India has carved a place for itself in many exclusive niches — it is among the very few countries to have achieved moon landing, to possess anti-satellite missile systems, intercontinental ballistic missile (ICBM) and ICMBs with multiple independently targetable re-entry capabilities. And now, the country is on course to join a very select club of countries that have their own long-flight, High-Altitude Platforms (HAP).

On May 7, the public funded research body, CSIR-NAL (National Aerospace Laboratories), test-flew a vehicle first to a height of about 3,000 feet (with a ‘synthetic aperture radar’ payload, made by the start-up, Galaxeye Space). It came back and took off again and flew to 25,000 feet (about 8 km), without the payload. The “subscale flight” was a milestone event in the journey towards the development of a full-scale HAP. The learnings from the flight would be used to build a bigger vehicle, which is the target of the ‘High Altitude Platform Program’, by December 2025. When NAL achieves the feat, India might be only the second or the third country to have HAP (depending upon other countries’ progress.)

What is HAP?

HAP is an unmanned aerial vehicle. Think of it as a big drone, but with two essential differences — the HAP typically operates at a height of 18-20 km above earth, clearing all air traffic and weather, and is equipped to stay up there for longer periods than drones — from several hours to even months.

The HAP that NAL is developing is being designed to stay airborne for 90 days — unless other HAP hopefuls get better earlier, NAL’s HAP would set a world record.

A HAP can do many things that a satellite can — surveillance, imaging the earth below, for both civilian and strategic purposes and can also be used, where economics work out, to provide telecommunications and broadband services over a chosen region — all at a fraction of the cost of a satellite. Of course, a satellite can be packed with far more capabilities and can stay up there for years — so HAPs won’t replace satellites but will complement them. Potentially, HAPs could compete with Elon Musk’s Starlink.

A satellite does not need to be powered for its orbital flight — it does this by gravity. But a HAP needs some propulsion system, with on-board energy generation capability. This, today, only means solar power, with batteries to store energy for night-time flight.

A new player in the sky

NAL’s HAP that was test-flown on May 7 was a much smaller version than the target vehicle. It stood 12-m across from wing-to-wing, had two motors with conventional solar panels and lithium-ion batteries to power them. This was just to collect data that would be used in the building of the bigger vehicle, Dr Abhay Pashilkar, told quantum.

The full-scale vehicle that is under development will be very different than the subscale model, in terms of size and energy-propulsion systems. With a wingspan of 35 metres (roughly same as of A-320 aircraft) and weighing 150 kg, the HAP will be capable of carrying payloads up to 15 kg.

The vehicle that NAL is developing is at the cutting edge of technology. Dr L Venkatakrishnan, NAL’s Program Director–High Altitude Platform, and Chief Scientist and Head, Experimental Aerodynamics Division, told quantum that the full-scale vehicle’s solar cells would not be the conventional silicon, but of gallium arsenide, and produced by the US company MicroLink. These can convert 30 per cent of sunlight into electricity; conventional solar cells do 20 per cent maximum.

And the battery of NAL’s HAP will pack much more energy. The energy density of the battery — likely to be a lithium-silicon or lithium-sulphur— will be 400-500 Watt-hour per kg of material; comparatively, Tesla’s batteries have about 270 Whr/kg. As for total battery power, Venkatakrishnan said, “a lot depends upon the design and payload — now we can only say what is the Whr/kg at cell level that we aim for.”

Furthermore, the solar cells and the batteries should be shielded from the extreme cold at an altitude of 20 km (minus 55oC).

Venkatakrishnan said the HAP could carry payloads weighing up to 15 kg. It will be geostationary, circling over a chosen region 24x7, at a speed of 100 kmph.

Taking the vehicle up there could be as challenging as making it. Since the HAP is basically a light-weight vehicle one must be careful about the structural integrity of the airframe. “It is like crossing a road,” Venkatakrishnan said, “once you get to the other side, you are safe, but while you are crossing you must be careful.”

Future plans

Between now and December 2025, NAL will be building a full-scale prototype, using the learnings from the recent sub-scale flight. The full-scale HAP will also be equipped with auto pilot capabilities — the hardware for it would be bought, but the software written in-house. To test the software, NAL might do one more flight of a sub-scale vehicle later this year.

“Over the next one-and-a-half years, a lot of ground tests will be done,” Venkatakrishnan said.

NAL is really aiming for the sky. No other HAP in the world is capable 90-day endurance. Airbus’ Zephyr has demonstrated 64 days flight. Others are being built. Phasa-35, made by UK’s Prismatic (a subsidiary of BAE Systems) last year demonstrated a stratospheric flight for 24 hours, but the company mentions “several months” as the HAP’s endurance. Lockheed Martin was working on a High Altitude Airship, a much heavier vehicle, but is said to have closed the program.

How much would the HAL cost? Venkatakrishnan did not wish to divulge the cost of developing the HAP, (nor the budget for the program which began in April 2021) but observed that the Zephyr had been offered for £8 million.

But then cost considerations take a backseat when it comes to a country’s security or disaster relief — the most likely uses of the HAP.