In 1963, Captain Engle was assigned as one of 2 Air Force test pilots to fly the X-15 Research Rocket airplane.
In 1965, he flew the X-15 to an altitude of 280,600 feet, and became the youngest pilot ever to certify as an astronaut.
Three of his sixteen flights in the X-15 exceeded the 50-mile (264,000 feet) elevation needed for astronaut rating.Image Credit: NASARead More.
Image Credits - NASA