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X-43A aircraft. Computer artwork of the unpiloted X-43A aircraft, or Hypersonic Experimental Vehicle (Hyper-X), flying on its Pegasus booster rocket. The X-43A (white, at rocket's tip) will test the first scramjet-powered jet engine to run at hyper- sonic speeds, those over 5 times the speed of sou- nd (Mach 5). Scramjet (supersonic-combustion ramjet) engines, which work at Mach 7-10, could be used in reusable spacecraft. Unlike rockets, which must carry both fuel and the oxygen in which it burns, scramjets use oxygen from air scooped up as the aircraft flies, increasing potential payload. Initial rocket propulsion is needed to reach oper- ating speeds. The X-43A is operated by NASA, USA. (KEYSTONE/SCIENCE PHOTO LIBRARY/NASA)