The Challenge: The Tyranny of Fuel
For decades, satellites have relied on chemical propulsion. Think of it as a series of powerful, controlled explosions. This method uses a liquid fuel and an oxidiser which, when combined, produce a powerful burst of thrust. This is great for big, fast
manoeuvres, like getting a satellite from its initial transfer orbit to its final position 36,000 km away. The major downside, however, is the weight. A significant portion of a satellite's launch mass is just this chemical fuel. For a typical 2,000 kg communication satellite, this could be hundreds of kilograms of propellant needed for its 12-to-15-year lifespan. This heavy fuel load limits the amount of scientific or communication equipment—the actual payload—the satellite can carry.
The Solution: A Gentle, Unrelenting Push
Electric Propulsion Systems (EPS) offer a completely different approach. Instead of powerful, short bursts, they provide a very gentle but continuous push. These thrusters use electrical power, typically generated by the satellite's solar panels, to create electric and magnetic fields. These fields then accelerate ions of a propellant—usually an inert gas like xenon—and expel them at incredibly high speeds, up to twenty times faster than from a chemical rocket. This is the core of how Hall-effect thrusters, a common type of EPS, work. While the force produced at any given moment is tiny (sometimes described as the weight of a sheet of paper), doing it continuously over weeks or months achieves the same overall change in velocity with far greater efficiency. It’s the classic tortoise versus the hare, where slow and steady wins the space race.
The Weight-Loss Secret: Extreme Fuel Efficiency
The magic of electric propulsion lies in its incredible fuel efficiency, a concept measured by 'specific impulse'. This is like the 'kilometres per litre' for a spacecraft; a higher specific impulse means you get more push for every kilogram of propellant. Electric propulsion systems can be five to six times more efficient than their chemical counterparts. This means a satellite might only need to carry 200 kg of xenon gas instead of over two tonnes of chemical fuel to perform its mission. This drastic reduction in propellant mass is how EPS helps ISRO cut satellite weight. A satellite that would have weighed four tonnes can be slimmed down to two, without sacrificing performance. This frees up mass and space for what truly matters: more transponders, better cameras, or more scientific instruments.
ISRO's Electric Fleet
ISRO has been strategically incorporating this technology for years. The GSAT-9 satellite, launched in 2017, was a key early adopter, using electric thrusters for in-orbit station-keeping—the small adjustments needed to maintain its precise orbital slot. This allowed it to fly with significantly less chemical fuel. Building on this, ISRO is developing more powerful, fully indigenous systems. The upcoming Technology Demonstrator Satellite (TDS-01) is set to validate a homegrown electric propulsion system for the entire process of orbit raising. This marks a major step towards self-reliance, moving away from imported components. Future satellites, like the planned GSAT-20, are envisioned as being 'all-electric', showcasing the agency's confidence in this game-changing technology.
More Payload, More Life, More Value
The benefits of shedding all that fuel weight are twofold. First, it allows for a longer mission. With fuel being the primary limiting factor for a satellite's operational life, the high efficiency of EPS means a satellite can perform its station-keeping duties for many more years. A 15-year mission could potentially be extended significantly. Second, the mass saved from fuel can be reallocated to the payload. This means a communication satellite can carry more transponders, generating more revenue and providing better services. For ISRO, this also means it can launch heavier, more capable satellites on its own rockets, like the GSLV, reducing dependence on expensive foreign launch providers and increasing the return on investment for each mission.
