The problem is that the likely sources of fuel.

Air Force Hopes to Cut Oil’s Role in Fuel

By DON PHILLIPS
Published: June 18, 2007

The United States Air Force has decided to push development of a new type of fuel to power its bombers and fighters, mixing conventional jet fuel with fuels from nonpetroleum sources that could eventually limit military dependence on imported oil.

The decision will open a contest between fuel refiners and other companies to produce a jet fuel composed of no more than 50 percent petroleum. The plan is to be announced at the Paris Air Show by the secretary of the Air Force, Michael W. Wynne; the administrator of the Federal Aviation Administration, Marion C. Blakey; and other American officials.

“The goal is to certify the entire fleet by 2010 with a 50-50 mix,” said Paul Bollinger, an Air Force official who is working on a shift to synthetic fuels.”

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Today’s most popular alternative fuel, made from corn, is not suitable for use in aviation. “Corn doesn’t have the B.T.U.’s for jet fuel,” Mr. Bollinger said, referring to the British thermal unit, a measure of energy. Richard L. Altman, executive director of an industrywide group called the Commercial Aviation Alternative Fuels Initiative, said fuels would most likely be developed in three phases, beginning with a focus on creating liquid fuels from nonrenewable resources like coal and natural gas.

Natural gas’s greatest utility is that it can be piped to point of use with very low losses, and coal liquification releases more C0₂ than burning it to generate electricity.