NASA's NB-52B carrier aircraft rolls down a taxiway at Edwards Air Force Base with the X-43A hypersonic research aircraft and its modified Pegasus® booster rocket slung from a pylon under its right wing. Part of a combined systems test conducted by NASA's Dryden Flight Research Center at Edwards, the taxi test was one of the last major milestones in the Hyper-X research program before the first X-43A flight. The X-43A flights will be the first actual flight tests of an aircraft powered by a revolutionary supersonic-combustion ramjet (''scramjet'') engine capable of operating at hypersonic speeds (above Mach 5, or five times the speed of sound). The 12-foot, unpiloted research vehicle was developed and built by MicroCraft Inc., Tullahoma, Tenn., under NASA contract. The booster was built by Orbital Sciences Corp., Dulles, Va.,After being air-launched from NASA's venerable NB-52 mothership, the booster will accelerate the X-43A to test speed and altitude. The X-43A will then separate from the rocket and fly a pre-programmed trajectory, conducting aerodynamic and propulsion experiments until it descends into the Pacific Ocean. Three research flights are planned, two at Mach 7 and one at Mach 10, with the first tentatively scheduled for late spring to early summer, 2001.