Model Answer

The Polar Satellite Launch Vehicle (PSLV) is India’s third-generation orbital launch vehicle and has earned the title of ISRO’s “workhorse” due to its exceptional reliability, versatility and sustained contribution to India’s space programme. Since its operationalisation in the mid-1990s, PSLV has consistently delivered satellites for domestic, commercial and scientific missions, making it the most frequently used launch vehicle of ISRO.

Design philosophy and key features

PSLV follows a four-stage, mixed-propulsion configuration, combining solid and liquid stages to optimise thrust, control and precision. This design ensures robustness during liftoff, stability during atmospheric flight and accuracy during satellite injection. Its modular design with strap-on boosters allows ISRO to customise thrust depending on payload mass and mission requirements, enabling launches into Low Earth Orbit (LEO), Polar Orbit, Sun-Synchronous Orbit (SSO) and even Geosynchronous Transfer Orbit (GTO). High redundancy, mature technologies and incremental upgrades have contributed to its high mission success rate, reinforcing global confidence in PSLV.

Stage-wise functioning

• The first stage (PS1) is the liftoff stage and uses a large solid rocket motor (S139) fuelled by HTPB. It provides the enormous thrust required to lift the rocket off the launch pad and carry it through the densest layers of the atmosphere. Depending on the mission, PS1 is augmented by strap-on solid boosters, which function like auxiliary engines, enhancing payload capacity and mission flexibility.
• The second stage (PS2) employs the Vikas liquid engine, using UDMH and nitrogen tetroxide as propellants. This stage provides controlled and sustained thrust after the violent solid-stage burn. Its liquid propulsion allows fine control over thrust and direction, ensuring trajectory correction and vehicle stability during ascent.
• The third stage (PS3) is again a solid propulsion stage using the S7 motor with HTPB fuel. It delivers a high-velocity boost required to take the launch vehicle close to orbital speed in the near-vacuum of the upper atmosphere. This stage plays a crucial role in energy build-up before final orbital insertion.
• The fourth stage (PS4) is a twin-engine liquid stage using MMH and MON propellants. It functions as a precision stage, enabling accurate orbital injection of satellites. PS4 can be restarted and used for multiple burns, allowing deployment of multiple satellites in different orbits. In recent missions, PS4 has also been utilised as an orbital platform for scientific experiments, reflecting the adaptability of PSLV.

Why PSLV is ISRO’s workhorse

The combination of reliability, adaptability and cost-effectiveness distinguishes PSLV from other launch vehicles. Its mixed propulsion architecture balances brute thrust with precision control, reducing mission risks. PSLV has successfully launched hundreds of satellites, including foreign payloads, underpinning India’s space commercialisation efforts through NSIL. It has also supported strategic missions such as Earth observation, navigation and interplanetary launches (e.g., Chandrayaan-1’s initial orbit).

Conclusion

PSLV’s well-calibrated stage-wise design, mission flexibility and proven reliability have made it the backbone of India’s space launch capability. By consistently delivering precise outcomes across diverse missions for over two decades, PSLV has justifiably earned its reputation as ISRO’s workhorse launch vehicle.