Before he became known as “The Fastest Man on Earth” for his rocket experiments, Captain John Paul Stapp, a medical doctor and member of the Air Force Aero Med Lab, was probably the coldest man on earth. He was testing out the effects of high-altitude flight on humans – on himself, mainly – and in the days before pressurized, temperature-controlled cabins, it couldn’t have been fun. Having launched himself into the stratosphere, Stapp proceeded to a next set of experiments, launching himself over ground at unfathomable speeds.

On this day, March 19, in 1954, Stapp boarded the “Sonic Wind,” a sled powered by six rockets, at Holloman Air Force Base, Alamogordo, New Mexico. The sled was mounted on a set of reinforced rails to smooth out the acceleration and deceleration: up to 420 mph within seconds, and then an abrupt stop by means of a water-catching vent.

Stapp’s goal in the experiments was to test the g-force effects on the human body on ejection while during supersonic flight. For half a second the gravitational pull on his body was more than 15 times that of normal, and for a fraction of a second it was 22 g’s, beyond the bounds of what was considered survivable by humans. Stapp go out the sled unphased. “I feel fine” he said,  “This sled is going to be a wonderful test instrument. I’m ready to do it again this afternoon.”