The workhorse of solid rocket motors, composite propellants consist of a heterogeneous mixture of fuel and oxidizer -- generally with the oxidizer as a crystalline material surrounded by a fuel-polymer matrix. A common example is ammonium perchlorate and hydroxyl-terminated polybutadiene binder.

"Ammonium perchlorate has been used in solid propellants for decades, and hydroxyl-terminated polybutadiene binder has been phased into production in the last decade, but fundamental questions remain about their combustion behavior," said Quinn Brewster, a professor of mechanical engineering at the U. of I. "Our work investigates the microscopic behavior of the propellant in a simplified geometry that allows easier measurement and comparison with theory."

To study combustion chemistry at the microscopic level, Brewster and his students first form a small propellant sandwich consisting of a layer of fuel between layers of oxidizer. The sandwich is then ignited in a laser-augmented, high-pressure combustion chamber, and the resulting reaction recorded with an intensified CCD (charge-coupled device) camera.

"An optical chopper permits the sequential acquisition of two nearly simultaneous images -- one of the flame emission alone, the other with the sample backlit with an ultraviolet source," Brewster said. "A narrowband filter rejects most of the light except that given off by excited hydroxyl molecules."

By studying the CCD images, the researchers were able to examine how the combustion behavior of propellant sandwiches varied with pressure and oxidizer width. "We found that the burning rate increased and the flame enlarged at high pressures," Brewster said. "Increased pressure and wider oxidizer layers also tended to cause the flame to split."

Through parallel computational simulation studies, the researchers further explored the flame structure and energy-transfer characteristics of the combustion reaction. "We used a two-reaction model where the oxidizer is allowed first to react and form an intermediate species which then combines with the fuel in the second reaction," Brewster said. "This double-reaction sequence provides a simple approximation of the complex flames seen in our laboratory experiments."

The simulation shows an initial flame and two leading-edge flames forming over the oxidizer portion of the propellant. "The initial flame splits due to the influence of these two edge flames, which are anchored close to the surface," said Brewster, who presented his team's findings at the Joint Army, Navy, NASA, Air Force (JANNAF) meeting in Cocoa Beach, Fla., on Oct. 22. "The edge flames provide additional heat which raises the reaction rate of the initial flame."

University of Illinois at Urbana-Champaign

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