WASHINGTON DC — The North American X-15 rocket-powered research aircraft is displayed in the Boeing Milestones of Flight Hall at the Smithsonian National Air and Space Museum. First flown in 1959, the X-15 bridged the gap between atmospheric flight and spaceflight, becoming the first winged aircraft to achieve Mach 4, 5, and 6 velocities and operate at altitudes above 30,500 meters (100,000 feet). The experimental aircraft was fabricated using Inconel X, a high-strength nickel alloy designed to withstand aerodynamic temperatures of 1,200 degrees Fahrenheit during hypersonic flight. Air-launche

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