Seattle-based Radical says it has put a full-size prototype for a solar-powered drone through its first flight, marking one low-altitude step in the startup’s campaign to send robo-planes into the stratosphere for long-duration military and commercial missions.

“It’s a 120-foot-wingspan aircraft that only weighs 240 pounds,” Radical CEO James Thomas told GeekWire. “We’re talking about something that has a wingspan just a bit bigger than a Boeing 737, but it only weighs a little bit more than a person. So, it’s a pretty extreme piece of engineering, and we’re really proud of what our team has achieved so far.”

Last month’s flight test was conducted at the Tillamook UAS Test Range in Oregon, which is one of the sites designated by the Federal Aviation Administration for testing uncrewed aerial systems. Thomas declined to delve into the details about the flight’s duration or maximum altitude, other than to say that it was a low-altitude flight.

“We take off from the top of a car, and takeoff speeds are very low, so it flies just over 15 miles an hour on the ground or at low altitudes,” he said. (Thomas later added that the car was a Subaru, a choice he called “a Pacific Northwest move, I guess.”)

The prototype ran on battery power alone, but future flights will make use of solar arrays mounted on the plane’s wings to keep it in the air at altitudes as high as 65,000 feet for months at a time. For last month’s test, engineers added ballast to the prototype to match the weight of the solar panels and batteries required for stratospheric flight. Thomas said he expects high-altitude tests to begin next year.

Thomas and his fellow co-founder, chief technology officer Cyriel Notteboom, are veterans of Prime Air, Amazon’s effort to field a fleet of delivery drones. They left Amazon in mid-2022 to launch Radical and have since raised more than $4.5 million in funding. September’s test of a full-size drone follows up on the 24-hour-plus flight of a 13-pound subscale prototype in 2023.

The company’s manufacturing operation is based in Seattle’s Ballard neighborhood. There are currently six people on the team, plus a new hire, Thomas said. “We’re still lean,” he said. “To make this airplane work, it has to be really efficient, right? Really efficient electronics and aerodynamics. And you also need a really efficient team.”

Thomas said Radical has attracted interest from potential customers, but he shied away from discussing details. “We’re working with groups in the government and also commercially,” he said. “Obviously there are applications at the end of this that span everything from imagery through telecommunications and weather forecasting. There are a lot of people really interested in the technology, and the thing that stops us from serving those customers is not having a product up in the sky. So, that’s what we’re working through.”

Radical’s solar-powered airplane, known as Evenstar, is just one example of a class of aircraft known as high-altitude platform stations, or HAPS. Thomas and his teammates use a different term to refer to Evenstar. They call it a StratoSat, because it’s designed to take on many of the tasks typically assigned to satellites — but without the costs and the hassles associated with launching a spacecraft.

Potential applications include doing surveillance from a vantage point that’s difficult to attack, providing telecommunication links in areas where connectivity is constrained, monitoring weather patterns and conducting atmospheric research.

“We have customers who are really excited about the way that this can improve how we understand Earth’s weather systems and climate,” Thomas said. “That’s an application that we’re really excited to get into.”

Evenstar will carry payloads weighing up to about 33 pounds (15 kilograms). “That was based on analysis about major use cases,” Thomas explained. “That payload is enough to carry high-bandwidth, direct-to-device radio communications, or to carry ultra-high-resolution imaging equipment.”

Radical isn’t the only company working on solar-powered aircraft built for long-duration flights in the stratosphere. Other entrants in the market include AeroVironment, SoftBank, BAE Systems, Swift Engineering, Kea Aerospace, Korea Aerospace Industries and NewSpace Research & Technologies. Airbus’ solar-powered Zephyr set the record for long-duration stratospheric flight in 2022 with a 64-day test mission that ended in a crash.

Among those who tried but failed to field stratospheric solar drones are Alphabet, which closed down Titan Aerospace in 2016; and Facebook, which abandoned Project Aquila in 2018.

Thomas said the outlook for high-flying solar planes has brightened in the past decade.

“The key supporting technologies have matured enormously,” he said. “Commercial battery energy density has doubled in that 10-year time period. Solar cells are 10 times cheaper than they were just 10 years ago. And then you have advances in compute and AI, and all of these things feed into the situation we have now, where it’s actually possible to make the models close — whereas when we run the 10-year-old numbers, we can’t close the models.”

The way Thomas sees it, the concept behind Radical isn’t all that radical anymore.

“Not only do our models say this will work, but we have flight data that agrees with our models, and says this is a technology that can serve its purpose and unlock the potential of persistent infrastructure in the sky,” he said. “I can see why other people are pursuing it. It’s not a new idea. It’s one that people have wanted to crack for a long time, and we’re at this critical inflection point where it’s finally possible.”