Lockheed Martin and LaserMotive, Inc., recently demonstrated the capabilities of an innovative laser power system to extend the Stalker Unmanned Aerial System (UAS) flight time to more than 48 hours. This increase in flight duration represents an improvement of 2,400 percent.
Stalker is a small, silent UAS used by Special Operations Forces since 2006 to perform intelligence, surveillance and reconnaissance missions.
Stalker UAV fly for 48 hours with wireless laser recharging
LaserMotive has demonstrated wireless power systems with a receiver specific power as high as 800 W/kg. By comparison, lithium-ion batteries used in small UAVs are generally used with a specific power in the range of 200-500 W/kg.
With current laser cells, the deliverable power is limited mainly by cell cooling, and it can easily exceed 6 kW/m2, or about 1 HP per square foot.
Current diode laser technology and reasonable apertures can produce useful beam intensity at the receiver out to a range of ~10km. Longer distances can be achieved by switching to higher-quality sources (such as fiber lasers), although there is a penalty in cost and efficiency can be achieved by switching to higher-quality sources (such as fiber lasers), although there is a penalty in cost and efficiency.
The biggest limitation of laser power beaming is that it requires a line-of-sight (either direct or reflected off mirrors) between the transmitter and receiver. Light weather can reduce efficiency and range, but heavy weather (heavy rain or snow, or fog) can block transmission altogether.
Overall system efficiency (DC power in through to DC power out) can be more than 25 percent, although in practice it is currently closer to 20 percent. Off the shelf diode lasers currently exhibit up to ~60 percent DC-to-light efficiency, and the DARPA SHEDS program demonstrated [http://www.nlight.net/nlight-files/file/technical_papers/SPRCS05_stanford.pdf] up to 85 percent efficiency. Current photovoltaics can achieve more than 50 percent light-to-DC efficiency, and monochromatic conversion efficiencies can theoretically go higher. We expect overall system efficiency to exceed 30 percent within several years, eventually approaching 50 percent
While the word “laser” conjures up the idea of a death ray to many, the power density used in the LaserMotive wireless power systems is in the range of only 10 to 30 times sunlight.
Tom Koonce, Lockheed Martin Skunk Works® Stalker program manager said “A ground-to-air recharging system like this allows us to provide practically unlimited flight endurance to extend and expand the mission profiles that the Stalker vehicle can fulfill.”
The Stalker UAS was modified for the indoor flight test to incorporate LaserMotive’s proprietary system that makes it possible to wirelessly transfer energy over long distances using laser light to provide a continual source of power to the UAS. At the conclusion of the flight test, held in a wind tunnel, the battery on the Stalker UAS had more energy stored than it did at the beginning of the test. The test was concluded only because the flight had already surpassed the initial endurance goals set by the team.
“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.”
Laser Motive is steadily increasing the power and distance that they can transmit.
Video of Laser Motive power a quadcopter from one year ago