A Skydweller solar-powered drone just successfully completed a series of test flights with U.S. Navy funding. This time the longest flight was ‘just’ 22.5 hours, but in operation the aircraft will fly patrol missions lasting for weeks, powered by solar cells in the day and batteries at night.
The road to solar aircraft with effectively unlimited endurance has been a long one, littered with the flimsy fragments of failed projects. The Navy is finally closing in on an important new capability, one that could transform its long-range, long-endurance missions – and kickstart a whole new field of commercial aviation.
Made of lightweight carbon fiber, Skydweller has a wingspan of 236 feet (72 meters) but weighs just 5,620 pounds (2,550 kilograms), less than many light aircraft, with up to 800 pounds of payload. Robust, flexible wings are essential.
“In this field you need to really understand what it means to fly large flexible structures,” Skydweller Aero CEO Robert Miller told me. “If the people who are developing it do not understand aeroelasticity you can get wings popping off.”
Skydweller is strong and flexible enough to survive where others have literally disintegrated.
Survivor In The Death Zone
In principle building a perpetual solar aircraft is easy. Just find a lightweight powered glider, cover the wings with solar cells and add electric motors and batteries. In practice it is far from simple. The big, fragile aircraft are prone to break apart when hit by turbulence or other stresses encountered while getting to and from the stratosphere, a challenge has defeated some of the best in the business.
“There is ‘death zone’ for these aircraft from five to thirty thousand feet,” says Miller, and aircraft behave in complex ways. “Aerodynamics scale, but aeroelastics don’t.”
NASA failed. Its groundbreaking solar-powered HELIOS prototype flew to over 90,000 feet in 2001, but broke up in midair in 2007, and the project was terminated.
Boeing and DARPA failed. After Boeing won an $89m contract for their Solar Eagle aircraft from DARPA, they ran into difficulties and the program ended in 2012, with DARPA abandoning flight tests in favor of basic research into energy management.
Google failed. Their Solara 50 was supposed to provide internet connectivity to remote areas, but during flight tests crashed shortly after takeoff in 2015. The aircraft suddenly gained speed and “the left outboard wing section separated from the aircraft” according to an NTSB report. Google gave up with the project shortly afterwards.
Facebook failed. Their Aquila was another project intended to provide internet from the skies, but a prototype a broke up in mid-air in 2016. While Aquila was coming into land, a sudden gust of wind caused the right wing to break off. The Aquila program did not recover and was canceled in 2018.
Skydweller started with two big advantages over the others. One is that it is derived from the Solar Impulse crewed aircraft which flew around the world in 2016. While other solar aircraft only carry a few pounds, Solar Impulse had a crew capsule weighing several hundred pounds, meaning it was built with safety in mind and had plenty of capacity when it was converted to uncrewed operation. The other advantage is the level of expertise.
“Fundamentally, my team has a lot more experience in aeroelasticity than all the other solar programs combined,” says Miller. “ Sometimes they don’t know what they don’t know. My team has worked on aircraft that have a million plus hours in the stratosphere.”
This expertise has produced a robust, reliable aircraft suitable for military service.
Solar Flight Plan
Miller says that the recent flight tests collected all the data they needed to validate the result of simulations. The campaign was sustained by funding from Naval Air Warfare Center Aircraft Division (NAWCAD) to evaluate autonomous maritime patrol aircraft.
The next stage will involve flying with the sensors and communications that the aircraft will use in operation with the Navy.
“In the winter we’ll be flying short flights of 15 to 20 hours in the Gulf of Mexico and the Caribbean with full U.S. Navy payloads, to demonstrate the utility of the system and how we can collect and distribute data,” says Miller. “In the Spring we will be flying 7-to-15-day flights, which is when our objective will be fully achieved.”
Skydweller gives the Navy an aircraft with unprecedented endurance. Their current long-range drone is the MQ-4C Triton, a conventionally-fueled aircraft with an endurance of 30 hours. But this type of aircraft is expensive buy and expensive to fly, costing an estimated $35,000 per flight hour to operate — or a million dollars for a 30-hour mission. The Skydweller is far cheaper, with no fuel requirement and very little maintenance — “It’s flight hours per maintenance hour, rather than maintenance hours per flight hour,” says Miller.
Even with recent upgrades, Skydweller is not pushing the limits of what is technologically possible for this type of aircraft,
“The fly-by-wire control system has been upgraded and it is now fully redundant with no single point of failure. The batteries have been upgraded to the state-of-the art of what is now in production,” says Miller. “We will shortly be upgrading the engines with high efficiency motors that have magnetic gearboxes, that will bring ultra-reliability and about an 8-10% increase in efficiency.”
Solar aircraft benefit from being at the intersection of several technologies, all of which are improving,
“There has been a lot of growth in the technology in general, “ says Miller. “With massive industry investment in solar, in batteries and in motors. The water is rising and all the boats are rising with it.”
That rise could enable a whole new solar-powered aviation industry.
Commercial Future
Skydweller will initially be a military project, carrying a variety of Navy sensors (typically radar) and communication packages. As we previously noted, a suite of AI software from Palantir will handle, fuse and analyze complex data collected by the sensors. Rather than beaming back gigabytes of video of empty ocean, Skydweller only needs to communicate when it sees a ship or other item of interest.
But as Google and Facebook had observed, long-endurance solar aircraft also have tremendous commercial potential as airborne communications platforms.
“The military will be early adopters of this technology, and we can grind out the rough edges with them before we roll out to a wider base,” says Miller. “It is very much like satellite communications, where the military were early adopters. They burned down the risk and the market took off, now it is 99% commercial.”
Miller sees solar aircraft as a complementary capability to LEO satellite communications rather than a direct rival. In certain parts of the world — Miller mentions specific regions his team have looked at — the economics could favor a network of solar aircraft circling overhead to provide continuous coverage.
At present Skydweller Aero just have one aircraft but demand is likely to drive production.
“We hope to start building another aircraft next year, and then several more after that,” says Miller.
After that, production could accelerate rapidly into big numbers. Lightweight aircraft with no jet engines can be manufactured quicker than airliners or other aircraft.
There is a commercial rival in the form of the Zephyr series made by Airbus, a type which has been under development for more than 20 years. This is a smaller, lighter aircraft; the Zephyr 8 has an 82-foot wingspan and carries a payload of about 11 pounds compared to 800 pounds for Skydweller. Zephyr is marketed as a potential “flying cellphone tower” but the project has experienced difficulties. Three aircraft have been lost, the latest in an accident in June 2022 when, like so many others, it broke up in mid-air.
In solar aviation, only the strong survive. And Skydweller, originally built to be robust enough to carry human pilots safely, looks like the strongest drone in the field. After decades of dead ends, solar aviation may finally be taking off.
