Since it must use such large extenal boosters to place it into the air, it is not feasible to launch the shuttle for less than orbital flight. To provide for "local" hops, NASA obtained a giant Boeing 747 airliner from American Airlines, and alter edit to carry the shuttle in a special cradle mounted to the fuselage. In addition,a pair of vertical stabilizers was installed to increase the stability of the 747 during the captive flights, and to provede the necessary vertical surface for safe flight in the event the shuttle colleded with the 747's large tail fin.
The 747 was specifically modified for the initial flight testing of the spaceship. To determine the flight characteristics of the shuttle, it was mounted to the airliner and carried to altitude. During captive flights, in which the shuttle remained attached to the 747, the controls and other systems were tested as the pair sruised over Edwards Air Force Base. Eventually, the shuttle was carried aloft was flown back to Edwards on its own, the final proof of its aerodynamic ability.
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