An artist's concept of a launch vehicle powered by an air-breathing rocket engine. Photo: NASA

Taking another step toward making future space transportation more like today's air travel, NASA's Marshall Space Flight Center in Huntsville, Ala., and its industry partners have completed a series of successful tests on air-breathing rocket engines.

The latest ground testing focused on engine performance during low-speed portions of the flight, when high thrust levels are needed to push the air-breathing rocket through Earth's atmosphere.

An air-breathing -- or rocket-based, combined cycle -- engine inhales oxygen from the air for about half the flight, so it doesn't have to store the oxygen on board. That reduces the vehicle's weight at launch, resulting in significant cost savings.

At launch, the engine is powered by specially designed rockets strategically placed in a duct that captures air. Once the vehicle reaches twice the speed of sound, the rockets are turned off and the engine relies totally on oxygen in the atmosphere to burn its fuel. When its speed increases to about 10 times the speed of sound, the engine converts to a conventional rocket-powered system for the final push to orbit.

Similar testing by Aerojet Corp. of Sacramento, Calif., and Rocketdyne of Canoga Park, Calif., showed that recent modifications to the engine's internal geometry improved performance. Aerojet conducted tests at its newly refurbished facility in Sacramento, while Rocketdyne conducted tests at the General Applied Sciences Laboratory (GASL) on Long Island, N.Y.

Meanwhile, the Marshall Center's academic partner, Pennsylvania State University of University Park, finished the first phase of its experimental work on air-breathing rocket engine development in mid-March and immediately started a second phase of activity. The experimental research now under way will examine the effect of two rockets in a duct and the use of hydrocarbon fuels, instead of hydrogen.