Mid-air refuelling of manned aircraft has been taking place for decades. However, the process requires flying a massive tanker plane and connecting the tanker to an aircraft that needs fuel via a delicate mid-air link.
In recent years, it has become possible to refuel unmanned drones in mid-air via laser refuelling systems that deliver energy from a station on the ground. The key to this system is an array of photovoltaic cells that are explicitly calibrated to an infrared laser’s wavelength. A ground-based system can then direct a laser beam onto the array, transferring enough energy to keep the craft aloft.
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For years, the concept of sending energy with lasers was out of reach due to such low efficiency of available lasers, as well as losses associated with transferring and transforming power. Then, in the early 2000s, the development of infrared lasers with an efficiency 40 to 50 percent, in addition to with highly-effective photoelectric systems made a laser refuelling system possible.
A commercial breakthrough
In 2012, Lockheed Martin announced that a modified version of its Stalker Unmanned Aerial System made use of a laser refuelling system to maintain continuous flight for 48 hours.
The laser refuelling system used in the Lockheed trials actually won NASA’s XPRIZE and was developed by LaserMotive, which is now known as PowerLight Technologies. The system consisted of a power source, laser power supply, laser, beam director, tracking sensors and safety sensors. Meanwhile, the system on-board the Stalker consisted of a laser-receiving “solar array,” which was connected to a battery and motor.
Lockheed's Stalker was customized for indoor flight and put into a wind tunnel set to replicate flight conditions. The experimental setup meant the laser system didn't have to track a moving aircraft.
Under the experimental conditions, the laser system shipped a relentless flow of power to the Stalker, keeping it running for much longer than even the test guidelines required. The Lockheed team only ended the trial because the Stalker and the laser system had already far surpassed the goals they had attempted to meet.
Additionally, the trial ended with the Stalker's battery having more power than when it began, indicating that the system is capable of sending enough power to keep a craft aloft under more stressful flight conditions.
The capacity for persistent flight would lead to operational cost savings and safety, partly by decreasing the number of take-offs and landings, which are the highest-risk situations during any drone flight. Moreover, the laser system is not dependent on magnetic or electric fields. This would allow the drone to reach considerably greater distances than crafts using electric or magnetic systems.
Longer ranges and persistent flight capabilities has significant implications for unmanned aerial surveillance and privacy concerns associated with drone surveillance.
While a laser refuelling system appears to hold great promise for aviation, the same technology can also be applied elsewhere, including in telecommunications, infrastructure maintenance, disaster response and law enforcement.
In 2016, a team of Russian scientists announced they were in the midst of developing a laser refuelling system that could be used for both aircraft and small micro-satellites in orbit about Earth. In fact, such a system might more effective for space-based purposes because nothing absorbs laser radiation in the vacuum of space.
Currently, nearly all space-based assets get energy from solar batteries, even though their enormous size creates many difficulties. In outer space, the massive and advanced solar batteries must continuously face the sun. Laser supplies of energy could help extend the lifetime of micro-satellites regardless of location and orientation.
A laser refuelling system could also facilitate scientific experiments in space. For instance, the International Space Station does not have a steady 'zero gravity' environment. On-board gravity oscillates and undergoes micro-acceleration as a result of various devices operating in it, like engines used to turn the solar batteries. This keeps the successful conclusion of many trials, such as those involving the growth of crystals. Laser refuelling could help produce ideal gravity conditions for such trials on autonomous modules.