All About G Forces by Peter Tyson
What's behind gravity forces,
and how much of them can we take?
Three
summers ago I took my then nine-year-old daughter on a glider ride. Midway
through, as we soared over a coastal landscape, I casually asked the pilot
whether he could do any tricks. Without a word, he threw the plane into a dive.
We were accelerating straight towards the ground. My daughter and I shouted and
grabbed the armrests. Suddenly we were hit with that thrill-inducing pressure
familiar from rollercoasters—tensed facial muscles, light-headedness, a
sense of altered reality.
The pilot
pulled up, and all we could see through wide-open eyes was sky. We zoomed
straight up until the glider ran out of pizzazz, then the pilot tipped it over
into another sheer drop. Again, squeezed faces, dizziness, otherworldliness.
After two or three loop-the-loops, the thrill became dread: Would he ever stop?
My daughter was laughing, but I thought I would pass out.
What was
going on? What happens to us physiologically when we start "pulling
G's," as pilots label what we were feeling? Why was the sensation
most pronounced as we swooped out of a dive? Might the glider pilot, I wondered
at the time, pass out himself?
"Fainting in the air"
Before
the advent of airplanes, which could accelerate the human body like nothing
before, people rarely experienced G forces. So-called gravity forces first
became a concern during World War I, when pilots began mysteriously losing
consciousness during dogfights. As early as 1919, a doctor wrote up this
strange phenomenon for the literature, calling it "fainting in the
air."
With the
development of faster and more maneuverable planes, G forces became more
dangerous. Based on rates of survival (or lack thereof) during crashes, it became
accepted wisdom that no pilot could withstand more than 18 G's, or 18
times the force of gravity at sea level. So cockpits were designed to withstand
only 18 G's. Yet pilots sometimes walked away from crashes in which the G forces were calculated to have been much
higher.
In the
mid-1940s, an Air Force physician named John Stapp began to suspect that it was
the mangling effects of a crash and not the G's that killed pilots.
Hoping to improve cockpit safety, Stapp set out to determine just what humans
could take in the way of G forces. He built a rocket-powered sled, the
"Gee Whiz," which accelerated a tightly strapped-in
body—initially a dummy but soon Stapp himself—to extraordinarily
high speeds along a track before coming to an almost unimaginably abrupt stop.
By the
late summer of 1948, Stapp had done 16 runs himself and withstood up to 35
G's. He lost dental fillings, cracked a few ribs, and twice broke a
wrist, but he survived. Still he was not satisfied. Eager to know what pilots
ejecting at high speed could endure in terms of sudden deceleration, Stapp
built a new sled called "Sonic Wind" in the early 1950s.
There is a limit to what anyone can take. Princess Diana tragically
proved that.
On what
became his final run, in December 1954, Stapp decided to pull out all the
stops. Firing nine solid-fuel rockets, his sled accelerated to 632 miles per
hour in five seconds, slamming him into two tons of wind pressure, then came to
a stop in just over one second. A witness said it was "absolutely
inconceivable anybody could go that fast, then just stop, and survive."
But Stapp did—in fact, he went on to live another 45 years, dying quietly at home
in 1999 at the age of 89—and he
experienced a record-breaking 46.2 G's. For an instant, his 168-pound
body had weighed over 7,700 pounds.
Stapp's
efforts put him on the cover of Time,
and he was called "The Fastest Man on Earth." More importantly, his
work led to greatly improved safety in both planes and cars, and he gave us a
much-improved understanding of human tolerance to G forces.
A
matter of acceleration
Even
before Stapp it was well-known that G
forces have less to do with speed than
with acceleration—the change in speed over time. If speed alone could
cause the thrill that comes from feeling G
forces, then simply driving on the highway would suffice.
When most
of us think of acceleration, we think of, say, a Jaguar doing 0 to 60 in six
seconds. But acceleration is technically any change in the velocity of an
object: speeding up, slowing down, and changing direction are all types of
acceleration. That's why, on a rollercoaster, you feel G forces when you
round tight bends and are thrown against the side of your seat (a change in
direction) as much as when you plunge from the heights (accelerate) or grind to
a halt (decelerate).
You feel
the thrill, but don't black out, because the coaster's creators
designed it to be well within the G-force tolerance of the average person. The
amount of G forces that are tolerable differs by individual. But for all of us
it depends on three factors: the direction in which the G forces are felt, the amount of G's involved, and
how long those G's last.
Blood
pressures
Depending
on which way your body is oriented when it accelerates, you can feel G forces front-to-back, side-to-side, or head-to-toe. (Or,
in each case, vice versa—for example, toe-to-head.) Each of us can
tolerate the two horizontal axes a lot better than the vertical, or head-toe,
axis. Facing forward in his seat on that final run, Stapp felt front-to-back
G forces as he accelerated and
back-to-front G forces as he
decelerated, and as we've seen, he endured well over 10 times the
G's my daughter and I encountered in the glider.
