Microwave Rocket is the one that utilizes microwave bemaed energy transmitted from the ground. This type of propulsion is also called "Beamed Energy Propulsion (BEP)". In particular, those that use laser beams have been vigorously researched and developed in the United States and Russia. On the other hand, research on the Microwave Rocket has not progressed much because of the lack of an oscillator that can generate a high-power microwave beam. However, Gyrotron, which can output MW-class power with a single unit, was developed at the Institute of Applied Physics, Russian Academy of Sciences (IAP RAS) in the 1960s. Accordingly, our group demonstrated the principle of the Microwave Rocket in 2003 using a gyrotron of the current National Institutes for Quantum and Radiological Science and Technology research (QST). The photo is the launch test with a 900 kW microwave beam applied to a 10 g miniature rocket.

　　The main feature is its low cost. Current so-called "rockets" (types that convert the chemical energy of fuel into propulsive energy) have theoretical performance limitations. This is apparent from the Rocket Equation shown by Tsiolkovsky, which gives us the amount that can be carried into space with a single launch. Therefore, if the satellite is transported to space multiple times, the cost will increase accordingly. On the other hand, in the case of Microwave Rocket, since the energy source can be placed on the ground, any heavy object can be theroretically transported to space at once and the ground equipment can be used repeatedly. Therefore, the cost of the access to the space is drastically reduced.

　　In recent years, member states of the United Nations have advocated SDGs (Sustainable Development Goals), and Microwave Rocket contribute to their realization. Many of the existing rockets were disposable, and some were the types that could cause air pollution. However, Microwave Rocket doesn't need to load onboard fuel and doesn't cause air pollution because it uses the surrounding air for propulsion. Furthermore, the development of Microwave Rocket will enable to construct large structures such as Space Solar Power Systems (SSPS) satellites, and consequently provide clean energy to all humankind.

　　Based on these characteristics, Microwave Rocket can be said to be an innovative system that expands the sphere of human life and enables sustainable human development. Therefore, in order to realize Microwave Rocket, I am engaged in a wide range of research and development from the elucidation of its propulsion principle to the development of a gyrotron that is indispensable for ground beam stations, using numerical numerical simulations and experiments.

　　For more details, please take a look at this paper.

　　Microwave Rocket utilizes surrounding air as its propellant when flying in the atmosphere. Due to its features, it is essential to design a air-breathing sysytem that can take in air efficiently.

　　Previous study has shown that using a reed valve, which is also used in pulse detonation engines, improves the air-breathing performance and consequently increases the propulsive force. In this study, we have modeled the reed valve operation accurately using the finite element method and three-dimensional CFD. As a result, a code that predicts the pressure, temperature, and density distribution inside the thruster's tube has been completed.

　　On the other hand, one of big problems was that the propulsive force during high altitude flight could not be investigated experimentally. Therefore, taking advantage of the features of numerical calculation, we have also performed calculations for high altitude flight. At high altitudes, it is inevitable that the air-breathing performance decreases due to the decrease of atmospheric density. Therefore, we tried to improve the performance by installing a plenum chamber on the side wall of the thruster's tube that can compress and take in air. Consequently, despite the disadvantage of air drag, the air-breathing performance has been improved, and as a result, (thrust force - air drag) has been greatly improved.

　　For more details, please take a look at this paper.

　　High-power millimeter-waves are applied to a cylindrical propulsion thruster, inside of which is filled with air, and high-pressure and high-temperature plasma can be generated. This is called "millimeter-wave discharge plasma". Since the outside is at atmospheric pressure, the hot air inside the aircraft pushes up the rocket and rises little by little. In the future, in order to optimize the shape of the propulsion system and conduct detailed trajectory analysis, it is important to correctly estimate the propulsive force. For that reason, I have been studying "millimeter-wave discharge plasma" using two approaches.

　　One is to use a high-speed camera to capture the propagation velocity of the plasma and its plasma structure in detail. The photograph was taken by conducting a discharge experiment using a 28 GHz gyrotron at the Plasma Research Center at the University of Tsukuba. Experiments with different atmospheric pressures and millimeter-wave beam intensities revealed various plasma structures. Past studies have shown that thrust force changes significantly with structural changes. Therefore, this study is of much importance in terms of elucidating the conditions of these structures transition, and the mechanism by which these structures are created.

　　In addition, we are using an Optical Emission Spectroscopy (OES), in which the emission of plasma is decomposed for each wavelength and its characteristics can be known. In millimeter-wave discharge plasma, nitrogen molecules, which account for 80% of the atmosphere, mainly emit light. The luminescence, called the Second Positive System, is a band spectrum often used to measure the vibrational and rotational temperatures of nitrogen molecules, and was used for analysis in this study. The figure shows the spectrum measured in this study and the result of fitting calculation based on theoretical schemes. It is possible to perform fitting using a commercial code, but problems often occur that commercial codes cannot handle, such as different temperatures for each particle and the need to consider a continuous spectrum. To sove those problems, we developed our own fitting code and used it for analysis.

　　Through such researches, the behavior of millimeter-wave discharge plasma with respect to atmospheric pressure and beam intensity changes has been elucidated in detail. This means that the thrust of Microwave Rocket can be estimated. In the future, we will optimize the shape of the thrtuster and confirm its performance through experiments.

　　For more details, please take a look at this paper.

　　A gyrotron capable of oscillating a 600 kW microwave beam has been installed at the University of Tokyo through JSPS KAKENHI(S), which began in fiscal year of 2015. Until now, gyrotrons have been developed for the purpose of heating fusion plasma, but there is no unprecedented development of gyrotrons for applications to aerospace engineering.

　　In order to observe the filamentary structures and detonation propagation structures in detail, which are one of the major features of millimeter-wave discharge, the oscillation power, frequency, and pulse width were set to 600 kW, 94 GHz, and 100 μs. Until now, we have been conducting rocket research at QST, Plasma Research Center at University of Tsukuba, and the Resaerch Center for Develoment of Far-Infrared region at the University of Fukui. In the future, we will use dedicated gyrotrons to conduct intensive research to elucidate the principle of Microwave Rocket.

　　The gyrotron is a prototype for future beam stations development. Past atudy has estimated that Microwave Rocket would require beam stations with as much as 1 GW power to replace current chemical rockets. The output per gyrotron is currently at most about 2 MW, which means to require an order of 1000 gyrotrons and consequently, the initial cost of building a beam base is enormous. One solution is to reduce the cost of the gyrotron itself. It is important to reduce the cost of superconducting magnets, which are the cost drivers for gyrotrons. Therefore, in the development of this gyrotron, we purchased a superconducting magnet (SCM) with a small bore diameter, which has lower cost than conventional gyrotrons.

　　The gyrotron element test was completed during teh fiscal year of 2019. In the future, we will check the 600 kW oscillation after installing the gyrotron on the superconducting magnet.

　　For more details, please take a look at this paper.