Scientific American
November, 1998
Volume 279, Number 5
Pages 50-51

Making thrusters for micromachines

Getting enough force out of silicon micromachines for them to do a useful amount of work has always proved a nettlesome challenge. A few researchers have begun to obtain more bang for the micron by making silicon chips with tiny cavities, filling them with explosives or rocket propellant and setting them afire.

Micropyrotechnics, it is conjectured, may one day power or reorient satellites and pulse drugs through the skin. Coupling ignitable materials with microelectromechanics (MEMS)--the technology that fashions submillimeter, electrically driven machines through standard chip fabrication methods--has begun to advance beyond the concept stage in a few laboratories.

The Defense Advanced Research Projects Agency (DARPA) last year awarded a $3.5-million contract to TRW, Aerospace Corporation and the California Institute of Technology to come up with a prototype for a propulsion system that could be used to position or propel microsatellites for space, defense and communications applications. Micropyrotechnics takes advantage of the ability of silicon fabrication methods to produce lots of little devices at once.

The TRW-led team has so far built a chip that contains 15 thrusters--an array of five-by-three elements. A thruster is essentially a silicon box that measures about 700 to 1,000 microns on a side and is filled with a propellant such as lead styphnate.

Figure 1. Photo of a Digital MicroPropulsion Chip. (38k JPEG)

Each box has a microscopic electrical resistor that heats up when it receives a signal from control circuitry. This action lights the fuel, providing enough force to rupture one of the outer faces of the box, which is made thinner during manufacturing than the other side walls. A thruster element can be used only once, but arrays of thousands or millions of thrusters might keep a satellite going for a few years. The existing prototype, for instance, might be developed into a panel that would measure 100 square centimeters (almost 16 square inches) and contain roughly a million thrusters.

Adjusting propulsion in precise increments by lighting different numbers of thrusters has lent the technology the name "Digital Propulsion." "It's typically difficult to make engines have arbitrarily small units of thrust, but we can do that," says David H. Lewis, a TRW research engineer who invented the system with Erik K. Antonsson of Caltech. Microsatellites may measure as little as 10 centimeters along one edge, weigh one or two kilograms (up to 4.5 pounds) and be deployed from the space shuttle or a rocket.

Figure 2. A Time-Series of High-Speed Photographs of a Digital MicroPropulsion Chip during Firing. (20k JPEG)

The National Aeronautics and Space Administration has considered them for space science. The Department of Defense is interested in them for use in ballistic-missile interceptors; a space-based projectile of less than a kilogram could accelerate to several kilometers per second, fast enough to obliterate a warhead traveling at an even higher speed. Communications companies could deploy clusters of thousands of satellites that could function as a reconfigurable antenna whose position might change on command from a standard-size orbiting satellite.

Figure 3. A 3-D Simulation of a Pair of Cooperating MicroSpacecraft Controlled by Digital MicroPropulsion Chips. (76k JPEG)

Other work on micropyrotechnics continues at a French laboratory that has been involved with the technology for both space and medical applications. The Laboratory for the Analysis and Architecture of Systems (LAAS)/CNRS has received a patent for a micropyrotechnic device that could replace a hypodermic needle or a transdermal patch. It consists of a microscopic hole in a silicon chip that might be filled with an explosive chemical, such as glycidyle azide polymer, which is used to set off air bags. Igniting the polymer would expand a silicon membrane. The movement of this membrane would send a volume of liquid at high velocity out of the device and through the skin. "If you consider an actual mechanical syringe, the speed of the injection is very slow by comparison," notes Carole Rossi, the LAAS researcher who developed the concept. "The duration of the pyrotechnic injection would be much shorter, and the pain would be diminished."

Micropyrotechnics could lead to still more ambitious schemes. Kristofer S. J. Pister of the Berkeley Sensor and Actuator Center at the University of California at Berkeley heads a research group that has begun DARPA-funded work on what he calls "smart dust." Investigators at Berkeley are fashioning small packages of temperature, pressure and other sensors that could be lifted for brief intervals by microthrusters to monitor weather or air quality or a battlefield. The sensors on these MEMS motes, each no more than a cubic millimeter in size, could then be interrogated by aircraft or unmanned aerial vehicles.

The Berkeley researchers, who have expanded on Rossi's work, want to send a smart dust particle a few hundred meters aloft using a single thruster. At its apex, the speck would deploy silicon wings coated with solar cells. The power generated could control the direction and rate of descent. Integrating sensors with electronics may let silicon chips see, hear and even smell. And adding micropropulsion will allow them to soar.

--Gary Stix

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