Can lasers be used to power aircraft or even space rockets? BILL READ FRAeS looks how recent experiments have already proved that such a system is possible.
In a recent RAeS conference on innovation, two speakers, Arava Anmol Manohar AMRAeS and Ritvik Anand, described a possible future in which aircraft could be powered by ‘E-viation’ laser beams generated by renewable energy. Each aircraft would be fitted with photovoltaic (PV) cells which would be powered by energy transmitted from a network of ground-based lasers. The PV cells would convert the lasers into electricity which would drive the aircraft’s electric motors.
While the technology to power large aircraft using laser beams does not yet exist, there have been a number of experiments which show that such a system can work and has been used to fly small UAVs.
The principles of power transfer from ground station to UAV. (Lasermotive)
Laser power beaming works by using a laser to send concentrated light through the air or fibre optic cable to a remote receiver that converts the light to electricity. Using power from an electrical supply (generated from fossil fuels or renewable energy), the laser is shaped by a set of optics to define the beam size at its destination. The beam is propagated through either air, space or fibre optic cable until it reaches a photovoltaic (PV) receiver which converts light to electricity. A PV receiver can generate electricity from the sun but a laser has the advantage of being more powerful and being available 24 hours a day.
The idea of power beaming has been around for some time with early proposals put forward for its use in space-based solar farms to supply energy to Earth. However, in recent years, it has also been suggested as a method to recharge batteries on electric-powered propeller UAVs. Recent advances in laser technology, solar power generation systems and rechargeable batteries have now made these suggestions not only possible in theory but also in practice.
The first experiments in laser propulsion began in the 1970s. In 1972, an experiment conducted by the UK Atomic Energy Authority (UKAEA) was able to propel small micro-foil targets in a vacuum to high velocities. More recent experiments at the US Naval Research Laboratory and other military and civilian laboratories claim to have propelled targets up to small fractions of the speed of light. At the fourth High Power Laser Ablation (HPLA) conference in 2002, Japanese scientists from the Tokyo Institute for Technology reported that they had been able to use lasers to laser to evaporate water off a paper plane, forcing it forward.
A test of the Myrabo Lightcraft showing plasma formation after illumination from a single high-energy laser pulse. (LightCraft)
Lasers have also been used to propel non-flying objects into the air. Professor Leik Myrabo conducted experiments at the private US research Rensselaer Polytechnic Institute in which he projected a laser into the base of a reflective funnel-shaped craft causing the air to heat up to 30,000 deg . The heat was channelled towards the centre and expanded the air underneath it to generate lift. To keep the platform stable, a small jet of pressurized nitrogen spun the craft at 6,000rpm.
LightCraft launch. (LightCraft)
After a series of tests, a new record was set in 2000 at the US Army White Sands missile range in New Mexico in which a 6in LightCraft achieved a 10.5sec flight which reached a height of 236ft (72m) (https://www.youtube.com/watch?v=-Nm16wp0kMs) However, the range of the LightCraft was limited by the power of lasers and the small amount of propellant that the small craft can carry.
NASA’s first tests powered a model aircraft using a theatrical spotlight. (NASA)
In 2002 indoor tests were conducted at NASA Dryden in which the beamed visible light from a large theatrical spotlight was used to power a small model aircraft. The following year, researchers at the Dryden Flight Research Center and the University of Alabama flight-demonstrated a small-scale radio-controlled MOTH 1 and MOTH 2 model aircraft made from balsa wood and carbon fibre tubing and covered with cellophane-like film.
First flight of the MOTH2 UAV with laser light illuminating its PV array. (NASA)
In the 2003 tests, engineers manually directed the beam from a 1kw laser at a panel of 17% efficient infrared-sensitive photovoltaic silicon cells mounted on the underside of the 2m wingspan MOTH 2 to power a 6wt motor as it flew circles inside the building. The power source used as a 1.5-kW 940-nanometer (nm) diode array with high (50%) efficiency infrared laser at a range of 15.2m. MOTH 2 made several indoor flights of over than 15 minutes each, although these could have theoretically continued indefinitely using an autotracker. Two months after the initial laser-powered flight demonstrations, NASA used the same laser system on a rotorcraft version operated along guide wires.
These experiments were followed by outdoor tests at Redstone Arsenal on the MOTH 2 but these were not successful because of problems with seeing the laser target spot and the light weight and limited power of the aircraft made it difficult to control in gusty winds.
