X-15 test pilots

Meet some of the X-15 test pilots (there were 12 overall) in 1966, from left to right: Air Force Capt. Joseph Engle, Air Force Maj. Robert Rushworth, NASA pilot Jack McKay, Air Force pilot William "Pete" Knight, NASA pilot Milton Thompson, and NASA pilot Bill Dana.

Test pilots have a well-earned reputation for being cool under pressure, but even they feel some stress sometimes. According to NASA, during X-15 flights, the pilots' heart rates ranged from 145 to 185 beats per minute, well above the 70 to 80 beats during test missions in other aircraft up to that time.

For more, see "Photos: Looking back at NASA's X-15 aircraft."