Strange Brew

Researchers cook up gelled rocket fuels

Engineers and food scientists are teaming up to develop a new type of gelled fuel the consistency of orange marmalade to improve the safety, performance, and range of rockets for space and military applications.

"This is a very multidisciplinary project," says Stephen Heister, a professor of aeronautics and astronautics who is leading one of two teams on the project, funded by the U.S. Army Research Office.

The team includes researchers from mechanical engineering, aeronautics and astronautics, food science, and agricultural and biological engineering at Purdue, as well as researchers from the University of Iowa and the University of Massachusetts.

Gels are inherently safer than liquids because they don't leak. They also would allow the military to control rockets better than is possible with the solid fuels now used, according to Heister.

Paul Sojka, a professor of mechanical engineering and an associate director of the project, is building an experiment containing a transparent window to take high-speed videos of the gelatinous fuel's behavior. Jets of the gel form during the fuel-injection process.

"These jets are wiggling, and there are pulsations. Those pulsations, we believe, lead to the formation of specific spray patterns and droplet formation," Sojka says. "The fluid mechanics of gels are quite challenging. The viscous properties of the gel changes depending on how fast it's flowing, which is not true of liquids."

The project will tap the expertise of food scientists and "food engineers," who are accustomed to working with gels. "Gels are more complex than ordinary solids and fluids," says Carlos Corvalan, an associate professor of food science. "Fluids are characterized by viscosity, and solids are characterized by elasticity. Because gels share properties of both solids and fluids, they possess viscoelastic properties."

Food science and agricultural engineering researchers will study these viscoelastic properties and create simulations describing how the gels behave.

Future rockets could require that gelled propellants be sprayed by fuel injectors into a motor's combustion chamber at rates of thousands of pounds per second. Using the gelled propellants, however, will require a thorough knowledge of how the fuel breaks into droplets as it is being sprayed into the chamber.

Most of the Army's ground-to-air missiles currently use solid propellants, which have inherent limitations. Unlike conventional rockets running on solid propellants, motors using gelled fuels could be throttled up and down and controlled more precisely, Heister said.

"You can turn the engine on and off, you can coast, go fast or slow," he says. "You have much greater control, which means more range for missiles. The gelled propellants also tend to have a little more energy than the solid propellants."

Researchers will first work with water-based gels that simulate fuels and will eventually conduct experiments using actual propellants at the university's Maurice J. Zucrow Laboratories.

"It's kind of like orange marmalade without the rind," Heister says. "We are going to make this gel and push it through holes and study how it flows and how big the drops are. Eventually we'll study the real gelled fuels, which can be quite hazardous and reactive, so we will use them in small quantities and under tightly controlled conditions."

The three-year, $6.4-million "spray and combustion of gelled hypergolic propellants" project is a U.S. Army Multidisciplinary University Research Initiative, or MURI, which began last summer and could be extended another four years to 2013. Another team is led by Pennsylvania State University.

-Emil Venere