"It is a technology in which Britain is currently leading the world," writes technology expert David Hambling.
Unmanned solar-powered aircraft circling at the edge of space could give British forces a vital edge in future conflicts.
Sometimes called near-space platforms, High-Altitude Pseudo-Satellites (HAPS) can keep flying over a given area for days, weeks or months at a time.
It is a technology in which Britain is currently leading the world, with aircraft built for the RAF in Farnborough.
Satellites are invaluable for both intelligence gathering and communication, but they are rare and precious assets.
Because of the limitations of orbit, a satellite may only be able to view a given spot briefly every few hours.
“Satellites are brilliant at global coverage, but they have a problem with persistence. Aircraft are good for local stuff, but they’re not very persistent and they’re very local,” says Paul Brooks, head of HAPS business development at Airbus Defence and Space.
The solution is Zephyr, which Mr Brooks calls “a satellite with wings.”
Zephyr is a lightweight unmanned aircraft with an airframe made of carbon fibre, the upper surface of its wings crammed with solar cells. It was originally developed by QinetiQ in 2003 and the program has since been taken over by Airbus.
There are two versions of Zephyr, the current Zephyr 8 or S and the future Zephyr T. The S version has a 25-metre wingspan but weighs less than 75 kilos – so light it is launched by being thrown into the air by the ground crew after a take-off run.
It cruises at around 70,000 feet, twice the altitude of most airliners and far above clouds and bad weather. This ensures continuous sunlight during daytime to power the eight propellers and recharge the batteries.
At night, the Zephyr runs on a bank of special high-capacity lithium-ion batteries which make up almost a third of the aircraft’s weight. These store about 30% more energy than conventional batteries, and the silicon nanowire technology behind them is being developed for electric cars and other applications.
On its maiden flight in Arizona last summer, the Zephyr S flew continuously for over 25 days, shattering the previous record of 14 days – which was set by a previous version of the same aircraft.
The Zephyr S carries a five-kilo payload, enough for a camera able to distinguish objects about 15 centimetres across on the ground, making it as good as the best current satellites.
The difference is that while a satellite can only take a few snaps as it whizzes past on each orbit, the Zephyr hangs around taking as many pictures as needed and can home in on objects of interest before they disappear.
Another advantage of Zephyr is that, unlike a satellite, it can easily return to base for repairs or upgrades to sensors or other equipment.
The Zephyr T model will be slightly larger, with a twin tail and a 33 metres wingspan. It will weigh almost twice as much as the S at 140 kilos, including a significantly bigger payload.
The T version is likely to be optimised for maritime surveillance and communications relay.
The RAF has taken delivery of three Zephyr S. They will not be used in action but are Operational Concept Demonstrators to test the capabilities of this type of aircraft.
In September, 99 Squadron RAF transported two Zephyrs to Australia for testing. Airbus has set up a base for Zephyr operations at Wyndham airfield in Western Australia.
Zephyr is remarkably cheap for such advanced technology at only £4m an aircraft, so large numbers may be acquired to support operations.
Other nations are pursuing similar capabilities; the US military has been working with solar-powered drones for decades, but their programs have always run into problems.
In 2010, the Pentagon’s research agency DARPA awarded $89 million to Boeing for a Solar Eagle demonstrator aircraft as part of its Vulture program.
However, the program has now been scaled back and is now focusing on solar power and energy storage technology; there are no plans for an actual aircraft.
Aurora Flight Sciences, now owned by Boeing, is continuing to develop a solar unmanned aircraft as a private venture.
Known as Odysseus, it is scheduled for its first flight in April. BAE Systems are developing their own solar drone, known as Phasa-35, also due to fly this year.
Success is not guaranteed: technology giant Facebook quietly cancelled its plans for a fleet of Aquila solar-powered communication relay drones last year.
Meanwhile the Chinese may be catching up.
The Caihong T-4 first flew in 2017. It is bigger than Zephyr, with a 40-metre wingspan, making it the size of a 737 airliner. It has double-bodied fuselage to help distribute its weight of some 400 kilos across the airframe.
It flies at 65,000 feet and the designers say it will ultimately be able to stay in the air for months at a time, though so far the T-4 has been restricted to relatively short flights.
High-altitude pseudo-satellites may be a game changer on land and at sea.
They have the potential to provide a stream of high-resolution images in real time and leaving the opposition with nowhere to hide.