Chuck Yeager: Well, number one, they built the airplane with very thin wings so that the airplane could go faster before it ran into the buffeting problems. It was rocket-powered, which meant that you had full thrust at altitude — jet engines decrease in thrust the higher you go — and it was built about two-and-one-half times stronger than airplanes that we were flying at that time. The airplanes that we used in World War II, and the ones that were built immediately after World War II, were stressed for Gs, or times the pull of gravity and, if you overstressed them, they would break, obviously. The wings would break off and the like. But the X-1 was stressed for 18 Gs, positive or negative. So it would stay together in case you run into a problem. And also, it had a moveable, horizontal stabilizer. The tail-plane on all airplanes just stabilizes, and you have elevators on the back to make the airplane go up and down. Well, they built the capability into the X-1 to move the whole angle of the horizontal stabilizer, change the angle with that. That really was the big secret on how we got the airplane through the speed of sound.