Atomspheric Dscharge using a High Power Millimeter Wave and its Applicaltion to Rocket Propulsion

Microwave rocket is an application of atmospheric discharge using a high power millimeter-wave beam. When a high power pulsed millimeter-wave beam is provided into a focusing reflector, atmospheric discharge arises in the vicinity of the focal point. The induced plasma absorbs the following part of the millimeter-wave pulse and propagates towards the beam source while generating a shock wave. The shock wave drives impulsive thrust. Because propulsive energy is provided by a microwave beam transmitted from outside, the vehicle is not necessary to load an energy source by itself. A flight experiment was conducted using a 1MW-class 170GHz-gyrotron. The measured momentum coupling coefficient C_m, defined as a ratio of propulsive impulse to input power, was over 400N/MW with a thruster model with cylindrical tube. In this study, the condition for the propagation of a shock wave supported by an atmospheric plasma in the microwave rocket was observed. Pressure histories in the rocket model were measured for detection of shock waves and their propagation velocities were estimated. At the same time, the propagation velocities of ionization front were measured using a high fra ming speed camera. As a result, the propagation velocities of the shock wave and the ionization front were found identical as same to the laser supported detonation. The minimum pow er density condition for the shock wave propagation supported by the ionization front was S>80kW/cm², which was lower than that of the laser detonation.