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Inconel X, a ferociously strong nickel alloy, gives the X-15 its gun-metal black color. Inconel was chosen for the airplane's skin because it retained its strength up to 1,200 degrees Fahrenheit, a temperature the X-15 would routinely experience at high speeds. (Eric Long/NASM)

Why We Miss the X-15

Not only was it the fastest. It may have been the best flight research program ever.

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