Recently great interest has developed in both military and private sectors in micro-devices and solutions. Significant investments have been made towards achieving the breakthroughs that make these devices possible. Micro-Electro-Mechanical-Systems (MEMS) manufacturing technologies, which are themselves based on integrated circuit manufacturing processes, are the enabling technology fueling this new micro-revolution. MEMS based devices have broad applicability. The devices and concepts based on the technology find use in almost every major field of study and have become the cornerstone of research endeavors both military and private.

Within the space community, perhaps the most aggressive applications of this enabling technology are in developing micro-propulsion solutions for micro-spacecraft. These craft in range mass from less than 1kg to a nominal 100kg with sizes ranging from roughly that of a tennis ball up to about a meter on the longest side. Because there are so many different missions with differing requirements, the constraints and requirements for these micro-spacecraft and the propulsion systems that power them are very demanding and vary according to stated mission goals. J. Mueller in his paper, “Thruster Options for Micro-spacecraft: A Review and Evaluation of State-of-the-Art and Emerging Technologies” (see references page), attempts to classify micro-spacecraft according to there sizes and probable missions. This sparingly classification is summarized here.

Partly owing to the stringent requirements for the propulsion systems for micro-spacecraft and partly due to the possibilities presented by MEMS, numerous attempts at developing micro-propulsion solutions for micro-spacecraft are currently underway – either in the conceptual stages or in production or in flight-testing. A few of these are presented here.

The hottest topic today for use of micro-spacecraft is for deploying micro-satellites in constellations or formations. The vision is to arrange these micro-craft in formations for stereoscopic imaging. Another application would be as a tool for mapping the magnetosphere. Since these satellites are in constant communication with each other and ground control, it is usually said that they are semi-autonomous since they are controlled both ways. Some of the groups working on this concept are the Orion group at Stanford, the 3Corner Sat groups at New Mexico State University, Arizona State University, and University of Colorado, and the TechSat 21 team at the Air Force Research Laboratory (AFRL). More information is available at http://www.nanosat.usu.edu/index.html

MEMS

“Micro-Electro-Mechanical Systems (MEMS) is the integration of mechanical elements, sensors, actuators, and electronics on a common silicon substrate through the utilization of micro fabrication technology. “ This quote from the MEMS Exchange web page is most probably the best definition of MEMS. This new technology is viewed as an enabling technology for many reasons. MEMS are a diverse, multi-disciplinary technology that could potentially change every genre of commercial and military products to s significant extend. Today MEMS devices finds usage for everything ranging from blood pressure monitoring to active suspension systems for cars to propulsion systems for micro-sized satellites. Furthermore, MEMS is a multi-disciplinary field because it is the meeting ground of almost all fields of engineering. MEMS technology blurs the distinction between complex mechanical systems and integrated circuit electronics.

The electronics are fabricated using integrated circuit manufacturing processes and sequences. The micro-mechanical parts are fabricated using etching and surface machining processes that selectively etch away parts of the silicon wafer or add new layers to build the mechanical and electromechanical devices.

The phrase “sacrificial silicon etching” or micro machining is often used to describe MEMS production processes. Sacrificial polysilicon surface micro machining is emerging as a technology that is enabling the mass production of complex MEMS systems by themselves or integrated with microelectronic systems. There are several distinct methods employed in manufacturing the components that will make up a useful MEMS device. These methods or processes (listed by rough categories) are wet etching, dry etching, “left off”, LIGA, and a relatively new approach with excimer lasers. In all processes the basic procedures are the same. A thin film of material to be “machined” is placed on a substrate (usually silicon wafers) and then a mask is placed over selected portions of the substrate. Selected portions are removed around the mask. When the mask itself is removed what is left a part ready for use in MEMS device or for combining with another part through bonding of the silicon wafers.

Details on popular manufacturing processes for MEMS can be found at: http://www.dbanks.demon.co.uk/ueng/what.html and more general information on MEMS can be found at: http://www.mems-exchange.org/MEMS/, http://www.sandia.gov/mems/micromachine/overview.html

A few Micro-Propulsion Thrusters

After reviewing the literature in the references page an incomplete list of current thrusters available for micro-spacecraft can be assembled. Most of these thrusters are at best in the early stages of development and are almost all MEMS-dependent. These thrusters easily satisfy the strict mass and volume requirements for class I micro-spacecraft and allow for an unprecedented level of integration between different propulsion components and required control electronics. This is a direct feature of the MEMS characteristics of these thrusters.

FEEPS THRUSTERS: (ion propulsion)

Field Emission Electric Propulsion (FEEP) thrusters have the ability to produce the very low impulse bits necessary for attitude control of micro-satellites. These thrusters generate thrust by accelerating ions with electrostatic forces. The ions are produced by field emission from a liquid metal surface. Capillary forces in the emitters force the liquid metal to the tip of the emitter where a high current discharge of a capacitor “plasmarizes” the liquid metal and the ions are shot out of the accelerator as shown in the picture.

Digital Micro propulsion Chips by TRW/Aerospace Corp

These chips consist of a high number of one-shot thrusters placed onto a silicon wafer. These thrusters produce small enough impulse bits to make them ideal for attitude control applications. The expectation is that 104-106 thrusters can be mounted on a 10-cm diameter wafer. Each thruster is made up of a cavity etched into an Is wafer where the propellant (inert gas or combustible material) is stored, sealed on one side by a heater element made of polysilicon, and sealed on the other side by a thin membrane. The concept is simple. When the heaters are activated, the temperature and pressure of the propellant increases until the membrane is ruptured and the high-pressure propellant is exhaust – creating thrust. An individual thruster is shown in the picture. Go to the http://design.caltech.edu/micropropulsion/ for a lot more on this thruster.

