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.
