Analysis, fabrication and testing of a MEMS-based micropropulsion system

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 microdevices. 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 37 micron 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 lsp 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 83 seconds. 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.

Department dc:contributor.department

Massachusetts Institute of Technology. Dept. of Aeronautics and Astronautics.

Grantor dc:publisher

Massachusetts Institute of Technology

Year dc:date.issued

1999

Author dc:creator

• Bayt, Robert Louis, 1970-

Advisor dc:contributor.advisor

• Kenneth S. Breuer.

• Aeronautics and Astronautics.

Statement dc:rights

• M.I.T. theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission. See provided URL for inquiries about permission.

Licence dc:rights.uri

• http://dspace.mit.edu/handle/1721.1/7582

Language dc:language.iso

eng

Handle dc:identifier.uri

http://hdl.handle.net/1721.1/8970

OAI identifier oai:identifier

oai:dspace.mit.edu:1721.1/8970