The X-43A 'Hyper X' was a sub-scale prototype that NASA had developed to test a supersonic-combustion (scramjet) engine.4V3 As the aircraft had to be accelerated prior to starting its engine, it was to be boosted by a hybrid version of the Pegasus (designated the HXLV) using only the first stage. The avionics normally carried on the second stage were relocated to the booster. There would be no aerodynamic shroud, the 12-foot-long aircraft would ride on the exposed tip. The rocket was to climb to an altitude of 100,000 feet and accelerate to Mach 7, whereupon a gas-driven piston would release the payload. The X-43A An artist's impression of the X-43A 'Hyper-was to run its engine for 10 seconds to X' on the nose of its Pegasus booster. sustain its speed.44 The fact that the scramjet required air meant that rather than making its usual zooming climb to 200,000 feet, the Pegasus would require to pursue a 'depressed' trajectory, and its control system had to be considerably reinforced because flying so fast within the atmosphere would impose aerodynamic loads 50 times greater than usual. In addition, because there were no upper stages and the payload was so light (1,275 kilograms), some 2,500 kilograms of ballast had to be carried to prevent the booster exceeding the desired speed. Although a hybrid, this was not to be a one-off test, as the MicroCraft Company had built four copies of the X-43A.45 The first vehicle was released by NASA's B-52 off the coast of California on 2 June 2001. About 8 seconds later, the booster began to shed debris, and it was destroyed by command when it veered off course. A replay of the video showed what appeared to be a piece of the reinforced tail fin snapping off.46 The investigation concluded that the mishap was probably due to multiple causes related to the ad hoc requirements imposed on the hybrid booster.47 When the programme resumed on 27 March 2004, both vehicles performed flawlessly.48 A follow-up flight on 16 November attained a speed just short of Mach 10.49,5°
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