Fabrication and Testing of Micron-Sized Cold-Gas Thrusters

Contoured converging-diverging micronozzles have been fabricated and tested for the acceleration of gas flows to supersonic velocities. Extruded two-dimensional devices, with minimum throat widths averaging 19 microns and 35 microns, are fabricated by deep reactive ion etching. Mass flow efficiencies through the nozzles range from 87% to 98% and are within experimental error to those predicted by a 2-D Navier-Stokes fluid simulation for Reynolds numbers 350-3800. The thrust of a 16.9 to 1 expansion ratio nozzle was measured to be 11.3 mN at 97 psia chamber pressure. This corresponds to an exit velocity of 650 m/s, which is Mach 4.1 for a chamber temperature of 300 K, and an Isp efficiency of 93%. Thrust and Isp efficiency are found to deviate considerably from the 2-D numerical model at low Reynolds, and possible causes are offered, but the definitive mechanism has not been identified. NOMENCLATURE a Speed of Sound Cd Coefficient of Discharge D Throat Width To Appear: Journal of Propulsion and Power, 2000