NASA was set over the weekend to conduct drop testing on the second of three unpiloted X-43A vehicles the Hyper-X program. The test is a dress rehearsal for its free flight currently scheduled for Feb. 21, 2004.

Pending thorough evaluation of all flight data, the captive-carry test could lead to the February launch of the X-43A stack. The stack, consisting of the X-43A and its modified Pegasus booster will be air-launched by NASA's B-52 carrier aircraft at 40,000 feet altitude. The booster will accelerate the X-43A to Mach 7 at approximately 95,000 feet altitude. At booster burnout, the X-43 will separate and fly under its own power on a preprogrammed path.

Distinctive to the X-43A is the blending of its integrated airframe with a scramjet or supersonic combustion ramjet engine, intended to make the X-43A the first air-breathing hypersonic vehicle in free flight. The hydrogen-fueled aircraft has a wingspan of approximately 5 feet, measures 12 feet long and weighs about 2,800 pounds.

The flight of vehicle two is programmed for Mach 7 - seven times the speed of sound. The third vehicle of the series is planned to reach Mach 10. Using an air-breathing scramjet engine instead of conventional rocket power, the X-43A could be the forerunner in providing faster, more reliable and less expensive access to space. A scramjet uses oxygen from the atmosphere, unlike rockets that must carry oxidizer onboard. This could enable scramjet vehicles to carry bigger payloads, travel farther, or be smaller than comparable rocket vehicles.

On June 2, 2001, the first X-43A vehicle was lost moments after the stack was released from the wing of the B-52. Following booster ignition, the combined booster and X-43A vehicle deviated from its flight path and was deliberately terminated.

Investigation into the mishap showed that there was no single contributing factor but the root cause of the booster going off its trajectory was resolved through development of better analytical models and modification of the control system in the booster. Now, the booster will carry less propellant and will be released from 40,000 feet instead of 20,000 feet as was done in the first flight.

Pegasus's normal launch altitude of 40,000 feet is beneficial due to the lower air density at this altitude. This reduces loads on the booster fins. To launch at this altitude and still end up at the correct test conditions of 95,000 feet and Mach 7 required removal of about 3,500 pounds of the propellant from the solid rocket motor.

The experimental aircraft will fly in the Naval Air Warfare Center Weapons Division Sea Range over the Pacific Ocean off the coast of southern California, where after powered flight, it will glide to a safe impact and sink. NASA's Langley Research Center, Hampton (VA), and Dryden Flight Research Center, Edwards (CA), conduct the Hyper-X program jointly.

Allied Aerospace Industries in Tullahoma (TN), built both the vehicle and the engine, and Boeing North American in Huntington Beach, Calif., designed the thermal protection and propulsion control systems. The booster is a modified Pegasus rocket from Orbital Sciences Corp. Chandler (AZ). The Hyper-X program is part of NASA's Next Generation Launch Technology program.

FMI: www.nasa.gov