In a study published in the Journal of Spacecraft and Rockets, researchers led by the University of Tsukuba in Japan have demonstrated wireless power transmission via microwaves for a free-flying drone and determined the efficiency of this process.
Previous analyses of this kind were carried out decades ago and mostly considered microwaves of a low frequency of a few gigahertz. Power transmission efficiency increases as the operating frequency is raised, so the team behind this latest research used microwaves with a relatively high frequency (28GHz). The team’s drone weighed roughly 0.4kg and hovered for 30 seconds at a height of 0.8m above the source of a microwave beam.
“We used a sophisticated beam-tracking system to ensure that the drone received as much of the microwave power as possible,” lead author Kohei Shimamura said in a statement. “Moreover, to further increase the transmission efficiency, we carefully tuned the phase of the microwaves using an analogue phase shifter that was synchronised with GPS units.”
The researchers measured the efficiencies of the power transfer through the beam (four per cent), the capture of microwaves by the drone (30 per cent), the conversion of microwaves to electricity for propulsion (40 per cent), and other relevant processes. Based on this information and an analytical formula, they calculated the overall power transmission efficiency in their experiment to be 0.43 per cent. In a previous study, the team measured the total transmission efficiency for a fixed-position (rather than free-flying) drone to be 0.1 per cent.
“These results show that more work is needed to improve the transmission efficiency and thoroughly evaluate the feasibility of this propulsion approach for aircraft, spacecraft, and rockets,” Shimamura said. “Future studies should also aim to refine the beam-tracking system and increase the transmission distance beyond that demonstrated in our experiment.”
The team said microwave-powered rocket propulsion is still in its early stages, but it could become a superior way to launch rockets into orbit given the high onboard-fuel demands of conventional propulsion techniques.