The Sound Barrier
A whip was the first object to break the sound barrier. The snapping sound heard when cracking
a whip is a sonic boom. The tip of the whip travels at a speed in excess of 740 mph.
Mach – the ratio of the speed of a fast-moving object to the speed of sound; used to represent the
speed of an object compared to the speed of sound – Mach 4 means four times the speed of
sound; when the speed of the moving object is greater than the speed of sound (Mach 1 and
higher), a shock wave is created and a sonic boom is heard.
Mach 
v object
v sound
shock wave – bow wave; cone shaped (conical) envelope created from the splattering of air
molecules at or above the speed of sound; not usually visible to the naked eye.
Jet fighter planes with conical shock waves made visible by condensation
sonic boom – sound heard when traveling a multiple of the speed of sound; not heard until you
are in the envelope of the conical shock wave.
Discussion

The quest to create the fastest vehicle has been documented for over 100 years and pursued
by many. Some lost their lives while others achieved great success. Exceeding the speed
of sound has been the dream of many vehicles, but success has only been achieved in the
air and on land. On October 14, 1947, 24 year old Air Force test pilot Chuck Yeager
became the first person to break the sound barrier. He achieved this monumental feat
while piloting the Bell X-1, a rocket-powered jet. On October 15, 1997, British fighter
pilot Andy Green became the first person to break the sound barrier on land. He achieved
this high-speed feat while driving the Thrust SSC, a modified, jet-powered car. (The
average cruising speed of a commercial airplane is 500 mph.)
Activity

After reading Breaking the Sound Barrier and The Fastest Vehicles on Land, Sea, and Air,
pose the question “Are We Going To Fast?” Discuss various opinions. Have each student
write a few paragraphs explaining their opinions regarding the benefits and dangers
involved with high speed travel. Include the advantages and disadvantages they foresee
with America moving to a high speed travel society.
Summary Review – The Sound Barrier

Shock or bow waves are created when an object exceeds the speed of sound through a
medium. This speed is measured in units of Mach.

Chuck Yeager was the first person to break the sound barrier (1947).

The fastest moving aircraft (2001) travels around 7,000 mph. (Mach 10)

