Coal: The cool fuel for future jets.

Say the words "coal-powered engine," and images of men shoveling black rock into the fiery belly of a steam engine come to mind. However, liquid fuel made from coal instead of oil may shoot the next generation of supersonic jets across the sky.

If flight speeds are to increase, jets will require new fuels that don't fall apart chemically in engines that become extremely hot, says fuel chemist John M. Andresen of the Energy Institute at Pennsylvania State University in State College.

Currently, the F-15 Eagle jet fighter can reach speeds two and a half times the speed of sound. The U.S. Air Force is working to build a jet that will fly at eight to nine times the speed of sound, Andresen said last week at the American Chemical Society's spring national meeting in San Francisco. The engines in such future supersonic jets could get hotter than 450 [degrees] C.

Today's petroleum-based jet fuels can handle the 300 [degrees] C temperatures of a normal jet engine, but they tend to "crack" when the going gets really hot, Andresen says. At high temperatures, these fuels break down into solid carbon waste called coke that can plug up fuel systems and cause catastrophic engine failure.

"Before we can put Luke Skywalker in the cockpit, we have to make sure the Force is with him," Andresen says. The Penn State scientists are creating coal-derived fuels to deliver the punch needed to get new jets safely up to speed.

Using reactors that simulate conditions inside a jet engine, the researchers heated chemicals modeling petroleum-derived or coal-derived jet fuels. The coal fuel, decahydronaphthalene, stood up to temperatures as high as 500 [degrees] C. That's about 50 [degrees] C higher than the temperature that reduced the petroleum fuel, n-tetradecane, to coke, says Andresen.

Coal's secret to keeping its cool is its structure, Andresen says. The carbon molecules in coal-derived fuels form rings that join into chainlike molecules a called cycloalkanes, while petroleum fuels have mainly linear molecules. Cycloalkane chains are much harder to break apart than petroleum's shoe-stringlike molecules, Andresen says. Some coal-derived jet fuels may be able to withstand temperatures up to 800 [degrees] C, he speculates.

The next step is to test the model fuels s in actual engines, the researchers say.

The new jet fuels will not only have to withstand engine heat but also serve as a coolant for the jet's mechanical and electrical systems, says Harold H. Schobert, the director of the coal-jet-fuel project at Penn State. Because fast planes must be light, engineers strip them down to bare essentials. "You need something that can soak the heat up, and about the only thing you've got on the aircraft is the fuel," says Schobert.

So far, methods aren't available for making large amounts of coal-derived fuels cheaply. "It's all a question of economics," says fuel scientist Gerald P. Huffman of the University of Kentucky in Lexington.

To combat the cost, Anne E. Fickinger of Penn State and her team have converted a mixture of petroleum and coal into a fuel that contains heat-resistant cycloalkanes.