Summary

There are several different thrusters available for micro-propulsion applications. Interested persons should view the references list for create sources of information about micro-propulsion thrusters. The last table in the presentation linked below summarizes table summarizes several thrusters and their range of applicability according to what size craft they are best suited for.

References:

C.-M.Ho, P.-H.Huang, J.M.Yang, G.-B. Lee and Y.-C.Tai, "Active flow control by micro systems", FLOWCON, International Union of Theoretical and Applied Mechanics (IUTAM) Symposium on Mechanics of Passive and Active Flow Control, Gottingen, Germany, Sep. 1998, pp.18-19.

Campbell, Mark, "UW Dawgstar: One Third of Ion-F", 3th AIAA/USU Conference on Small Satellites Paper SSC9-III-4, Logan Utah, 1999

F.-G. Tseng, C. Linder, C.-J. Kim, and C.-M. Ho
"Control of Mixing With Micro Injectors for Combustion Application"
Proc. MEMS (DSC-Vol.59), Application of Microfabrication to Fluid Mechanics, ASME International Mechanical Engineering Congress and Exposition, Atlanta, GA, Nov. 1996, pp 183-187.

German et al, "An Evaluation of Green Propellants for an ICBM Post-Boost Propulsion System", Defense Technical Information Center, 2000

Ho, C.M., Tai, Y.C., "Micro-Electro-Mechanical Systems and Fluid Flows", Annual Rev. Fluid Mech, 1998

Horan et al, " Three Corner Sat Constellation - New Mexico University", 13th AIAA/USU Conference on Small Satellites Paper SSC99-VI-7, Logan Utah, 1999

K. C. Pong, C. M. Ho, J. Q. Liu, and Y. C. Tai, "Non-Linear Pressure Distribution in Uniform Microchannels," Application of Microfabrication to Fluid Mechanics 1994 presented at 1994 ASME International Mechanical Engineering Congress and Exposition, Chicago, IL, pp. 51-56, Nov. 6-11 (1994).

Ketsdever et al, "Predicted Performance and Systems Analysis of the Free Molecule Micro-Resistojet", Micropropulsion for Small Spacecraft, Progress in Astronautics and Aeronautics, Vol 187, edited M. Micci and A Ketsdever, AIAA, Reston VA 2000, Chap5

Ketsdever, A, "System Considerations and Design Options for Microspacecraft Propulsion Systems", Micropropulsion for Small Spacecraft, Progress in Astronautics and Aeronautics, Vol 187, edited M. Micci and A Ketsdever, AIAA, Reston VA 2000, Chap4

Kimura et al, "Measurement of Wall Shear Stress of a Turbulent Boundary Layer Using a Micro-Shear-Stress Imaging Chip, The Japan Society of Fluid Mechanics and Elsevier Science, 1999

Kiraly, Z., Engberg, Brian, et al, "The Orion MicroSatellite: A Demonstration of Formation Flying in Orbit", 13th AIAA/USU Conference on Small Satellites Paper SSC99-VI-8, Logan, Utah, 1999

Lewis et al, "Digital Micropropulsion", Sensors & Actuators A, 2000 p143-154

Liu et al, "A Micromachined Flow Shear Stress Sensor based on Thermal Transfer Principles", IEEE/ASME J. of Micro-electro-mechnical systems (J. MEMS), 1999

Marcuccio et al, "Attitude and Orbit Control of Small Satellites and Constellations with FEEP Thrusters", Electric Rocket Propulsion Society, 1997

Marcuccio et al, "Flight Demonstration of FEEP on Get Away Special", 33rd AIAA/ASME/SAE/ASEE Joint Propulsion Conference and Exhibit, AIAA Paper 98-3332, Cleveland, OH 1999

Mueller, J, "Thruster Options for Microspacecraft: A Review and Evaluation of State-of-the-At and Emerging Technologies", Micropropulsion for Small Spacecraft, Progress in Astronautics and Aeronautics, Vol 187, edited M. Micci and A Ketsdever, AIAA, Reston VA 2000, Chap3

Mukerjee, E.V., Wallace, K. Y., et all, "Vaporizing Liquid Microthruster, Sensors and Actuators Paper 83(2000) 23-236, Elsevier Science S.A., 2000

Reichbach, Jeffrey, "Micropropulsion System for Precision Formation Flying", MIT Masters Thesis, MIT 2001

Tai et al, "Micro Heat Exchanger by Using MEMS Impinging Jets", Proc. 12th Annual International Workshop on Micro Electro Mechanical Systems, pp. 171 - 176, 1999, Orlando, FL

Yang, Xue'en, "A MEMS Valve for the MIT Microengine", MIT Masters Thesis, MIT 1999

Yashko, Gregory, "Ion Micro-Propulsion and Cost modeling for satellite Clusters", MIT Masters Thesis, MIT 1998

http://mems.sandia.gov/scripts/index.asp

http://www.atlanticresearchcorp.com/docs/space_biprop.shtml

http://www.darpa.mil/mto/mems/

http://www.dbanks.demon.co.uk/ueng/liga.html

http://www.howstuffworks.com/spy-fly.htm

http://www.nanosat.usu.edu/index.html

http://www.nanospace.org/

http://www.redwoodmicro.com/publications.htm#PAP