In 1963, Captain Engle was assigned as one of two Air Force test pilots to fly the X-15 Research Rocket aircraft. In 1965, he flew the X-15 to an altitude of 280,600 feet, and became the youngest pilot ever to qualify as an astronaut. Three of his sixteen flights in the X-15 exceeded the 50-mile (264,000 feet) altitude required for astronaut rating. https://www.nasa.gov/image...