John Paul Stapp was a physician in the Air Force who was fascinated by speed and the effects of G-Force on the human body.
G-Force is the measurement of acceleration due to gravity. On Earth, gravity causes us to accelerate towards the center of the planet at 32 ft per second squared, which is a force of 1G.But if you hop onto a roller coaster, or into an airplane, the increased acceleration will cause more g-force and perceived weight on your body.
So, how much G-Force is safe? In the 1940's, it was assumed that pilots could only withstand up to 18 G's. But Stapp believed humans could withstand more. Much, much more. To test his theory, Stapp built a rocket sled that propelled down a track at hundreds of miles an hour. At first he used dummies, but eventually he tested it on himself.