In addition to biofuels, early efforts included developing liquid fuels from coal and natural gas for the Air Force, the largest energy user of the armed services. But the gas and coal fuels would not meet cost or environmental requirements, officials said. The Defense Department focused on advanced biofuels, which are generally made from plant and animal feedstocks that don’t compete with food uses, which is a concern with common renewable fuels like the corn-based ethanol used in cars.

The federal Renewable Fuel Standard, which sets targets for renewable fuel production and requires a certain amount to be blended into conventional gasoline and diesel, has been the main catalyst for the growth of several companies exploring new technologies.

Investors, however, have been leery of the enormous amounts of cash it can take to bring the fuels from the lab to the gas tank. Industry officials say that having a large, steady customer like the military could attract other investors to help finance large refineries that would bring costs down through economies of scale. Military officials say that their purchases of small amounts for testing has already helped reduce the cost. In 2009, the Pentagon spent roughly $424 a gallon on algae oil from Solazyme.

“Finding a user like the military can rapidly help to scale technologies that then are used in the civilian marketplace — it becomes a catalyst,” said Bob Johnsen, chief executive of Primus Green Energy, which is developing fuels from biomass and natural gas. “If the military becomes a buyer, that becomes a means by which the production facilities can be financed.”

The Defense Department is always vulnerable to charges of overspending — remember the $7,600 coffee maker? — but military leaders argue that what they are putting into biofuels is a blip given the potential benefits of reducing their need for Middle Eastern oil, with all its volatilities.

“Our primary rationale is not economic,” said Sharon E. Burke, assistant secretary of defense for operational energy plans and programs. “Our job is to defend the country.”