WASHINGTON — While plans for a military space plane remain somewhere off in the future, the U.S. Air Force is moving ahead with work on the air-breathing rocket engines that could power such a vehicle.

In late September, the Air Force Research Laboratory awarded contracts to five companies to begin working on improvements to existing air-breathing engine technology under an effort with a total budget of $49 million through 2010.

The goal of the program is to pave the way for engines that are more powerful and durable than the engines that could be built with existing technology, making it possible to conduct multiple satellites launches with the same vehicle.

Plans for the Robust Scramjet program are focused on ground demonstrations, and do not involve a major flight experiment at this point, said Bob Mercier, deputy for technology in the aerospace propulsion division at the Air Force Research Laboratory’s propulsion directorate at Wright-Patterson Air Force Base, Ohio. While the total current budget for the program is $49 million, that figure could rise if program officials decide to expand the scope of the work, he said.

Program officials will evaluate the progress on an annual basis to decide which areas look most promising, and what areas need more work, Mercier said. While the contracts are all options after the first year, program officials may suggest new areas for companies to examine if what they are working on does not pan out, he said.

Scramjet engines use oxygen from the atmosphere instead of carrying heavy tanks filled with the oxygen that has to mix with hydrogen fuel to provide engine thrust.

Eliminating the oxygen tanks makes rockets light, which makes it possible to launch heavier payloads.

The Air Force began flight testing the engines in 2001 with a small-scale version of a missile, according to an Air Force fact sheet.

The Robust Scramjet effort will help pave the way for the much larger engines that will be needed to launch spacecraft, said Albert Boudreau, chief of the propulsion technology branch at the laboratory.

The companies involved in the work are Johns Hopkins University Applied Physics Laboratory, Baltimore, Md.; GenCorp Aerojet, Sacramento, Calif., Pratt and Whitney, West Palm Beach, Fla.; GASL, Inc., Ronkonkoma, N.Y.; and Innovative Scientific Solutions, Inc., Dayton, Ohio.

Work under this effort is intended to pay off farther into the future than on systems like the X-43C experimental aircraft, a project jointly funded by the Air Force Research Laboratory and NASA that is scheduled to begin flying in 2007, Mercier said.

While rocket engines today are typically used just once, the Robust Scramjet effort is intended to develop engines that can ultimately be used 50 to 500 times, Boudreau said.

Program officials will explore the use of new methods of cooling the engines and composite materials that can withstand high temperatures to facilitate repeated flights, Boudreau said.

One possibility is using a circular or elliptical shaped combustion chamber in the engine, which may be more durable and help burn the fuel more efficiently than traditional rectangular shaped combustion chambers on previous scramjet designs, Raffoul said.

The program will also examine ways to most efficiently distribute fuel throughout the engine, Mercier said.

This will likely involve designing different points to inject the fuel to meet the air as the engines that carry satellites in the future will be far larger than those that the Air Force has experimented with thus far, he said.