Design and initial testing of a miniature microwave electrothermal thruster (MiniMET)

The feasibility of a low power 14.5-GHz microwave electrothermal thruster was examined through design and experiment. The design was scaled down from previous lower frequency versions developed at The Pennsylvania State University, and it was tested for plasma ignition and stabilization. Adjustments to the functionality of the previous designs were made to improve the tangential swirl of the propellant and subsequent plasma stability. The incorporation of a thick dielectric insert in order to accomplish this improvement caused repeatability and performance problems. Cold flow and hot fire tests were conducted into a vacuum. The hot fire tests were conducted while inputting a maximum of 14.3 W at the maximum mass flow rate of 2.15 mg/s with helium as the propellant. This maximum mass flow rate was limited by the mass flow controller used. The ratio of hot fire chamber pressure to cold flow chamber pressure was used to calculate the chamber temperatures. The ratio reached a maximum of only 1.1, resulting in only slightly increased performance values from cold flow to hot fire. The corresponding maximum thrust and specific impulse achieved by the system assuming an infinite nozzle expansion were 4.2 mN and 197 s, respectively. Recommendations for future designs to improve plasma repeatability and performance are discussed.