An Ammonia Microresistojet (MRJ) for Micro Satellites

Very small satellites are generally power limited, and, in addition, are usually subject to strict first cost constraints. When the missions require electric propulsion, these constraints tend to direct the propulsion system selection to simple, light weight options with high thrust/power for the modest Δv levels normally associated with such missions. The ammonia microresistojet (MRJ), the development of which is described, is intended to answer these needs. In addition, with ambient temperature vapor pressure of 0.85 MPa (123 psi), ammonia can also serve as the propellant for the attitude control system, with a specific impulse of approximately 80 seconds. The paper describes the development of the ammonia MRJ of nominal 25 watt power level with a specific impulse level of between 150-210 seconds and thrust of 5-12 mN. The ammonia is convection heated to a maximum temperature of 1100C while flowing through a thin (.01-.02 cm) annulus between coaxial platinum tubes. The machined platinum expansion nozzle with 0.010-0.015 cm throat and 23/1 area ratio is welded to the outer platinum tube. The total platinum mass is approximately 10 grams. The heat source is a stock tantalum co-axial heater which is simply inserted inside the inner platinum tube. A ten layer radiation shield is wrapped around the outer tube. Total device weight is about 15 grams. Successive performance improvement development steps are described. A successful 100 hour Engineering Model (EM) test of the MRJ with operating sequences simulating actual MRJ operation has been carried out. Recent tests leading to improved performance are described.