Analysis, Fabrication and Testing of a MEMS-based Micropropulsion System
Author(s)Bayt, Robert L.
Various trends in the spacecraft industry are driving the development of low-thrust propulsion systems. These may be needed for fine attitude control, or to reduce the mass of the propulsion system through the use of small lightweight components. The nozzle converts the stored energy in a pressurized gas into kinetic energy through an expansion. The nozzle efficiency is characterized by the amount of kinetic energy leaving the nozzle, and is governed by the exit velocity. Because of the increase in viscous losses as scale is reduced, it was feared that high Mach number supersonic flows could not be generated in micro devices. However, a scaling analysis indicates that the reduction in throat area can be offset by an increase in operating pressure to maintain a constant Reynolds number. Therefore, thrust can be decreased by reducing the nozzle scale, and viscous losses mitigated by running at higher chamber pressures. In order to operate a supersonic nozzle efficiently, the geometry must be contoured to guard against flow separation and reduce the boundary layer thickness at the throat. Deep Reactive Ion Etching enables extruded flow channels of arbitrary in-plane geometry to be created at scales an order of magnitude smaller than conventional machining. These channels are encapsulated by anodically bonding glass to the upper and lower surfaces. Testing indicates that 11.3 milliNewtons of thrust is generated for a nozzle with a 37micron throat width, 308 microns deep, and a 16.9:1 expansion ratio. The exit velocity was 650 m/s, which corresponds to an exit Mach number of 4.2, and an Isp of 66 seconds. This is 100 m/s higher than previously achieved in a micromachined device and demonstrates that supersonic flows can be generated at this scale. The performance of the system is increased by electrothermal augmentation. By resistively heating fins present in the chamber, a thruster temperature of 700◦C has been achieved. This will increase the theoretical Isp to 145 seconds. However, the reduction in Reynolds number with increased chamber temperature causes viscous dissipation to increase and thruster efficiencies to decline. The efficiencies vary with Reynolds number in the same fashion as their unheated counterparts, which confirms that Reynolds number is the governing similarity parameter. The thruster was operated at a temperature of 420◦C, and demonstrated an Isp of 83seconds. This represents an Isp efficiency of 79% for an 8.25:1 area ratio nozzle. These results suggest that MEMS-based micropropulsion systems offer higher performance at lower mass, when operated at Reynolds numbers above 2500 for both heated and unheated thrusters.
Aerospace Computational Design Laboratory, Dept. of Aeronautics & Astronautics, Massachusetts Institute of Technology
ACDL Technical Reports;FDRL TR-99-1