We've been flying subsonic, 400mph to 500mph jets since the late 1950s and early 1960s.

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The B-52s are long-range bombers capable of flying at subsonic speeds at altitudes of up to 50, 000 feet.

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Unlike the ill-fated Concorde, it can also cruise as efficiently as existing business jets at high subsonic speeds.

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In other words, the boundary between the supersonic and subsonic regions would do for sound waves what the event horizon does for light.

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Flying at subsonic speed, the needle-nosed jet flew without passengers on Saturday from a military air base in Istres, near Marseille, and landed at Paris' Orly Airport.

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At subsonic speeds, the aircraft guzzles fuel.

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The airlines are not prepared to say what fares they will charge, but the 3, 500 regular Concorde users (many of whom fly back first class on subsonic flights after a day's work in America or Europe) are unlikely to be price-sensitive.

ECONOMIST: Any day now the supersonic jet will be cleared for take-off