After World War II, the US Air Force needed to know if pilots could eject from supersonic jets without facing certain death because of the shock of rapidly decelerating from the speed of sound to a near standstill. The transition exposed pilots to forces of over 40 or 50 Gs. (One G equals the force of gravity at the surface of the earth; 40 Gs is like a 7000-pound elephant falling on top of you.) Many doctors believed that 18 Gs was the most a human body could endure, but no one knew for sure. Flight surgeon John Paul Stapp volunteered to serve as the guinea pig in a series of physically brutal experiments to find out.
Dr. Stapp decelerating
Dr. Stapp decelerating
At Holloman Air Force Base in New Mexico, Stapp designed a rocket-powered sled that blasted down a 3500-foot track at speeds up to 750 mph before slamming into a pool of water that brought it to an abrupt halt. It went from 750 mph to zero in one second. Strong restraints made sure that the passenger didn’t continue their forward trajectory, though the restraints didn’t always work. One test dummy came free of the harness and was catapulted 700 feet through the air.
For his inaugural rocket sled ride, in 1947, Stapp went at a gentle 90 mph. The next day he advanced to 200 mph. And subsequently he kept signing up for more rides, upping his speed, probing the limits of human endurance. Over a period of seven years he rode the sled twenty-nine times.
Each time he rode the sled, the force of the deceleration hammered his body. He repeatedly endured blackouts, concussions, splitting headaches, cracked ribs, dislocated shoulders, and broken bones. One time, in a show of bravado, he set a broken wrist himself as he waited for medics to arrive. The greatest danger was to his eyes. Rapid deceleration causes the blood to pool with great force in the eyes, bursting capillaries and potentially tearing retinas. Even more disturbingly, when a human body comes to a stop that abruptly, there's a real possibility the eyeballs will simply keep going — popping out of the skull and flying onwards.
An early version of the rocket sled
On Stapp's final ride on 10 December 1954, this almost happened. Nine rockets propelled him to 632 mph, faster than a .45 calibre bullet. He outran a jet flying overhead. And when the sled hit the water, Stapp experienced a record-breaking 46.2 Gs of force.
Stapp survived, but he later wrote of the experience, "It felt as though my eyes were being pulled out of my head... I lifted my eyelids with my fingers, but I couldn't see a thing." He feared he'd permanently lost his vision, but thankfully his eyesight gradually returned over the next few days. However, on account of that final ride, he suffered vision problems for the rest of his life.
- Ryan, C. (1995), The Pre-Astronauts: Manned Ballooning on the Threshold of Space, Annapolis: Naval Institute Press.
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Ed Murphy (of Murphy's Law fame) was also involved with the rocket sled experiments.
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