There is a name you need to know: John Paul Stapp. If you have been in a car accident, it is likely that you owe your life and health to him. That is, if you were wearing a seat belt.
Step back about 70 years. World War II had just ended, and a young physician was wondering why so many military pilots were dying, when they didn't have to. During that war, getting shot down was a death sentence in one of two ways: you died when the plane crashed, or you died trying to exit the plane. After the war, ejection seats were found to be, far too frequently, tickets to oblivion. Their design was based on, at best, random guesses about the amount of stress the human body could survive, and the forces the aircraft frame could handle.
Dr. Stapp set out to gather accurate and usable data. What he did and how he did it are detailed in the first half of Sonic Wind: The Story of John Paul Stapp and How a Renegade Doctor Became the Fastest Man on Earth, by Craig Ryan. The second half shows what he, and the country, did as a result.
Before the 1940s, a smattering of centrifuge experiments had established that, with training and with minimal support from a flight suit, a fighter pilot could avoid blacking out at accelerations of about 6 G's. The G is a one-gravity acceleration force. If you weigh 150 lbs (68 kg), that is the force a mattress must apply to hold you up. If you and the mattress are put in a centrifuge and spun so as to apply a 6 G acceleration, the centripetal force the mattress (and the frame holding it) must now apply to hold you is 900 lbs (408 kg). When your body weight is spread out by a mattress, if the area of your body against the mattress is about 5.4 sq ft (0.5 m²), you'll feel a pressure of about 28 lb/ft² or 136 kg/m². That comes to about 0.19 psi. Now, multiply that by six, and you'd feel almost 1.2 psi. If your normal blood pressure is 120/75 (what doctors currently recommend, but maybe yours is higher), that 120 mm translates into 2.3 psi, and the 75 mm into 1.5 psi. So you can see that sustained acceleration of 6 G's tends to draw the blood in your body towards the mattress. If you are sitting rather than lying down, it doesn't take long for an acceleration of 6 G's to pull the blood from your brain, and you black out.
At this point it is all about sustained G forces. It makes sense that you could survive larger forces if they occurred briefly and were rapidly abated. Somehow, a factor of three became dogma, so that a brief acceleration of 18 G was considered the threshold of death. Yet, common observations of people surviving falls calls this into question. One of my brothers fell 20 feet out of a tree, landed on his back on the lawn, and had the breath knocked out of him. But he got up after a minute or so and was OK. Now, a grassy lawn is softer than landing on concrete, but it doesn't have much give. The main thing keeping this from being an "instant stop" (physically impossible) was the flexibility of the body, which squishes out briefly. I calculate that my brother's body touched the ground going about 24 mph (39 kph) and stopped in a distance of about 4 inches. That works out to a stopping force of 60 G's. If instead we allow him a little more flexibility to squishing, perhaps the stopping distance was 6 inches, and he experienced 40 G's. Either number is a far cry from 18 G's.
Over about a decade, Dr. Stapp used himself as the primary experimental subject (not the only one; he also used chimpanzees and on rare occasions, another volunteer) in rocket sled experiments. The rockets would get the sled going to some high velocity, and a braking system would then stop it over a prescribed distance. Here are parameters that might describe a typical experiment:
- Rocket acceleration: 4 G's
- Burn time: 4.6 s
- Burn distance: 410 m (1,340 ft)
- Peak speed: 644 kph (400 mph)
- Stop distance: 20.5 m (67 ft)
- Stopping time: 0.23 sec
- Average stop G's: 20
- Peak stop G's: 30 (measured by camera)
Early experiments were conducted with the seat on the sled facing backward, so the subject was pressed into the seat by the stopping forces. Experiments were also conducted with the seat in various orientations, including "butt forwards", to determine the forces of an ejection seat's kick-off blast.
Later experiments were conducted with the seat facing forward, and the subject exposed first to the wind blast, and then to deceleration against the webbing holding him into the seat. Dr. Stapp used chimps to determine the edge of lethality, though it turned out that they are much, much tougher than humans, so getting the calibration right for human experiments was tricky. With humans (mostly himself), he gradually raised the G forces and observed his own feelings and had doctors note what injuries he sustained. Thus, as time went along, the design of the seat was improved to avoid points that exerted extra forces and were causing injury. Over time these design changes were implemented in pilot seats.
