Since the first aeroplane wobbled off the ground a hundred years ago, engineers have dreamed of making their magnificent machines fly faster and faster. But for a long time the world speed record has not changed. The SR-71, an American spy plane, has been the fastest publicly acknowledged aircraft for almost 40 years. It has a top speed of just over Mach 3-in other words, three times the velocity of sound, or about 3,000 kilometres per hour. But if all goes well, this will soon change. On March 27th, NASA, America's aerospace agency, will conduct a test flight of an aircraft called the X-43A at Edwards Air Force Base in California. This vehicle is, admittedly, a disposable, unmanned drone. But its top speed is supposed to be a remarkable Mach 7.
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