LaserMotive laser-powered helicopter. (Lasermotive)
More recent tests have been carried out by Seattle-based LaserMotive, a research and development company specialising in laser power beaming for commercial applications which won a NASA Centennial Challenge Prize for beaming power over 1km in 2009.
LaserMotive laser-powered helicopter in flight. (Lasermotive)
In 2010 LaserMotive successfully flew a model laser-powered Ascending Technology Pelican quadcopter UAV for 12 hours. However, the company admitted that there was a problem with using lasers to power helicopters because they cannot glide if they lose power. This was followed in 2012 by tests in which LaserMotive used a laser to power a specially modified Lockheed Martin Stalker, a small UAV used by Special Operations Forces for intelligence, surveillance and reconnaissance (ISR) missions. By using the laser to constantly recharge its batteries, the Stalker was kept airbourne in a wind-tunnel for 48 hours. Outdoor tests were also carried out in which the Stalker reached a height of 600m.
LaserMotive used a laser-transmitter demonstrator consisting of two gallium arsenide-based diode laser arrays which were merged into a single laser beam which was focused into a gimbaled mirror and directed toward the UAV. The laser system was able to transfer between 20-25% of the electricity from its ground source. The Stalker UAV was fitted with a photovoltaic panel on the underside of the 9.5ft wing. The first tests successfully transmitted power from a distance of 30 ft over a period of two days. By the end of the test the UAV’s battery held more energy than when it began.
LaserMotive has also been working on other challenges associated with laser-power aircraft. One problem is keeping the laser focused on the aircraft and the company has developed computer software that uses video signals to keep the laser pointed at the receiver as the drone flies. Transmission distances are also being increased. LaserMotive is planning to conduct outdoor tests with larger aircraft over greater distances.
Laser flight - pros and cons
While the technology needed for laser propulsion of aircraft is still in its early stages, the system does offer a number of advantages. Although, it is still a long way off being practical for large aircraft, power beaming could be used to recharge batteries in small surveillance, observation or signal transmitting UAVs to prolong their range. In 2011 The RAND Corporation published a paper for the US Air Force on the feasibility of laser power transmission to a high-altitude UAV (http://www.rand.org/content/dam/rand/pubs/technical_reports/2011/RAND_TR898.pdf). The report concluded that the concept was technologically feasible but there were still some limiting factors.
Lasers could be used to continuously recharge a UAV to extend its endurance. (Lasermotive)
The report said that, while solar-powered UAVs have the advantage that they can fly higher and longer than those using conventional fuel, they do have the disadvantage of not being able to receive power at night and having to rely on batteries. A high-altitude laser-powered PV UAV could be powered by lasers from the ground at ranges of up to 40km. However, the intensity of the power received would vary depending on the angle of the beam. There is also the disadvantage that, currently, the aircraft needs to be close to the beam source. The power of the beam can also be degraded by clouds. One solution is not to transmit the beam from the ground but from the air - such as from another aircraft which could fly above the clouds. According to the RAND report, this could increase the potential range of a laser transmission beam to hundreds of kilometres.
The RAND report was also critical of suggestions that PV UAVs could be powered from another aerial platform, pointing out that such platforms could be used for the same missions without the need for duplication. Regarding the problem of what happens when the laser beam is blocked by another aircraft or the power is diminished by clouds, the report suggested that a PV aircraft could be targeted by a system of several lasers at once, making its movement between ground stations easier (an idea also suggested by NASA). Using a distributed system also means that each individual laser could be less powerful, reducing any possible danger from the beams to other aircraft or birds.
In addition to recharging UAVs, a number of other proposals for laser transmission have also been proposed. Laser-powered UAVs or high-altitude airships could be used as low-cost alternatives to satellite networks to relay mobile phone signals, TV broadcasting or providing Internet connections. They could also be used for Earth observation applications, such as remote sensing of the surface or monitoring the atmosphere. There have also been proposals for using lasers to power swarms of UAVs or for low-altitude scout aircraft monitoring a military combat area or flying in front of a convoy to warn against an ambush.
There have also been proposals for space applications, one is which for a ‘modular laser launcher’ in which a large number of ground-based ‘beam modules’ are used to transmit small amounts of power to a heat-exchanger rocket vehicle. The system would be scaleable and could be expanded by adding additional beam modules, offering a low-cost alternative to conventional rockets. Other ideas include orbiting power stations which could beam lasers down to ground installations or vehicles either on the Earth or on other planets. Lasers could also be used to recharge satellites which were not receiving enough solar power to recharge their batteries.