More Innovation, Less Debris: ISRO’s Repurposing Of PSLV Rocket’s Spent Final Stage

An ingenious side experiment initiated by the Indian Space Research Organisation (ISRO) in the middle of 2022 is taking centre stage this new year of 2024.

Last week, space companies like Hyderabad-based Dhruva Space and Bengaluru-based Bellatrix Aerospace, as well as ISRO’s own Thiruvananthapuram-based Vikram Sarabhai Space Centre (VSSC), reported successful technology demonstrations using a part of an ISRO rocket whose destiny in the years past was to become space debris.

It all began on New Year day with ISRO's launch of an X-ray polarimeter satellite — an incredible achievement — using its workhorse Polar Satellite Launch Vehicle (PSLV).

After the precise injection of the satellite into the intended, 650-kilometre (km)-orbit, the process of lowering the PSLV rocket’s “spent fourth stage” began. This is the part which earlier had no use than to be lost to the sands of space and time.

On this occasion, the terminal stage was lowered to a 350-km circular orbit. At this address, which will be its home for about a month, the terminal stage transformed into a laboratory for in-orbit experiments.

It turned into an "orbital platform" for testing and space validation of 10 experimental payloads belonging to six companies and academic institutes, as well as two ISRO centres.

This mighty-useful avatar of the used PSLV stage is called the PSLV Orbital Experimental Module (POEM).

The POEM experiment accomplishes several important objectives, most notably vacating prime orbit space for other space missions (including ISRO's future missions), minimising the grave and growing space debris problem, and opening up a garage laboratory in orbit, if you will, for swift use by the startup, student, and scientific communities seeking to access space. If not for POEM, orbital experiments would be practically out of reach for said communities.

Spurring Space Innovation

Within two days of launch, Dhruva Space announced the successful space qualification of its P-30 satellite platform called ‘Launching Expeditions for Aspiring Payloads - Technology Demonstrator’ or the LEAP-TD mission.

With the success of their LEAP technology demonstrator, Dhruva Space revealed they were "open for business as a hosted payload solution provider."

The company explained that a “hosted payload service” can be a part of a satellite, like a sensor, an instrument, or a set of communications transponders owned by an entity other than the primary satellite operator.

The hosted section operates independently of the main spacecraft, but shares the satellite’s power supply, transponders, and, in some cases, even the ground systems.

Dhruva Space said the first LEAP satellite mission (LEAP-1) has already been conceived and will launch soon.

Similarly, using ISRO’s beneficial orbital platform, Bellatrix Aerospace gained the capability of supplying propulsion systems globally after the successful space qualification of their Arka and Rudra thrusters.

Both these propulsion systems, which were developed with the mentoring and support of the Defence Research and Development Organisation (DRDO), represent India firsts — while Arka is said to be the country’s first privately developed Hall Effect thruster, Rudra is dubbed the first high-performance green propulsion system.

The 200-Watt Arka electric engine is pitched as ideal for satellite missions with high efficiency requirements, while the 1-Newton Rudra green (environmentally friendly) propulsion system works best for satellite missions requiring high thrust.

On the 2024 POEM mission, both Arka and Rudra operated in space as per design specifications and performed in line with ground test results.

“Upon firing of Rudra, a noticeable displacement of the POEM-3 platform was recorded, which was in-line with our simulations. Arka was validated upon successful ignition and functioning of the thruster and heaterless hollow cathode,” Rohan M Ganapathy, the chief executive and technology officer at Bellatrix Aerospace, said in a statement.

Not just the thrusters, the performance of the entire spectrum of subsystems was successfully validated.

Test Of ISRO's Key Technology

Besides the private players, successful technology demonstrations were reported by ISRO’s VSSC centre.

The operation of a 100-W-class polymer electrolyte membrane fuel cell-based power system was assessed and all the relevant data collected to support the design of such systems for future missions.

“These fuel cells are the future for power production in space habitats,” ISRO said on X, explaining in a statement that fuel cells are the “ideal power source for Space Station as it provides both power and pure water.”

Following the successful deployment of XPoSat, ISRO Chairman S Somanath said fuel cells were a “precursor to our future powering of space station and other areas where we need power generation on board.”

In another demonstration, the VSSC validated 10 Ah silicon-graphite, anode-based, high-energy-density lithium-ion cells as a low-weight, low-cost alternative to the cells currently used.

While conventional lithium-ion cells use pure graphite as anode material, the use of a silicon-graphite composite lends higher energy density to the cells. The innovative battery system finds use both on the ground and in space.

Thanks to the repurposed orbital platform, the capability of these cells to survive and perform in the harsh space environment was demonstrated successfully. “Based on the confidence gained through this performance, these cells are poised to be used in upcoming operational missions where 35-40% battery mass saving is expected,” ISRO said.

The ‘POEM’ Experiment

This was only the third time that ISRO used the PSLV rocket’s fourth stage, after its propulsion duties had concluded, for in-orbit experiments and space debris mitigation.

The POEM mission debuted on the fifty-fifth flight of the PSLV. The PSLV-C53 mission, which took off on 30 June 2022, saw the launch of three Singapore satellites as part of a commercial mission to be executed by ISRO’s commercial arm NewSpace India Limited (NSIL).

After the completion of the primary mission, ISRO sought to demonstrate the use of the PSLV’s spent upper stage as a stabilised platform for scientific payloads and experiments.

For the basics, the PSLV is a four-stage rocket. The first three stages propel a spacecraft along the planned trajectory towards the desired orbit and separate from the vehicle, one by one, to fall back into the ocean.

The fourth and final stage, however, ends up in space as space junk or debris after satellite separation.

To cut down space debris while also enabling greater private participation in the space industry, ISRO decided to repurpose the terminal PSLV stage as an orbital platform for scientific experiments.

POEM provides power, attitude control, communication, data storage, and limited avionics capabilities all by itself, reducing the worries of the nervous payload owners. Two liquid engines and reaction control system thrusters provide POEM with propulsion and control.

The platform derives power from solar panels and a lithium-ion battery. Its navigation is guided using four Sun sensors, a magnetometer, gyroscopes, and the Indian satellite navigation system NavIC.

The POEM Goes On

After the success of the first POEM experiment, the sequel launched about 10 months later, on 23 April 2023.

Coincidentally, that mission also involved the deployment of Singapore satellites as part of yet another dedicated NSIL commercial mission.

Yet again, after the completion of the primary mission, the planned in-orbit scientific experiments began on POEM.

A Bellatrix payload made its debut and Dhruva Space returned with another payload on this second POEM mission.

The Indian Institute of Space Science and Technology (IIST) and the Indian Institute of Astrophysics (IIA), Bengaluru, contributed the other payloads on this experimental mission.

In POEM’s third and latest instalment, the spent upper stage of the PSLV rocket was, in a first, lowered to a 350-km orbit from the original 650-km orbit in which the primary satellite was deposited.

The orbit was lowered so that “as a responsible space agency,” ISRO didn’t “create debris in the process,” Somanath said in his address at the Satish Dhawan Space Centre–Sriharikota (SDSC SHAR).

Ten payloads featured on this POEM mission with the objective of space-qualifying new systems with novel ideas. Dhruva Space, Bellatrix Aerospace, and ISRO’s VSSC have reported successes with testing and space qualification so far.

Thanks to its space smarts, a sense of responsibility as a leading space power, and a commitment to private space-sector innovation, ISRO is confidently penning a POEM that would make for great reading for Indian space in the years to come.