Late last year, DARPA researchers upped the standard two-hour endurance of Lockheed Martin's Stalker small unmanned aerial system (UAS) by a factor of four using a propane-fueled compact solid oxide fuel cell (SOFC). Now the flight time of the aircraft has been improved by a whopping 2,400 percent, with a test flight lasting more than 48 hours using a laser power system to wirelessly transfer power to the UAS from the ground.

The indoor flight test saw the Stalker UAS modified to incorporate a Power Link system developed by Kent, Washington, based company LaserMotive. The system sends a beam of laser light from a ground station, through the air to a receiver of photovoltaic cells on the UAS. The system can provide continuous power to the UAV while it stays within range of the ground station and can also charge batteries onboard the UAS to allow it to fly beyond the range of the power link.

In fact, the indoor flight test, which was held in a wind tunnel, finished with the Stalker's battery having more energy stored than it did when the flight began. With the system providing the potential for practically unlimited flight time, the flight test was only concluded because the initial endurance goals set by the team had been met.

"This test is one of the final steps in bringing laser-powered flight to the field," said Tom Nugent, president of LaserMotive. "By enabling in-flight recharging, this system will ultimately extend capabilities, improve endurance and enable new missions for electric aircraft. The next step in proving the reality of this technology is to demonstrate it outdoors in an extended flight of the Stalker."

With the ground-based laser powered by a standard industrial electrical outlet or a generator, there is potential for it to be positioned at a base or on the back of a vehicle in the field. While there is a significant loss in energy when transferring power between two points near the ground using the system due to turbulence and dust, LaserMotive says that power beamed vertically suffers relatively little energy loss as it quickly gets away from ground effects. The company is even working on systems to beam power from the Earth's surface to satellites and even the Moon.

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