A boost for rocket fuels

  • 15-Mar-2009 01:32 EDT
Timothee Pourpoint (second from left) is in charge of designing and operating a new Purdue lab to test gelled rocket fuels designed to improve the safety, performance, and range of rockets for space and military applications. Because the fuels being tested are toxic, the lab will be equipped with a specialized ventilation system.

Researchers with various backgrounds of expertise are teaming to develop a new type of gelled fuel designed to improve the safety, performance, and range of rockets for space and military applications. Gelled fuels also could be used in thrusters to position satellites and on NASA space missions.

Gels are inherently safer than liquids because they do not leak, and they also would allow the military to better control rockets than is possible with solid fuels now used. Motors running on gelled fuels could be throttled up and down and controlled more precisely than conventional rockets that use solid propellants, said Stephen Heister, the Purdue University Professor of Aeronautics and Astronautics who is leading one of two teams on the project, which is funded by the U.S. Army Research Office.

“You can turn the engine on and off, you can coast, go fast or slow,” he said. “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.”

The five-year, $6.4 million “spray and combustion of gelled hypergolic propellants” project is a U.S. Army Multidisciplinary University Research Initiative, or MURI. Another team is led by Pennsylvania State University.

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

Paul Sojka, Professor of Mechanical Engineering and an associate director of the project, will devise a system 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, there are pulsations, and those pulsations, we believe, lead to the formation of specific spray patterns and droplet formation,” said Sojka. “The fluid mechanics of gels are quite challenging. The viscous properties of the gel change depending on how fast it’s flowing, which is not true of common liquids such as water or gasoline.”

According to Carlos Corvalan, Associate Professor of Food Science, “Gels are more complex than ordinary solids and fluids. Fluids are characterized by viscosity, and solids are characterized by elasticity. Because gels share properties of both solids and fluids, they possess viscoelastic properties, or a combination of both.”

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.

The fuels are hypergolic, meaning they require no ignition source but ignite spontaneously when mixed with an oxidizer. The fuel and oxidizer tanks each feed into a separate fuel injector. As the streams of fuel and oxidizer mix, they form droplets that ignite.

“There is an unsteadiness of these two jets, and these fluctuations can have all sorts of ramifications in terms of engine performance,” said Sojka, who is focusing on what occurs between the fuel and oxidizer streams as they form elliptical sheets, spaghetti-like strands, and droplets.

When fuel and oxidizer jets collide, “impact waves” result.

“We are trying to understand the source of those waves and be able to control or capitalize on the unsteadiness to make smooth combustion,” said Heister.

Experiments being conducted at Purdue’s Maurice J. Zucrow Laboratories are aimed at developing a comprehensive spray model that describes the precise behavior of propellant droplets in a rocket motor.

One aim is to be able to consistently create the relatively small, uniform droplets that would be needed for rocket propulsion. Food scientists are familiar with processes used to create droplets in foods.

“The texture of those foods is closely associated with the average size and range of sizes of droplets,” said Osvaldo Campanella, Professor of Agricultural and Biological Engineering. “In a combustion chamber you also want to control droplet size, but for a different reason—to precisely control combustion. You want uniform combustion, and for that you need controlled drop size.”

Corvalan will lead work to develop simulations that determine the viscoelastic behavior of the gels and droplets.

Researchers will first work with water-based gels.

“We are going to make this gel, which has a consistency of orange marmalade without the rind, push it through holes, and study how it flows and how big the drops are,” said Heister. "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.”

Not only will the viscoelastic properties of the gels be experimentally measured, but also molecular models representing the gels will be developed to enable researchers to predict the behavior of gels and optimize their formulations for improved combustion.

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