Sonic Booms

Program Information

Series: Destination Tomorrow
Program: Episode 20
Segment Number: 4 (Watch entire program)
Duration: 00:08:38
Year Produced: 2005
Description:

Fourth segment of episode 20 contains the How It Works segment which describes NASA research on sonic booms. The Sonic Booms segment describe the research underway aimed at making super sonic over land flight possible.

NASA's Destination Tomorrow™ is a series of 30-minute programs that focus on NASA research. Each exciting program gives the audience an inside look at NASA and demonstrates how research and technology relate to our everyday lives.

For more information visit: http://destination.larc.nasa.gov/

Transcript

Today in our busy world,
one of the key prerequisites
for many people
in personal and business life
is speed.

This is especially true
when it comes to aviation.

Although air travel
is almost always
the fastest means of travel,
many would like it to become
even faster.

Though the technology exists
for aircraft to fly at speeds
faster than the speed of sound,
today's aircraft don't because
of the problem with sonic booms.

To help lessen
the impact of these booms,
NASA researchers are attempting
to find a way
to help aircraft move faster
without causing
disruptions on the ground.

Our own Johnny Alonso spoke with
researcher Dr. Kevin Shepherd
at NASA Langley Research Center
to learn what a sonic boom is
and find out how it works.

(Alonso)
In the early days of flight,
having an aircraft
that could fly
even as fast as 30 miles
per hour seemed revolutionary.

But a goal that pushed
virtually
every aircraft designer,
engineer, and pilot at that time
was to find a way to increase
the speeds of their aircraft.

As new designs began to emerge,
aircraft were
continually getting
stronger, safer,
and, above all, faster.

By the mid-1940s,
aircraft technology
had advanced to the point
that breaking the sound barrier
was finally in sight.

After numerous attempts
and failures,
the world's first sonic boom
was heard on October 14, 1947,
when Chuck Yeager flew the X-1
aircraft into history
over the desert
near Edwards, California.

From that point on,
military and civilian
test pilots
were regularly breaking
the sound barrier
in fighter aircraft
and in specialized test vehicles
like the X-15.

But it wasn't until 1976
that civilian passengers
finally got their chance
to fly supersonically
with the introduction
of the famed Concorde.

Concorde had the ability to fly
at over 11 miles high,
1,350 miles per hour,
and travel from Paris
to New York in only 3 1/2 hours.

Unfortunately,
one of the major drawbacks
from the Concorde's
incredible speed
was the amount of noise
it produced.

Not only was it noisy
when taking off and landing,
but once it reached
supersonic speeds,
it created
a very loud sonic boom.

Sonic booms are so disconcerting
to most people on the ground
that commercial aircraft have
only been given the clearance
to break the sound barrier
over water.

So are we just relegated to
flying below the speed of sound?

Well, maybe not.

To help us understand
what causes a sonic boom
and if there's anything
we can do to lessen its impact,
I spoke with Dr. Kevin Shepherd
at NASA Langley Research Center
to find out how it works.

Any vehicle traveling faster
than the speed of sound
creates a sonic boom.

What actually happens is,
shock waves which pressurizes
and develop near the airplane,
and as those travel
to the ground,
what we perceive as a noise
in fact is
this sudden pressure jump,
much like a rifle crack
or a balloon popping.

In fact, what you hear
are two booms
closely separated in time:
boom, boom.

You could visualize it
as two rifle cracks
or as two claps of thunder
closely spaced in time.

What is the speed of sound,
and how do you measure
the speed of sound?

We like to say
Mach 1 is supersonic.

Everyone knows that expression.

Mach 2 is
twice the speed of sound;
Mach 3, three times,
and so forth.

The actual speed depends
on the atmospheric conditions.

So if you're near the surface,
where it's typically quite warm,
speed of sound
is 700, 750 miles an hour.

When you're at altitude,
where airplanes fly,
it's a little lower,
maybe 600 miles an hour.

So for example,
Concorde traveled at Mach 2.

