In the cockpit of the sleek, black aircraft slung underneath the wing of the B-52 bomber, my interphone crackles. "Ah, Robert, it’s a lovely morning," says Jack Allavie, the commander of the B-52 launch aircraft. "Yes it is, Jack," I respond while running through the preflight checklist for our July 17, 1962 mission. The North American Aircraft X-15 was designed to investigate flight at hypersonic (Mach 5-plus) speeds and extremely high altitudes, and the effects of aerodynamic heating on aircraft surfaces. It was the first aircraft to fly Mach 4, Mach 5, and Mach 6—and I had the good fortune...