But keeping them aloft continuously means solving an awkward energy problem. Solar power works during the day when the sun is shining, and can be helped with batteries bridging the gap overnight.
But both are heavy – and as anyone in the aerospace industry knows, weight is money.
A new £5.3m project involving Scalable Laser, Durham University and the University of Glasgow wants to lighten the load by leaving part of the power system on the ground, and firing the energy into the air with a laser.
The LIVINGSTON project plans to convert electricity into laser light, transmit it through the atmosphere up to 25km, then turn it back into electricity using photovoltaic receivers mounted on the aircraft. It’s one of 18 projects funded through ARIA’s £70m Enduring Atmospheric Platforms programme, which wants to make aircraft that can operating constantly in the stratosphere economically viable. The specific goal of the project is to prove an aircraft can remain aloft above the UK for a whole week, all while powering a 300W payload.
The project has three core areas of interest. One team is developing the flat-panel laser while another is specialising in developing the photovoltaic receiver. Then a third group working in the partnership is tackling the challenge of the optics needed to keep such a high-intensity beam focused on an aircraft that could be flying up to 25km away.
All the parts are tricky, but it’s the optics – and keeping in contact – that could be the trickiest. “A satellite goes on a very, very predictable [path], and you can kind of see where it is,” says John Tyrer, professor emeritus of optical instrumentation at Loughborough University and managing director of Laser Optical Engineering. But aircraft aren’t quite as predictable given they by definition move, and often find their journeys are buffeted by turbulence. “The first thing is being able to lock in to the target or the object,” he says – and at the range the LIVINGSTON project is aiming for, that becomes difficult.
The group plans to tackle that using engineering – and specifically adaptive optics. A reference beacon attached to the aircraft will help the ground station measure how heat and atmospheric turbulence are distorting the laser beam, then compensate in real time using a high-speed deformable mirror.
It’s a method that has worked before. In 2013, researchers, including one working on the current project, demonstrated the ability to beam laser power over 30 metres, turning nearly half of the received optical power into electricity using tailored photovoltaic cells. However, 30 metres and 25 kilometres are quite different distances.
It’s not just turbulence that could cause issues for the laser-powered project. Raindrops, fog, snow and clouds all can refract or block out light. “If it’s a nice sunny day, that means there’s no clouds, there’s no fog, there’s no mist,” says Tyrer. Under those conditions, transmitting a laser is possible. But when dense cloud gets in the way, he says, things become far harder. “If you can’t see it, you can’t hit it,” says Tyrer.
There are some ways to try and mitigate the impact of atmospheric conditions. Different wavelengths and larger beam diameters can reduce the losses, but adaptive optics can’t stop clouds getting in the way – at least not without producing a laser beam powerful enough to punch through the cloud cover that could also harm whatever its ultimate destination is.
That’s before getting to the problem of hitting a moving target and maintaining contact with it. Tyrer has spent decades working with lasers, including systems designed to track aircraft and drones. Even at ranges of hundreds of metres to a kilometre, he says, doing so reliably is “bloody hard”.
Aircraft accidentally crossing the beam path would also need to be detected. Helicopters could pose a particular issue, Tyrer says, because some have windows beneath the cockpit to help pilots see the ground during landing.
All of which makes the idea seem like more hassle than it’s worth. But there are good reasons to try to overcome the hurdles. ARIA sees power beaming, alongside better batteries, novel materials and other technologies, as one potential route for aircraft to remain in the stratosphere rather than repeatedly returning to Earth to refuel or recharge. It’s just a case of whether we can make the atmosphere co-operate. “If you can see it, fine,” says Tyrer. “If you can’t see it, you can’t communicate with it.”
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