1,200 miles an hour is roughly
the speed it traveled at.

(Alonso)
A common misconception
about the sound barrier is,
once it has been broken,
there is just one quick noise,
and then the noise dissipates.

One reason this misconception
is so prevalent
is that most people
hear a sonic boom
when they're standing
in a stationary position
on the ground.

What actually happens is,
when the aircraft
breaks the sound barrier,
it continues to break it as long
as it's flying supersonically.

Any observer on the ground,
you know,
hears the airplane go by.

If you picture a boat
in the middle of a creek
and the bow wave from the boat,
you watch the boat go by,
and a little while later,
that bow wave passes you
on the riverbank.

People further down
the riverbank
have the exact same
experience.

So what's happening is,
in the case of the airplane,
it's dragging this boom carpet
behind it
all the way across the country.

(Alonso)
Depending on weather
and altitude,
the sonic boom
created by the aircraft
can be heard in a path
of about 60 miles wide
for the entire distance
of the flight.

So if an aircraft is flying
from New York to Los Angeles,
the sonic boom will be heard
consistently across the country
in a 60-mile-wide path.

This is the foremost reason
supersonic flights are not
allowed to fly over land
in the United States.

(Shepherd)
Yeah, most people find
the sonic boom unacceptable.

They're too-loud sounds.

They're startling.

They're annoying.

They tend to shake buildings,
rattle windows,
and so based on experience
with Concorde, for example,
it just doesn't happen.

There is no commercial
overland supersonic flight.

(Alonso)
But revolutionary steps
now being taken by NASA
may change that in the future.

So, Dr. Shepherd,
are we stuck with the fact
that we'll never be able to fly
over land at supersonic speed?

We're hopeful
that's not the case.

The current programs
we're working on
are aimed at allowing
supersonic overland flight.

The hope we have is based
on a recent flight test
which demonstrated that we can
in fact shape the airplane
in such a way that
we can shape the sonic boom,
and it will sound different,
sound more acceptable.

This has been known in theory
for 40-plus years,
but it was only demonstrated
within the last couple of years
with a real flight vehicle.

Now, that's
part of the story.

The real issue is, can we get
the boom low enough
for people to find it
acceptable?

We think we can reduce it.

Can we reduce it enough?

We're hopeful,
and we're hoping
we'll have a flight demonstrator
within the next few years.

So, Dr. Shepherd,
how do you test sonic booms?

I mean,
is it always in flight,
or can you also test it
on land?

We'd love to do it in flight,
but building vehicles, as you
can imagine, is very expensive,
and you don't get to do it
very often.

So if you've got a theory
that this kind of vehicle
will make a different kind
of boom than this,
yeah, we'd like
to build the vehicles,
but that's not going to happen.

So in terms of figuring out what
people might find acceptable,
we simulate the sonic booms
using ground-based simulators,
which are basically
loudspeaker systems,
where we can produce the sounds
that would be developed
by certain vehicle types,
and the simulators
that we have here at Langley,
they're being used for that,
because we hope that will guide
the design of the airplanes
to ultimately lead
to an acceptable sonic boom.

Can you give me some examples
of what you test
in these simulators?

These simulators are basically
loudspeaker-based systems,
so we can make sounds,
and we can design them
to make sounds
that sound very much
like real sonic booms.

We bring in human test subjects,
members of the public,
and in essence,
they give us their opinion:
this sonic boom versus another,
which actually corresponds
to one airplane versus another,
because we're trying
to design airplanes
to give us
the right sonic boom,
and so the characteristics
of the boom
is what they're assessing
with their ears.

If we can solve
the sonic boom problem,
then we can have
supersonic flight over land.

People and goods can get
from place to place quicker,
because our overall aim here
is to make
the air transportation system
more efficient, safer,
in this case faster,
but also
environmentally acceptable.

That way, we save time;
we save money.

We have a more efficient system.

That's it for this edition
of NASA's Destination Tomorrow.

I'm Kera O'Bryon.

For all of us here at NASA,
we'll see you next time.