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Quest for new rocket fuel leads scientists to kitchen

March 16, 2009
Science, like politics, makes strange bedfellows. At Purdue University, the quest for a new missile and spacecraft fuel has brought together an oil-and-vinegar mix of rocket scientists and food scientists.

They joined forces to pursue a five-year, $6.4 million project funded by the U.S. Army Research Office to study the "spray and combustion of gelled hyperbolic propellants." Or, in layman's terms, how to use gelled rocket fuels (which are easier to store and handle than liquids) to power everything from missiles to spacecraft.

The stumbling block (well, one of the stumbling blocks) is learning how to burn it efficiently. In particular, making sure the fuel droplets sprayed into a rocket engine's combustion chamber are just the right size.

Getting a grip on the droplet issue demanded an eclectic team of experts in mechanical engineering, aeronautics, astronautics, food science, and agricultural and biological engineering — both at Purdue and at the study's other participating schools, Iowa State University and the University of Massachusetts.

The project needed rocket scientists because they know about rocketry. It needed food science and agricultural experts because they knew about handling gels, which are used in hundreds of products found on grocery store shelves.

"This is by far the most multidisciplinary team I've been associated with," said Stephen Heister, Purdue professor of aeronautics and astronautics and the leader of one of the program's two groups.

"The food science folks are incredibly knowledgeable and are wonderful colleagues to work with. In our application, however, we are pushing the gels at much higher velocities than one normally would find in a food processing plant. For this reason, it is an interesting problem for them as well."

The other "interesting" aspect is that food science people don't normally work with potentially toxic or explosive compounds. Still, tapping them for help with propellants isn't as odd as it might sound. When it comes to processing gel-like compounds on an industrial scale, they wrote the book.

"Orange marmalade, peanut butter and even mayonnaise can have mechanical properties similar to this rocket fuel," said Osvaldo Campanella, Purdue professor of agricultural and biological engineering. "In fact, we are looking at a material that has gel-like mechanical properties when it is stored and more liquid-like characteristics when it is pumped or sprayed."

Much of the work will be done using an inert compound that's the color and consistency of orange marmalade. Purdue scientists are experimenting with it at the university's Maurice J. Zucrow Laboratories in the Department of Food Science, and at the School of Agricultural and Biological Engineering.

One of the key goals is creating a fine spray of uniformly sized droplets that provides the maximum amount of boom when ignited. To see which spray techniques work best, a test chamber was constructed into which the fake fuel could be injected. Super-high-speed movies (approximately 10,000 frames per second) capture every nuance of its behavior.

So far, researchers have learned that gelled fuels are far easier to store than they are to actually use, said Paul Sojka, a Purdue professor of mechanical engineering and an associate director of the project.

"Anyone can form a gelled fuel," Sojka said. "The key question is, 'What's the right consistency?' This is complicated by the fact that the 'right' consistency is probably different when considering fuel-storage issues, fuel-pumping issues, fuel-injection issues and fuel-burning issues."

In other words, the perfect gel for, say, storage, probably isn't the perfect fuel consistency for, say, burning. Which is why it could be a while before NASA or the Department of Defense can use this technology.

"It would likely be eight to 10 years before a system could be fielded, as there are many hurdles such as long-term storage that require substantial time to evaluate," Heister said.

It will be worth the time if the project eventually pans out. Jason Lovell, manager of the state's Defense Development Division, said such odd-seeming synergies aren't that odd at all. It's all part of the sprawling world of Department of Defense research.

"Almost anything you'd develop for the commercial world can have a defense application," Lovell said. "Or vice versa."

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