DESIGN OPTIMIZATION AND DIRECT THRUST MEASUREMENT OF AN 8-GHz MICROWAVE ELECTROTHERMAL THRUSTER

Open Access
Author:
Hopkins, Jeffrey Robert
Graduate Program:
Aerospace Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
August 02, 2011
Committee Members:
  • Michael Matthew Micci, Thesis Advisor
  • Sven G Bilen, Thesis Advisor
Keywords:
  • electrothermal
  • electric propulsion
  • thrust measurment
Abstract:
The direct thrust output, efficiency, and specific impulse of an 8-GHz microwave electrothermal thruster (MET) was measured using a vertical thrust stand in an evacuated vacuum tank. Measurement was done by assembling a vertical thrust stand board inside a vacuum chamber. The thruster was mounted via flexible WR-112 waveguide to 1-kg load cell. Pressure was evacuated from the chamber to levels of 0.1 Torr and plasma was formed using ammonia and decomposed-ammonia simulated hydrazine as propellants. Thrust measurements were taken at previously determined parametrically optimized values and reconciled against predicted values computed from pressure differential calculations. The 8-GHz MET uses the resonant TMz011 mode to excite a propellant gas into a free-floating plasma. Due to density gradients and buoyancy issues in the plasma chamber, a vertical thrust stand was employed in order to obtain maximum thrust values. This constraint provided several challenges in thrust stand design, including cavity redesign, load cell sensitivity issues, and vibration noise issues caused by the vacuum chamber pump. Cavity redesign also allowed for materials optimization, including the reconstruction of the cavity using stainless steel for its better electrical properties, and a boron nitride separation plate. A converging–diverging nozzle with an area ratio of 25:1 was also included in the design to get more realistic operational values. Thrust measurements in most cases were within 20% of predicted values and yielded more repeatable results than previous thrust measurements using deflection cones and momentum traps. While some of this error could be accounted for with measurement uncertainty, this is still too large of a discrepancy between theoretical and measured values. Incorrect assumptions of the mixture dynamics for physical constants in calculations, thermal biasing of the load cell, vibration and electrical noise as sources of error all need to be explored and eliminated.