In the early jet fighter days, nobody knew how much G-force the human body could withstand. Testing the limits of survivability came down to human testing. Today, we owe much of that knowledge to Dr. John Paul Stapp, who strapped himself into a crazy rocket sled to see just what the human body could endure.
In a training exercise gone wrong back in 1989, Capt. Brian Udell had to eject from his F-15 travelling at supersonic speeds. The force of the air on his body was so strong that it nearly killed him. Here is Udell explaining what happened.