Guitars and rockets have a lot in common,
but what's good for a musician might spell trouble for an astronaut.
by Dr Tony Phillips
When the space shuttle
lifts off its pad at Kennedy Space Center, the roar is unbelievable.
Even miles away onlookers grab their seats and hold on tight. Sound
waves penetrate flesh and shake bones.
Recently, country music
star Clint Black recorded a public service message for NASA (listen).
According to Black, those launches remind him of something: himself.
"Did you know
my guitar is like a rocket?" he asks.
No. It's scientific fact. Black's guitar is like a rocket.
"They both resonate,"
explains aerospace engineer Rodney Rocha of NASA's Johnson Space
Center (JSC) in Houston.
"When you pick
up an acoustic guitar," Black demonstrates, "one of the
first things you'll notice is the body is basically an air chamber.
The shape of the chamber is designed to be 'in tune' with the sound
from the strings." He plucks the E string and the body of the
guitar vibrates, producing "sympathetic" E-frequencies
of its own.
Black visits with NASA.
"We call this
'resonance' and it's a big part of what makes a great guitar,"
Resonance may be great
for guitars, but "it can be disastrous for a spacecraft,"
"When the shuttle
lifts off, the main engines roar so loudly that a person standing
near the pad would be killed—not by the heat of the exhaust,
but by the sound of the engines," he says. The engines "strum"
the spacecraft with incredible force. Rumbling sound waves penetrate
the shuttle and its cargo, seeking, probing, shaking.
"We cannot let
these sounds [find] and over-excite a sympathetic resonance,"
says Rocha. If they do ... the sound is amplified, vibrations increase.
Bolts can become unscrewed, covers ripped off, joints loosened.
"It could really
shake up your mission," laughs Black.
The engines aren't
the only source of sound. After liftoff, the rocket rips its way
through the atmosphere en route to space. Rushing air creates strong
aerodynamic noise, which rattles the ship. "You can hear this
kind of noise by rolling down your car window while driving,"
and shuttles both have chambers
Even in space, the
noises don't stop. Vibrations can ripple through a spaceship when
it docks with another ship, or when it fires its manoeuvring thrusters.
With each bump or thrust, the rocket is strummed anew.
The goal of engineers,
says Rocha, is to make sure these vibrations die out quickly, before
they do any harm. In the language of musicians, "rocket designers
must avoid sustain."
When Black strums his
guitar, the sound lasts a long time. "That's the sustain,"
he explains. Long-lasting vibrations are encouraged by the fabric
of the guitar itself. "Notice how the guitar is made of lightweight,
flexible wood—a material that likes to vibrate," points
Rockets are made of
stiffer, heavier materials, that damp resonance's and reduce sustain.
But that's not the only trick spacecraft designers use. Sometimes
they modify the shape of the rocket, adding supports or filling
in empty spots. The purpose: to detune the rocket from itself.
Detuning rockets isn't
easy because, as instruments, they're much more complicated than
Consider this: A guitar
is constructed from dozens of parts: tuning knobs, clamps, the sides
and faces of the air chamber and, typically, six strings. The strings
produce six fundamental frequencies: 82 Hz, 110 Hz, 147 Hz, 196
Hz, 247 Hz, 330 Hz corresponding to the open notes of E2, A2, D3,
G3, B3, and E4.
A typical rocket, on
the other hand, is made of thousands of parts. The space shuttle
famously contains more than a million components. All these pieces
vibrating together produce a cacophony of frequencies ranging from
subsonic waves that only an elephant could hear to high-pitched
whines akin to fingers scratching a blackboard.
interferograms reveal vibrations in the body of a guitar.
Which frequencies might
do the most damage? What parts of the spacecraft are most vulnerable
to resonance? And how do you de-tune this complicated instrument?
To answer these questions,
NASA engineers have developed "sound studios" for spacecraft.
"These are huge chambers where we take pieces of our rockets
and expose them to loud noises." Really loud. "One of
our 165 decibel acoustic horns at JSC can make as much noise as
a space shuttle main engine," he says.
By observing the response
of "test articles" to the sounds, engineers can discover
resonance's and make changes to squelch them. "The most vulnerable
articles tend to have low mass and lots of surface area—like
a guitar," he notes.
Acoustic testing has
been a regular part of rocket design since the Apollo program four
decades ago. "In those days," says Rocha, "NASA engineers
blasted sections of Saturn rockets with loud sounds in special laboratories.
And when the space shuttle came along we tested its components in
the same way."
Now NASA is preparing
to build a new spaceship, the Crew Exploration Vehicle (CEV) to
carry astronauts back to the Moon and on to Mars. "I'm sure
the CEV will get its turn in a sound chamber, too," says Rocha.
Who'd have guessed
it? "Controlling your sound is just as important to rocket
scientists as it is to musicians," marvels Black, strumming
his guitar. If you listen carefully, you can hear the moonshot in