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June 16, 2004 – NASA made quite a bit of noise in late March when it successfully deployed its X-43A aircraft in a 10-second test flight. The craft reached a record-breaking — and ear-shattering — Mach 7 before beginning its descent toward the Pacific Ocean.
It was powered by a scramjet engine, the shortened term for supersonic combustion ramjet. NASA and the military are developing the engines for next-generation missiles, aircraft and space vehicles.
Taitech Inc., a research and development company in Ohio, hopes to help NASA and the military meet their goals using MEMS sensors.
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Scramjets, like conventional engines, burn fuel by combining it with oxygen from the atmosphere. But at high speeds, conventional engines cannot pull in enough oxygen to sustain combustion.
Scramjets succeed using inlets that draw in heated air. Scramjets have existed for decades, but there is still much to be learned about their inner workings. The Air Force and Beavercreek-based Taitech are attempting to solve those mysteries using a wind tunnel at the Wright-Patterson base and pressure sensors designed by Taitech.
The sensors measure the fluctuating pressure as air passes through the inlets and mixes in the engine chambers.
“We have to make sure the sensor can deal with harsh environments,” said Tzong Chen, chief executive of the 30-person company. “The temperature varies a lot, and that can change the performance.”
Don Abeysinghe, a researcher with Taitech and an adjunct faculty member at the University of Cincinnati, is designing MEMS pressure sensors on optical fibers that could be incorporated in scramjet simulations.
The MEMS-optics hybrid provides the sensitivity and size advantages of MEMS while eliminating the need for electrical signals, he said. Electromagnetic currents within the engine interfere with electrical signals and can affect the results.
Abeysinghe chemically bonds the glass fiber to a sensor head, a technique that bypasses the need for temperature-sensitive adhesives like epoxy. In the process, he creates a cavity between the fiber and silicon layer.
Pressure changes outside the device force the MEMS membrane to deform, a shift that can be detected when a light beam passes through the optical fiber and is reflected back. The engine’s electromagnetics don’t distort the signal.
The device is no larger than the tip of the fiber, which allows the Air Force to use arrays of sensors. “They want a sensor array because most of these environments are dynamic,” Abeysinghe said.
A clear understanding of the complex interaction of fuel and air before and during combustion will help scientists design safer, faster and more powerful engines. NASA’s X-43A reached Mach 7, seven times the speed of sound, and is designed to go up to Mach 10.
The Air Force and the Defense Advanced Research Projects Agency are trying to develop an unmanned war craft that can reach any part of the world within hours from a home launch, and have identified scramjet as the means to achieve that.
The Air Force and other branches of the military also consider scramjet a candidate for missiles.
Abeysinghe’s MEMS-optical fiber system still needs refining, though. The temperatures within a scramjet engine are too volatile for silicon-based MEMS, and he’s considering a switch to the more robust silicon carbide for the MEMS component.
He is in contact with NASA Glenn Research Center, a client of Taitech and a leader in silicon carbide technology.