The speed of sound was not just a psychological barrier to break — unlike, say, the 300 or 600 mph longstanding airspeed goals — but a technological one. At speeds approaching the sound barrier, the dynamic of flight changes completely: air compresses around the craft, affecting density and therefore lift on the wings. None of this was well understood during the war years, which led to pilots sometimes losing control of their planes during diving attacks. After the war, the newly-established Air Force devoted its full attention and resources to exploring and conquering that barrier.

On this day, October 14, in 1947, Charles E. “Chuck” Yeager, at the controls of his rocket-powered Bell X-1, named “Glamorous Glennus” for his wife, breaks the sound barrier. Yeager achieves a top speed of Mach 1.07 before descending safely.

The control breakthrough that allowed Yeager to go past the sound barrier was in the tail design. Air compression created a kind of “shadow” lift around the tail section, preventing him from changing altitude without slowing down. A redesign was performed on the X-2 to help it pass Mach 1, and further redesigns on the X-planes allowed them to reach “hypersonic” speeds above Mach 5.