Solar-powered aircraft seem doomed to repeat the myth of Icarus, whose homemade wings melted when he flew too close to the sun. High-profile crashes have terminated a slew of projects across three decades. The loss of the only Skydweller solar aircraft prototype last month might reinforce the impression that this technology will always be just out of reach. But Skydweller Aero CEO Robert Miller remains optimistic. Because, as he explains, this was very different to the accidents that ended other solar projects, and Sky dweller’s technology is not just mature but improving.
Fragile Flying Giants
An uncrewed aircraft using solar power and batteries can remain at high altitude for weeks or months at a time, providing the sensing and communication capability of a satellite at a fraction of the price. Unlike a satellite it can be repositioned at will, and can be landed and upgraded whenever needed. An eternal aircraft is a prize worth wining.
The problem is that solar aircraft tend to be fragile creatures. In the fifty years since the Astro Flight Sunrise made the first ever solar-powered flight, the type has presented engineering challenges. In 2003, NASA’s groundbreaking Helios solar drone ran into unexpected turbulence and disintegrated mid-air over the Pacific in 2003. Solar cells and skin were ripped off and debris fluttered into the ocean, and NASA’s solar dream came to an abrupt end.
Many others went down a similar path. Google’s solar eternal program was terminated shortly after a wing section broke off its Solara 50 aircraft in flight in 2015. Facebook’s Aquila project never really recovered after damage sustained in a heavy landing in 2016. A Zephyr solar drone operated by the British broke up after hitting severe weather in 2019 and the MoD placed their remaining Zephyrs in indefinite storage.
Fragility is hard to avoid because of the need for light weight combined with vast wing area for the solar array. The Zephyr S has a wingspan of more than 80 feet but weighs less than some electric scooters. Any turbulence can be catastrophic.
After all the industry casualties, three major projects remain: Zephyr, run by Airbus subsidiary AALTO; BAE Systems’ PHASA-35; and Skydweller. Skydweller is very much the odd one out of this trio. It has a wingspan of 232 feet, similar to NASA’s original Helios, but weighs around six times as much. This is because Skydweller was adapted from the Solar Impulse 2 crewed aircraft which flew around the world in 2016. The need to carry a heavy cockpit and crew, along with safety requirements that go with piloted aircraft, made it very different to drones intended to carry a few pounds of electronic communication gear.
Skydweller is significantly more robust than other solar planes and structural failure was not an issue. Miller says the loss last month was down to a simpler problem: they ran out of battery power.
The FLEX-26 Incident
The loss happened after a U.S. Navy fleet exercise in the Caribbean, during which the Skydweller flew for eight days and carried out the sort of activities it would do in a real-life mission.
“We participated successfully in the FLEX 26 exercise, tracking small boats using military radar, day-night video, and other sensors, distributing the data to US Navy networks,” says Miller.
The aircraft loitered south of the Cayman islands while a spell of bad weather passed by. When a break appeared, the operators directed Skydweller back towards Stennis Air Base.
“The weather window looked permissible, but we flew through more severe weather than expected – the sort of turbulence that would have had coffee on the ceiling in an airliner – and the only problem was that it consumed more energy than anticipated,” says Miller. “Everything was green except the battery level.”
As Skydweller battled strong winds as battery levels ran down. The sun was not high enough to give full power, and the batteries ran down to zero operators were forced to ditch Skydweller safely in the sea, still under full control.
The plane had been through similar weather situations before, but this time conditions were just too severe. This was partly because time pressure had prevented the a full set of new batteries being installed.
“If we had been flying with a complete set of new batteries we might have made it through the situation,” says Miller.
The next Skydweller, which is already under construction, will have a number of improvements which makes problems less likely. Better batteries are just one of the upgrades.
“The new engines on the next version are more efficient, the new solar cells are better, there are a number of things which would have helped us get through. All of these new technologies are available to us today,” says Miller.
It will also gain advanced new propellers. And the new aircraft might get sensors to prevent running into worse weather than it can handle.
“One possible upgrade is putting weather radar on the aircraft to detect some of this in real time,” says Miller. “That would give us better data than the current weather prediction tools.”
Seen in this context, the future of Skydweller looks far more secure. Unlike fragile giants like Helios, which was always at the mercy of a gust of wind while descending from the stratosphere, the more robust Skydweller is not likely to break up. As Miller notes, his team are world leaders in aeroelasticity. Their plane is designed to bend rather than breaking and was not damaged by the turbulence.
The prototype has been lost, but the program continues with lessons learned.
“We hope to have the next Skydweller aircraft flying in 12-18 months, with technology upgrades incorporated,” says Miller.
And while the previous failed solar aircraft projects were at the testing and experimental stages, Skydweller has proved that it is ready for operational use.
“The exercise showed this is a mature technology,” says Miller. “We could run the exact same kind of mission in the Strait of Hormuz, tracking small boats threats rather than drug boats. We can put an unblinking eye over that whole area.”
During the FLEX-26 exercise the Skydweller was fitted with the sort of sensor payload that might be carried by an MQ-9 Reaper. The US has lost literally dozens of Reapers in the conflict with Iran, so many that it is reportedly deploying old MQ-1 Predators. Drones need to be expendable. Skydweller, with its much lower operating cost and indefinite flight time, might well have a future in this role.
That could give Reaper some tough competition.
The Solar Scene
Meanwhile, other solar aircraft are unobtrusively making progress, including BAE Systems’ PHASA-35.
“Further trials are planned this year with the latest Block 20 standard variant of aircraft which will provide twice the solar power generation and storage capacity compared to previous iterations,” a BAE Systems spokesperson told me. “We continue to engage with customers as we look to make PHASA-35 ready for customer operations in the next few years.”
The “customer operations” here sound like a reference to the five-year contract quietly signed last year to operate PHASA-35 solar drones for US Southern Command. Its intended role is unknown but may be for long-duration surveillance. We may only find out about these operations from reports of unusual aircraft at high altitude.
China is also developing solar aircraft technology. The AVIC Qimingxing-50 (“Morning-Star 50”), a solar drone with six engines and a 164-fooe wingspan, made its maiden flight in 2022. Scattered information online mentions further test flights, but there have been no official statements since then. This is likely because, like Skydweller and PHASA-35, the aircraft is being developed for the military.
The Chinese have started late in this field, and the timing suggests they have learned from the West and now believe that the technology for solar cells, batteries and other elements is mature enough to be useful. Again, we may not know that Chinese solar aircraft are flying operationally until long after it happens.
Solar aircraft technology has definitely advanced to the brink of being operational. Perhaps Icarus is not, after all, the best Greek myth to represent solar drones. Maybe they are better illustrated by the Phoenix, a fabulous bird which rises up again renewed after its own destruction. Rather than an unattainable dream, perpetual solar flight starts to look more like an engineering problem that took some time to solve.