The final, most definitive experiment was conducted with a chase plane flying above the rocket sled, to observe and film it from above. The pilot was astounded when the sled outraced the plane, reaching a top speed of 639 mph (1,028 kph), or Mach 0.9. This earned Stapp the title of "fastest man on earth" in a ground-bound vehicle. The title stood for about 30 years. During the deceleration, though, he sat forward-facing, getting the full wind blast, and being jammed against seat restraints with a crushing 45 G's, peak, during a stop that lasted less than 1.5 seconds. He was a mess when he was helped out of the seat. His eyes looked like pools of blood; he was lucky they had stayed in his head. It took weeks for all his sight to return. He had several broken bones. Though he had the ambition to go 1,000 mph, or at least Mach 1 (about 715 mph; authorities vary), it was not to be. He had advanced to Captain, Major, and was now a Colonel, and was moved by the Air Force command to a more administrative role. His sled, named the "Sonic Wind", was retired.
What he did next is the subject of the second part of the book. Dr. Stapp had performed his experiments, often against opposition, on a shoestring. He had to scrounge and cadge for equipment and apply verbal tricks to get some semblance of permission. Such skills were even more necessary after about 1956. He had long lobbied and clamored to Air Force brass about the safety, and its lack, in fighter aircraft and also transports. One result of his nagging was that many transports in war zones had the seats for the troops facing backwards. Then they were much more likely to walk away from a crash. But even during his earlier experiments he was also lobbying for the use of seat belts in automobiles.
By 1956, about 36,000 Americans were dying every year in automobile crashes. The population was about half what it is today, so in proportion, there could now be 72,000 auto deaths yearly, but instead, there are about 33,000. It took Colonel Stapp and his allies another 14 years to bring about the changes, primarily in laws, that have, since about 1970, saved at least 800,000 lives. Over the last 17 years, some of the difference is also due to airbags, something Stapp heartily approved of; he died in 1999, the year after airbags were mandated.
During his "lobbying years", he fought resistance in both government and industry against mandatory seat belt installation and use. The auto manufacturers were a lot like the tobacco lobby of the same era, denying that their products' quality had anything to do with the deaths that were occurring. Fortunately, there were at least aftermarket seat belts available, and many members of the public didn't wait for Washington or anyone else. Over a decade's time, sufficient statistics were compiled that a growing number of lawmakers became convinced of the belts' value, and in 1968, factory-installed seat belts were required by law. I remember an ambulance EMT who said he'd never unbuckled a dead body.
I bought my first car in 1967, a 1964 VW beetle. A couple of years later I bought a set of aftermarket 3-point lap/shoulder belts and installed them. Fortunately, Europe had been ahead of the curve, and though the car didn't have belts already installed, it did have threaded mounting holes, so the installation was easy. I have used seat/shoulder belts ever since. But growing up, we did many road trips, hundreds of miles yearly, in a big station wagon with no belts, and a mattress in the "back-back" for us boys to nap on. We were lucky.
Since 1984, one after another of the U.S. states has passed laws requiring seat belt use. Compliance varies, but averages 85%. Nearly all of those 33,000 highway fatalities in recent years, has come from the 15% who don't wear seat belts. In spite of the air bag in most vehicles, they either crash around inside during a collision, or are ejected. Driving in California with my brother several years ago, we saw an SUV hit the median barrier on the freeway, and the driver burst through the side window and landed on the highway almost in front of us, on his head. One of us (I don't recall who) said, "We just saw someone die."
Two things to remember about Colonel Dr. John Paul Stapp: He risked his life, incidentally becoming the fastest man on earth, to gather safety data; then he used those data and traffic statistics to practically crowbar the United States into becoming quite a bit safer as a place to drive or fly. Craig Ryan's exciting biography brings us the man and the stories, a portrait of someone to whom you just might owe your life.