Science reporter, BBC News
Technical Director Chris Kelleher on the solar plane's 3 day flight
A UK-built solar-powered plane has set an unofficial world endurance record for a flight by an unmanned aircraft.
The Zephyr-6, as it is known, stayed aloft for more than three days, running through the night on batteries it had recharged in sunlight.
The flight was a demonstration for the US military, which is looking for new types of technology to support its troops on the ground.
Craft like Zephyr might make ideal platforms for reconnaissance.
They could also be used to relay battlefield communications.
Chris Kelleher, from UK defence and research firm QinetiQ, said Unmanned Aerial Vehicles (UAVs) offer advantages over traditional aircraft and even satellites.
"The principal advantage is persistence - that you would be there all the time," he told BBC News. "A satellite goes over the same part of the Earth twice a day - and one of those is at night - so it's only really getting a snapshot of activity. Zephyr would be watching all day."
Deployment close
The latest flight was conducted at the US Army's Yuma Proving Ground in Arizona.
The Zephyr flew non-stop for 82 hours, 37 minutes.
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That time beats the current official world record for unmanned flight set by the US robot plane Global Hawk - of 30 hours, 24 minutes - and even Zephyr's own previous best of 54 hours achieved last year.
However, the Yuma mark remains "unofficial" because QinetiQ did not involve the FAI (Federation Aeronautique Internationale), the world air sports federation, which sanctions all record attempts.
The US Department of Defense funded the demonstration flight under its Joint Capability Technology Demonstration (JCTD) programme.
This programme is designed to advance the technologies American commanders would most like to see in the field.
"We think Zephyr is very close to an operational system - within the next two years is what we're aiming for," Mr Kelleher said. "We have one more step of improvements; we trying to design a robust and reliable system that will really sit up there for months; and we want to push the performance."
Energy density
The trial, which took place between 28 and 31 July, also included the participation of the UK Ministry of Defence.
The 30kg Zephyr was guided by remote control to an operating altitude in excess of 18km (60,000ft), and then flown on autopilot and via satellite communication.
It tested a communications payload weighing approximately 2kg.
 Zephyr should be in commanders' hands within two years |
At first sight, the propeller-driven Zephyr looks to be just another model aircraft, and it is even launched by hand. But this "pilotless" vehicle with its 18-metre wingspan incorporates world-leading technologies.
Its structure uses ultra-lightweight carbon-fibre material; and the plane flies on solar power generated by amorphous silicon solar arrays no thicker than sheets of paper. These are glued over the aircraft's wings.
To get through the night, the propellers are powered from lithium-sulphur batteries which are topped up during the day.
"A lot of effort has gone into power storage and light-weighting the systems," explained Mr Kelleher. "Lithium sulphur is more than double the energy density of the best alternative technology which is lithium polymer batteries.
"They are an exceptional performer. We've worked with the Sion Corporation. They've had them in development for years. We're actually the first application in the world for them."
Vulture venture
Zephyr has demonstrated that it can cope with extremes of temperature - from the blistering 45C heat found at ground level in Arizona's Sonoran Desert, to the minus 70C chill experienced at altitudes of more than 18km (60,000ft).
The engineers from the Farnborough-based company are now collaborating with the American aerospace giant Boeing on a defence project codenamed Vulture.
This would see the biggest plane in history take to the sky, powered by the sun and capable of carrying a 450-kilo (1,000lb) payload.
US commanders say the design must be able to maintain its position over a particular spot on the Earth's surface uninterrupted for five years.
QinetiQ is also developing UAV technology for civilian uses.
It has been working recently with Aberystwyth University on field monitoring trials, plotting areas of ground that may or may not need fertiliser applications.
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