The PSLV-C57 mission lifts off from Sriharikota on September 2, 2023, carrying the ISRO Aditya-L1 spacecraft. | Photo Credit: PTI

At 5.59 am on May 18, the Indian Space Research Organisation (ISRO) launched a PSLV rocket in its XL configuration. It was carrying a satellite called EOS-09, which it planned to deposit into a sun-synchronous polar orbit (SSPO) around the earth. The mission was designated C61. But a few minutes after liftoff, the rocket suffered a problem in its third stage and the PSLV-C61 mission ended in a failure.

PSLV stands for ‘Polar Satellite Launch Vehicle’. It was originally built to launch satellites into SSPOs. It’s a four-stage rocket. Each stage is defined by a set of engines that often fire together. Once they are done, the rocket jettisons that stage so that the remaining stages have less mass to lift.

PSLV’s first stage engine burns hydroxyl-terminated polybutadiene-bound (HTPB) propellant to generate a peak thrust of 4.8 MN. (In its XL configuration, six strap-on boosters augment the first stage.)

The second stage is powered by a Vikas engine, which uses unsymmetrical dimethylhydrazine as fuel and nitrogen tetroxide as oxidiser to develop a thrust of 0.8 MN. The third stage is back to using HTPB as fuel whereas the fourth burns a combination of monomethylhydrazine and mixed oxides of nitrogen in two engines.

After the failure of PSLV-C61 became clear, ISRO chairman V. Narayanan said the chamber pressure in the casing that contained the third-stage motor fell during the flight, leading to the unfortunate outcome. ISRO will assess the reasons for the loss of pressure in the coming days and subsequently reattempt the mission to launch (another copy of) EOS-09.