back failures, two of ISRO’s PSLV missions have malfunctioned during the third of four stages. In May last year, PSLV-61 experienced a drop in pressure in the propulsion chamber during the third stage, leading to a deviation from the intended course. Last week, PSLV-62 also deviated off course towards the end of the same stage.

The third stage can be challenging because it uses a solid propellant, which does not allow the precise control that liquid propellants provide in the second and fourth stages. While the first stage also uses a solid propellant, the third stage is fired high in space, under conditions very different from those near Earth when the first stage ignites.

The failures should be seen more as an aberration than a trend, as PSLV’s track record has been outstanding for decades. ISRO’s workhorse has succeeded in most of its previous missions, demonstrating that all four stages can operate reliably. It is not yet clear whether a new factor has introduced uncertainty into the third stage.

The four stages

PSLV, in all its versions, is a four-stage rocket with an alternating pattern of solid and liquid propulsion systems.

Each stage comes with its own propulsion system, fuel, and structural elements, designed to provide what the vehicle needs at different phases of its trajectory. PSLV rockets use solid propellants—easier to store and ignite—in the first and third stages, and liquid propellants—better for precise orbital insertion and restarts—in the second and fourth stages.

The first stage lifts the rocket and pushes it through the lower atmosphere. It uses a solid propellant, producing very high thrust that allows the rocket to accelerate rapidly and climb through the atmosphere in about two minutes. Once the propellant is exhausted, Stage 1 separates.

In the second stage, the vehicle continues accelerating and stabilising its flight in preparation for Stage 3. Thrust is provided by a liquid-fuel engine, allowing precise control of thrust and direction. By this point, the heat shield is jettisoned. When the liquid propellant is depleted, Stage 2 separates after about 2–3 minutes.

The third stage switches back to a solid propellant, providing a strong but short burst of thrust. This is meant to increase the vehicle’s horizontal velocity, essential for achieving orbit. This stage should ideally burn for less than two minutes before separating.

After Stage 3 separation, the fourth stage is responsible for precise orbital insertion. It uses liquid propellant to fine-tune the vehicle’s velocity, altitude, and inclination. Payload satellites are then deployed into their intended orbits. The duration of this stage can vary depending on mission requirements, including possible coasting phases.

Twin glitches

An ISRO fault-assessment committee has probed the PSLV-61 failure. Although its findings have not been made public, it is known that the thrust required in Stage 3 could not be maintained. This resulted in a drop in pressure in the propulsion chamber, throwing the rocket off trajectory.

ISRO has not yet divulged preliminary findings on what happened during PSLV-62 either.
“Discussions are ongoing and we will come back to you once the analysis is over,” ISRO chairman V. Narayanan told The Hindu.

On the day of the malfunction, Narayanan cited disturbances in the vehicle leading to a deviation in its path. The glitch occurred when motors were providing thrust to propel the vehicle to the intended altitude during the third phase.

The rocket performed as intended through the first two phases and almost the entire third phase. However, after separation of the first and second stages, the upper stages started “rolling in orbit”.

“As the rolling rate of the upper stages increased, the fourth phase ignited and separated, but its direction was no longer in control,” said Manish Purohit, a former ISRO space scientist.

Fifteen of the 16 satellites on the payload were lost immediately. One sent signals briefly, but that too soon ended.

Looking for a fix

ISRO has started analysing the latest failure. With findings from the previous one still not public, it remains to be seen if the causes were similar.

“We do not know if any changes were proposed or if any were incorporated,” Purohit said.

Although no design changes to PSLV’s basic structure have been announced, each rocket is made for one-time use only. Components are expendable, and new hardware—often with variations—is used in every launch.

“The process is well laid out. These two were PSLV’s 63rd and 64th flights, but failure has been extremely rare over the previous 62 missions,” Purohit said.

PSLV rockets have been involved in landmark missions—from Chandrayaan-1 (2008) to the Moon, Mangalyaan (2013) to Mars, and a world record 104 satellites in a single flight (PSLV-C37, 2017).