Combustion Instability Innovation

The same type of white noise you hear from a TV set, properly applied, could prevent a rocketry catastrophe.

This has attracted engineers at Space-X to a University of Alabama in Huntsville (UAH) graduate student’s innovative new approach to suppressing combustion instabilities in rocket engines.

John Bennewitz, a Von Braun Propulsion Scholar at the UAH Propulsion Research Center who is working with advisor Dr. Robert Frederick, was a visiting lecturer at Space-X’s facility in Hawthorne, Calif., earlier this year. While there, he told employees of the firm that designs, manufactures, and launches advanced rockets and spacecraft about his efforts in applying band-limited white noise to neutralize combustion instabilities.
 

Wave of Bad News

Combustion instabilities are a wave of bad news for rocket designers. Instabilities can occur due to a feedback between unsteady heat release, acoustic fluctuations, and incoming propellant flow perturbations.

“What can happen is, when you have rough combustion with these instabilities excited at certain frequencies, the oscillations can get so large with amplitudes so great that it destroys the engine,” Bennewitz says. “Combustion instability is one of the largest issues with rocket engine development and design.”

Prior efforts to control combustion instabilities have focused largely on designing the engine baffles and injectors to smooth propulsion flows at critical points. Recently, researchers tested a fast-response actuating valve to deliver propellant to the engine’s combustion chamber modulated at certain frequencies to nullify instabilities. They found that approach worked in a set of narrow, targeted frequency ranges.

That research intrigued Bennewitz. He proposed a broader approach using a piezoelectric speaker located upstream of the combustion chamber and housed at the base of the oxidizer post of an injector, to broadcast white noise frequencies targeted to negate the instabilities.

“With a speaker, the band-limited white noise being generated is basically a package of frequencies,” he notes.

Bennewitz designed a model liquid rocket engine combustor and fabricated it with the help of senior aerospace student Jake Cranford. By experimenting with a gaseous methane/gaseous oxygen fuel mixture, the two found a productive sweet spot, which suppressed 2,400Hz instability by applying band-limited white noise between 500Hz and 1,000Hz.

“When these combustion instabilities get excited, they do so at a specific frequency,” Bennewitz states. “Essentially, we are sweeping right now from zero to 2,500Hz. We sent it 500 frequencies at a time, beginning with zero Hz all the way up to 2,500Hz, in increments of 100Hz and then back down again.”

Cranford, who was responsible for some design changes to the test stand, says the experience has piqued his interest in attending graduate school at UAH.

“As an undergraduate, I’ve been working with John and he’s been very forthcoming with the science,” Cranford comments.

Next up for the pair is testing narrower frequency bands to identify the most effective ranges for instability suppression. They are now applying 150Hz ranges to isolate the best frequencies more precisely.

Cranford is working on another part of the project, a system that will sense pressure oscillations and be able to offset them with a signal from the speaker through a band-pass filter and time delay, working in similar fashion to anti-knock sensors in cars. That development would provide greater in-flight adaptability to conditions, making the concept useful across a much broader range.

“The hope is that if we can get the filter up and working,” Bennewitz says, “then Jake can be the primary author of a conference paper about it.”

University of Alabama in Huntsville
Huntsville, Ala.
www.uah.edu

Elizabeth Engler Modic is managing editor of AMD and can be reached at 300.523.5344 or emodic@gie.net.

Learn More: Visit http://bit.ly/1gEojfg to watch the experimental video that shows the transition from unstable combustor operations to stable conditions with the instability being suppressed due to properly applied frequency bandwidth white noise signal.