The fastest moving land vehicle (1997) travels over 766 mph. (Mach 1.02)
Breaking the Sound Barrier
by Bryan Ethier
American History Magazine, September/October 1997
Only 44 years after Orville and Wilbur Wright’s first successful heavier-than-air-flight,
Captain Chuck Yeager became the first to fly faster than the speed of sound.
On October 14, 1947, U.S. Air Force Second Lieutenant Bob Hoover awoke early,
charged with a sense that history was in the making. He dressed, then made the short drive to
Muroc Air Base, an isolated Air Force outpost located in California’s Mojave Desert. Dotted
with creeping, arthritic Joshua trees, transient ramble weed, and a vista of dry lakebeds, Muroc
was a 28,000-acre world of stark desolation.
Hoover arrived at the aircraft hangar at about 8:00 A.M. and was greeted by fellow test
pilots Jack Ridley, Dick Frost, and Chuck Yeager. A stiff breeze swept through the hangar,
chilling the pilots and the few tracking engineers on hand. As technicians fueled the orange jet
before them, the quartet of flyers wondered if the powerful new plane would make its pilot,
Yeager, a hero or a memory.
The rocket-powered research jet known as the X-1 was built by Bell Laboratories to take
man beyond the speed of sound. Shaped like a bullet, it was constructed specifically to withstand
the severe aerodynamic stresses a plane encountered as it approached the speed of sound, Mach
1. During the plane’s preliminary flight tests, however, pilots had experienced buffeting so
severe that it became virtually impossible to control the craft. As a result, scientists hypothesized
that at Mach 1 the Bell X-1 would encounter air so stiff that the plane would literally fly into a
brick wall in the sky.
Nonetheless, Hoover knew that if any pilot could conquer the myriad mysteries of the
upcoming flight it was Yeager, a 24-year-old captain and World War II veteran from West
Virginia. Just three years earlier, during a mission over Germany, he had shot down five German
aircraft, earning the distinction of being the first pilot to record an ace (five kills) in one day. But
today’s opponent was no Messer Schmitt, and Yeager was not behind the controls of a P-51
Mustang. This time the enemy was an invisible obstacle called the sound barrier.
Bright with confidence, the crew reported to their respective planes to get this ninth test
of the X-1 underway. Yeager and Ridley boarded the B-29 bomber that would carry the test
plane into the clouds, then release it into flight. Frost and Hoover were to serve as Yeager’s
chase team, following behind in P-80 Shooting Star jet fighters. By the time the B-29
approached an altitude of 10,000 feet, Yeager had climbed from the mother plane and squeezed
into the frigid, cramped X- 1 that was suspended below its fuselage. Alone in the unnerving
silence, Yeager shivered with the cold and considered the hazards of the flight. Stored in a tank
behind him at minus-296 degrees Fahrenheit were six hundred gallons of liquid oxygen (LOX).
The LOX and water alcohol on board, the deadliest fuel ever used in an aircraft, rendered the
jet’s four rocket engines ticking time bombs. Yeager knew that if the pressure were not exact,
the X-1 and the B-29 would explode instantly. He scanned his pressure readings; everything
checked out.
At 20,000 feet, Yeager heard a familiar BANG, as the mother ship released the X-1 from
its bomb bay. He hit his craft’s ignition switch and was slammed back against his seat as, one by
one, the engine chambers ignited. The X-1 became a blur as it zoomed past .70 Mach, then .80
Mach. Hoover, who was downrange in his Shooting Star awaiting the X-l, spotted the streaking
orange bullet as it whizzed toward him. A high-speed camera, mounted on the nose of Hoover’s
plane, recorded for the first time the diamond-shaped sonic waves rippling off the passing
aircraft.
With a whoosh, the X-1 soared toward Mach 1 and 40,000 feet. At .96 Mach, Yeager
noticed that the ride was surprisingly smooth. Then, as the X-1 approached seven hundred miles
per hour, the needle in the mach meter began to vacillate wildly. Suddenly, it jumped right off
the meter.
On the ground some nine miles below, the earth rambled and rolled from the first-ever
sonic boom. The body-shaking thunderclap heralded the news: Chuck Yeager had just become
the first man to fly faster than the speed of sound.
Hours after Yeager had rewritten the aviation record books, President Harry S. Truman
pored over a series of photographs that foretold the future of aviation. Hoover’s photos of the
sonic waves rolling off the X-1 told a fascinating story. Unfortunately, as Truman knew, the
public would not learn of this historic flight, which the Air Force had labeled classified for
reasons of national security, for eight months.
Fifty years have passed since Chuck Yeager helped change the dynamics of flight.
During that time, other test pilots and astronauts have orbited the earth, landed on the moon, and
made space flight a common occurrence. Yeager’s shattering of the mythical sound barrier
unquestionably helped make these once-unthinkable feats seem possible.
“I felt, ‘My gosh, this is big,’” Hoover recently said of Yeager’s achievement. “Chuck
and I both knew this was going to be a big breakthrough in aviation. And it was disappointing to
me that he didn’t get the acclaim then and there. It was a monumental breakthrough. I knew it
was a historic flight, a milestone in aviation ranking right at the top. It wasn’t as if it was a
project without risk; it was fraught with disaster. There had been some [earlier test pilots]
killed.”
The Fastest Vehicles on Land, Sea, and Air
Automobile
Jet-Propelled: Thrust SSC – 767 mph (1997)
Rocket Propelled: Blue Fame – 622 mph (1970)
Internal Combustion Engine: Goldenrod – 409 mph (1965)
Steam: Stanley – 122 mph (1906)
Sea Craft
Jet-Speed Boat: Spirit of Australia – 318 mph (1978)
Airplane
Passenger: Lockheed A-12 Blackbird Oxcart – Mach 3.6
Manned: North American X-15 – Mach 6.7 (1967)
Unmanned: Hyper-X-43A – Mach 10 (2001)
Thrust SSC
Spirit of Australia
Hyper-X-43A
Homework – Sound Basics and Speed of Sound and The Sound Barrier
vs = 331 m/s + 0.6 m/s  T
Mach 
v object
v sound
1. How far away is a storm if you note a 3 second delay between a lightning flash and the sound
of thunder? (assume vs = 331 m/s)
2. What is the speed of sound traveling through a piece of steel at 0 C?
3. What is the velocity of sound in air at 37.5 C? At –11.5 C?
4. What is the temperature of air through which sound passes with a velocity of 342.8 m/s?
5. The jet-propelled Thrust SSC can achieve a maximum land speed of 767 mph (342.88 m/s).
Determine the vehicle’s speed in terms of Mach in sunny Los Angeles (35 C), frozen Green
Bay (-15 C), and overcast Pittsburgh (15 C).
6. The North American X-15 is the fastest manned aircraft reaching a top speed of Mach 6.7.
What is the speed of the X-15 assuming the air temperature is 0 C?
Jet fighter planes with conical shock waves made visible by condensation
Sound Answers
Sound Basics and Speed of Sound and The Sound Barrier
993 meters
4.97  103 m/s
353.5 m/s @ 37.5 C; 324.1 m/s @ -11.5 C
Mach 0.97; Mach 1.06; Mach 1.01
2.22  103 m/s
19.7 C