Want to see what Darpa's "stay aloft for five years" Vulture UAV might look like? Successful bidders Aurora Flight Sciences and Boeing Advanced Systems have released images of their concepts for the solar-powered stratospheric UAV, but nothing yet from Lockheed Martin's Skunk Works.

Aurora's approach looks the most unusual. Its Odysseus UAV has two flight configurations: "Z-mode", in which the wing is partially folded so the solar cells are canted to catch as much sunlight as possible during the day; and flat, so that wing span is maximised to reduce drag and power consumption during the night.

Odysseus - folded by day, flat by night. Photo: Aurora Flight Sciences

But what's most unusual about Aurora's design is that it's actually three air vehicles that take off separately and dock in the stratosphere. This avoids having to fly that huge wing through the turbulent lower atmosphere, and allows an individual section to undock and fly home for repair if needed. A replacement vehicle "module" would be launched to take its place.

Boeing's Vulture - a Zephyr on steroids. Photo: Boeing

Boeing's Vulture concept looks more conventional. It is a single, very large air vehicle designed to stay up there for the full five years. The manufacturer says its design will leverage technology already demonstrated by QinetiQ's (much, much smaller) Zephyr solar-powered UAV.