A microwave-powered rocket? Drone project suggests it may be possible

Researchers in Japan are using microwaves to power free-flying drones, a project that  could possibly pave the way for a new type of rocket.

Currently, most rockets generate thrust by using controlled explosions of a solid or liquid fuel source, which can make up 90% of their total weight. However, new research published in the Journal of Spacecraft and Rockets demonstrates the potential of using an alternative source of fuel: microwaves. 

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Microwaves are a type of electromagnetic radiation. As such, they're jam-packed with energy that can be converted into electricity, much as sunlight can be converted to power by solar panels. In the new study, researchers generated electricity to fuel free-flying drones by beaming microwaves directly at them.

"In drone experiments, microwave power is sent from the antenna on the ground to the antenna on the drone. A rectifier is used to convert RF [radio frequency] to DC [direct current], and the DC power is used to drive the drone's motors. We call it "rectenna" (rectifier + antenna)," one of the new study's authors, Kohei Shimamura of the University of Tsukuba, told Space.com in an email.

Previous studies exploring microwave-powered propulsion used low-frequency waves but found that when the frequency is raised, the power transmission efficiency also increases. Taking this fact into account, the research team  used high frequencies (28 gigahertz) to lift a 0.9-pound (0.4 kilograms) drone off the ground. 

Sitting directly above the source of the microwave beam, the transmitted power enabled the drone to reach a height of around 2.6 feet (0.8 meters) for 30 seconds. "We used a sophisticated beam-tracking system to ensure that the drone received as much of the microwave power as possible," Shimamura said in a statement. 

In the experiment, 30% of the emitted microwaves were captured by the drone, and 40% of those microwaves were converted to electricity for propulsion. 

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"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."

Although the new research showcases the potential of microwave-powered propulsion, the technology largely remains in its infancy, especially when considering its potential use for rocket flight. 

"The big challenge is to track the microwave to the rocket until it reaches an altitude of 100 km [approximately 62 miles]. In order to achieve this, it is necessary to control the phase of the thruster and microwave with high accuracy. Also, aligning the phase of multiple microwave sources with high power is a future challenge," Shimamura told Space.com via email. "Cost is a major issue as well as a technical challenge. Building a high-power source of several MW [megawatts] is equivalent to building a nuclear fusion power plant, and the cost of launching a rocket is currently very high." 

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