Zero rocket fuel with wireless power transfer Successful measurement experiment takes research one step forward – DG Lab Haus

　In recent years, as the space business by private companies has become more active, reducing the cost of launch vehicles has become a major issue.

　The cost of transporting artificial satellites into outer space is expensive, and it is said that 1 gram of satellite is 1 gram of gold. Recently, satellites have become lighter, and the number of small satellites weighing less than 100 kg is increasing, but even such satellites cost more than purchasing 100 kg of gold.

　One of the factors driving up the cost of launching rockets is poor fuel efficiency. Currently, rocket engines use liquid or solid fuel, but the ratio of fuel to the total mass is more than 90%, and it is said to be inefficient that it is said that "rocket is flown to fly fuel". It's becoming

　The "microwave rocket" is expected to eliminate this fuel and reduce the rocket launch cost to about 1/100. In a microwave rocket, microwaves (*) from the ground are supplied wirelessly, and propulsion is generated by generating plasma inside the rocket or by heating the fuel.

*Short-wavelength radio waves used for microwave ovens, etc.

However, there are still many technical issues with this. One of the challenges is that there has been no way to instantaneously measure high-power microwaves, and it is impossible to measure how much power is being sent from the ground to the rocket.

In this situation, a research group led by Kohei Shimamura, an assistant professor at the Information Systems Department of the University of Tsukuba, and Tsuyoshi Kariya, an associate professor at the Plasma Research Center of the University of Tsukuba, conducted rocket thrust generation experiments using microwaves with a frequency of 28 GHz (gigahertz). We succeeded in measuring the total propulsion efficiency including the power supply efficiency. We asked Mr. Shimamura about the content and background of the experiment, and what kind of impact this research might have on society and business in the future.

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　According to Mr. Shimamura, the microwave rocket concept itself is not new, and was invented nearly 40 years ago. The experiment was put on hold for a long time due to technical problems, but the recent progress in the development of a high-power microwave generator called a "gyrotron" has shed light on this attempt again. In the 2000s, the successful launch experiment by Professor Koya Komurasaki of the Graduate School of the University of Tokyo made Japan a leap forward in this field of research.

　However, most of the research on microwave rockets so far has focused on "how to convert microwaves into rocket thrust". Under such circumstances, Mr. Shimamura, who was researching wireless power supply using microwaves in general and was experimenting with flying drones using microwaves, focused on the extremely low efficiency of power transmission to drones. We thought that the inefficiency of wireless power supply would be an issue for rocket launches as well. Therefore, the first step was to create a mechanism to accurately measure the power supply efficiency.

　The propulsion efficiency of a rocket is multiplication, so no matter how much the technology for converting microwaves into thrust improves, if the efficiency of sending microwaves from the ground to the rocket remains low, the propulsion efficiency will not improve. Microwaves We were aware that this would be a bottleneck in the practical use of rockets, which led us to this experiment." (Mr. Shimamura).

　In the experiment, a 500 kW class gyrotron owned by the Plasma Research Center of the University of Tsukuba was used to irradiate microwaves towards a cylindrical propulsion machine with a diameter of 200 mm and a length of 600 mm. When Shimamura and his colleagues measured the microwaves inside the propulsion unit using a rectenna circuit (*) that they had developed independently, they found that the power transmission/reception efficiency from the outlet (power supply) to the rocket was about 6%.

* A reception circuit that integrates an antenna and a rectifier that converts alternating current radio waves into direct current.

"We are still at the starting point. First of all, we need to improve the power transmission and reception efficiency by devising the shape of the rocket and antenna. In addition, we need to develop technology to control the attitude of the rocket, etc., until the microwave rocket is put into practical use. , There are many technical elements that require research and development, just like ordinary rockets.”

Drones can be realized soon

What impact will research on wireless power supply using microwaves have on society in the future?

　According to Mr. Shimamura, launching rockets with microwaves and dramatically changing the business and space development environment is a possibility that ``may or may not happen in our lifetime.'' He said, "It is a long-term goal to launch a rocket into space with microwaves."

On the other hand, he said, "It's not that difficult to wirelessly power drones, small aircraft, flying cars, etc.", especially if the funds and development environment are ready, wireless power feeding to drones can be realized immediately. I can do it," he confidently said.

　Using a typical lithium-ion battery, the flight time of a drone is limited to several tens of minutes. Its use will expand rapidly.

"Generally, airplanes and rockets require the most fuel during landing and takeoff. This is where the most fuel is used, using microwave wireless power supply, and using batteries when flying. In other words, It's going to be a hybrid of battery and wireless power supply.In terms of current drones, even if the capacity of the battery can be greatly reduced, the fuel consumption will be sufficiently improved, and the utility value of the drone will increase significantly."

　However, in order to put wireless power supply using microwaves into practical use, there are some issues such as the development of laws and overcoming the impact on the ecosystem. Coexistence with communication networks is a particularly big problem. If high-power microwaves are emitted here and there, there is a risk of interference with other communication networks. For this reason, it is necessary to explore ways of coexistence, such as "placing wireless power supply equipment in mountainous areas with few people" in the future.

　By the way, in this measurement experiment, we used the same 28GHz frequency as 5G, but this is the same as 5G when Associate Professor Kariya, a collaborator, used the equipment used in his research on nuclear fusion power generation. Just because 28GHz was used, it doesn't mean that wireless power supply using microwaves can only be realized at this frequency. Coexistence with other communication networks is by no means impossible from the point of view that it can also be realized in other frequency bands.

"We would like to gradually commercialize it from where we can while overcoming the issues. First, drones, then small aircraft, flying cars, gradually increasing the distance and power. As we move forward, we will get closer to launching a rocket, and I hope that this will lead to the development of technology that will give back to society.”

　Currently, Mr. Shimamura's laboratory is receiving frequent requests and consultations for joint development from companies.