NASA is developing technology for air-breathing rocket engines that could help make space transportation safe, reliable and affordable for ordinary people. Powered by engines that breathe oxygen from the air, the spacecraft would be completely reusable, take off and land at airport runways, and be ready to fly again within days. The engines would get their initial take-off power from specially designed rockets, called air-augmented rockets, that boost performance about 15 percent over conventional rockets. When the vehicle?s velocity reaches twice the speed of sound, the rockets are turned off and the engines rely totally on oxygen in the atmosphere to burn the hydrogen fuel. Once the vehicle?s speed increases to about 10 times the speed of sound, the engine converts to a conventional rocket-power mode to propel the vehicle into orbit. This Quick Time movie features an aerojet engine which is the main propulsion system of the X-series future launch vehicles.