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Microwave Rocket is considered to be a future mass transportation system to space. Thrust is generated by high-pressure air isometrically heated by millimeter wave irradiation from the ground. High-power and high-frequency operation has been achieved mainly because of the improvement of a millimeter wave generator, gyrotron. Thrust was measured in a high-duty-cycle condition with a high-power millimeter wave. As a result, 30 N of time-averaged thrust was recorded at 91 kW of time-averaged power, which corresponds to 330 N/MW of thrust-to-power ratio.

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The Microwave Rocket is a candidate for a low cost launcher system. Pulsed plasma generated by a high power millimeter wave beam drives a blast wave, and a vehicle acquires impulsive thrust by exhausting the blast wave. The thrust generation process of the Microwave Rocket is similar to a pulse detonation engine. In order to enhance the performance of its air refreshment, the air-breathing mechanism using reed valves is under development. Ambient air is taken to the thruster through reed valves. Reed valves are closed while the inside pressure is high enough. After the time when the shock wave exhausts at the open end, an expansion wave is driven and propagates to the thrust-wall. The reed valve is opened by the negative gauge pressure induced by the expansion wave and its reflection wave. In these processes, the pressure oscillation is important parameter. In this paper, the pressure oscillation in the thruster was calculated by CFD combined with the flux through from reed valves, which is estimated analytically. As a result, the air-breathing performance is evaluated using Partial Filling Rate (PFR), the ratio of thruster length to diameter L/D, and ratio of opening area of reed valves to superficial area α. An engine cycle and predicted thrust was explained.