A Study of a Low-Power Microwave Arcjet Thruster Using Ammonia Propellant

A low-power microwave arjet thruster was studied using ammonia gas as propellant. The purpose of the project was to generate ammonia plasma using a 7.5 GHz magnetron and a cylindrical cavity thruster, which is resonant in the TM001 mode [1]. The desired pressure was atmospheric or above while generating a thrust of roughly 20mN and an Isp of about 500s operating at low power (<100 kW). Introduction/Overview of Electric Propulsion Two main parameters used to describe a rocket engine are the thrust and the specific impulse (Isp) generated by the rocket. Thrust is an exchange of momentum, momentum from the exhaust is transferred to the space vehicle which allows is to propel through space. Below is the derivation of the basic thrust equation where P = momentum, mdot = mass flow, ve = exhaust velocity, and FTHRUST = thrust force. P = mv dP = dm ve dP/dt = dm/dt ve FTHRUST = mdot ve Specific impulse is a performance parameter used for rocket engines, which compares the thrust to the amount of propellant used. The equation for the Isp is shown below where FTHRUST = thrust force, mdot = mass flow, and g0 = gravitational constant. Isp = Fthrust / mdot * g0