But
vertical forces are another matter, and it has everything to do with blood
pressure. At sea level, or 1 G, we require 22 millimeters of mercury blood
pressure to pump sufficient blood up the foot or so distance from our hearts to our brains. In
2 G's, we need twice that pressure, in 3 G's, three times, and so
on. Most of us would pass out with head-to-toe G forces of just 4 or 5 because
our hearts can't summon the necessary pressure. Blood pools in our lower
extremities, and our brains fail to get enough oxygen.
Fighter
pilots can handle greater head-to-toe G
forces—up to 8 or 9 G's—and for longer periods by wearing
anti-G suits. These specialized outfits use air bladders to constrict the legs and abdomen
during high G's to keep blood in the upper body. Fighter pilots can
further increase their G-tolerance by
training in centrifuges, which create artificial G's, and by learning
specialized breathing and muscle-tensing techniques.
All of
us, fighter pilots included, can handle only far lower toe-to-head, or negative, G
forces. Facing a mere -2 or -3 G's, we'd lose consciousness as too
much blood rushed to our heads.
Too
much and too long
Magnitude
and duration are as critical as direction. While John Stapp showed that people can
withstand much higher G forces than had
long been thought, there is a limit to what anyone can take. Princess Diana
tragically proved that. Experts estimate that, in the car accident that killed
her, the G forces on her chest were
about 70 G's (and 100 G's on her head). That acceleration was
enough to tear the pulmonary artery in her heart, an injury almost impossible
to survive. If Diana had been wearing a seatbelt, the G forces would have been
in the neighborhood of 35 G's, and she may have lived.
Astronauts in orbit are still subject to about 95 percent of the gravity
we feel on Earth.
Diana's
death notwithstanding, Stapp proved that people can often survive high G forces for very brief periods. We're all familiar
with this to a certain degree. According to a 1994 article in the journal Spine, the average sneeze creates G forces of 2.9, a slap on the back 4.1, and a plop down
into a chair 10.1. If you jump from three feet up and land stiff-legged,
write the authors of the book Physics of the Body,
you'll feel about 100 G's momentarily.
We suffer
no ill-effects from these everyday events because they're so brief. The
trouble starts when G forces linger.
That's why I began feeling worse with each dive the glider made.
It's also why, during launches of the space shuttle, controllers keep the
acceleration low—no greater than what generates about 3 G's—so
as not to unduly stress the astronauts.
Zero G's
Of
course, once the shuttle goes into orbit, astronauts no longer feel G forces.
They're in a zero-G environment, right?
Well, not
exactly. There's no such thing as zero G's. Even the two Pioneer
spacecraft, launched in the 1970s and now the most distant man-made objects,
experience a tug of one 10-millionth of a G from the solar system they've now left. Astronauts in
orbit are still subject to about 95 percent of the gravity we feel on Earth.
It's just that they're in a constant free fall. They're
falling towards Earth, but their speed—up to 25 times the speed of
sound—means that the planet is falling away from them just as fast.
Better to say they're in a microgravity, or weightless, environment.
Weightlessness
may be a gas, but it comes at a cost, because our bodies are used to
a 1-G environment. Each of us here on Earth is actually accelerating towards
the center of the planet at roughly 32 feet per second squared. We don't
feel we're accelerating because the ground holds us in place. But without
that customary pressure, our bodies take a beating. Over time, our cell walls
collapse, our muscles atrophy, our bones decalcify. (The opposite happens in hypergravity:
A 2001 study found that Australian fighter pilots who routinely felt G forces of 2 to 6
experienced, over the course of a year, an 11 percent increase in the bone mineral content
and density of their spinal columns.)
These
health effects of microgravity are of concern to NASA as it contemplates
sending astronauts to Mars, a trip that could take three months one-way.
On the way there, astronauts would need a centrifuge or other means to create
artificial gravity to ensure that any "small step for a man" onto
the Red Planet didn't result in a broken ankle. Visionaries are already
wondering whether people born in potential future colonies on Mars (38 percent
of Earth's surface gravity) or the moon (17 percent) could ever safely
come to Earth.
Give me gravity
Coming safely to
Earth was just what my glider-riding daughter and I began to wish for in the
worst way. (Later she admitted to feeling increasingly queasy, adding, "I
felt like my whole body was collapsing.") Fortunately, after four
figure-eights, the pilot tired of his sport and leveled off, and we returned to
the airport without further ado. One G never felt so
welcome—good old 32 feet per second squared.
Selected
sources
John
R. Cameron, James G. Skofronick, and Roderick M. Grant. Madison. Physics of
the Body. WS: Medical Physics Publishing,
1992.
Allen,
M., et al (1994). "Acceleration perturbations of daily living: A
comparison to ‘whiplash'." Spine, 19(11):1285-1290.
Naumann, F. L., et al. (2001). "The effects of
+Gz force on the bone mineral density of fighter pilots." Aviation,
Space, and Environmental Medicine,
72(3):177-81.
