Day Two – The Sound Barrier and How To Break It

More History:

The speed of sound was first broken in level flight by USAF Brigadier General Chuck Yeager on October 14, 1947 in a plane he named Glamorous Glennis. Two days before his historic flight, Yeager broke two of his ribs in an equestrian accident. He was unable to seal the plane’s hatch himself and was worried he would be pulled off of the mission. Yeager had a friend who jury rigged a broken broom handle as a lever so Yeager could close the hatch. Immediately after Yeager landed that flight, he rushed to the hospital.

The Sound Barrier:

Let’s talk more about why advancing through the speed of sound is so difficult. What makes accelerating from Mach .9 to Mach 1.1 require more force than accelerating from Mach .1 to Mach .3? The answer is the sound barrier, a wall of pressure that must be broken through to achieve super sonic speeds. Let’s look at an object travelling slower than the speed of sound again.

In this image the object is travelling to the right. This causes the sound waves on the left to be more spaced out than the waves on the right. If you remember from yesterday’s post, sound waves are formed by the compression of air particles. Because there is a higher concentration of sound waves in front of the object, there is more compressed air and thus higher air pressure. Next we will look back on an object right before it breaks through the sound barrier.

 

Now the sound waves are very tightly bunched in a plane perpendicular to the direction of motion. This “wall” that is the sound barrier increases in pressure as an object approaches the speed of sound, because all of the sound waves that emanated from the object occupy the same space. The object must break through this wall of pressure to achieve supersonic speeds. The instant it does is when you can hear the sonic boom.

 

In the case of supersonic travel, there is no longer pressure in front of the object, allowing it to accelerate without as much force as before breaking the sound barrier. A Mach cone is formed instead from the high pressure, as you can see in the two dimensional representation to the left. I will talk more about Mach cones later.

 

The Sound Barrier Fades:

Chuck Yeager had to deal with breaking through the sound barrier in his first flight, but modern supersonic aircraft are able to achieve the speed of sound without passing through a noticeable sound barrier. These adaptions include swept wings, sharp noses, and more, generally for the purpose of making the aircraft more aerodynamic to reduce drag. The biggest difficulty is that the aerodynamics of super sonic flight are much different than subsonic flight, so supersonic aircraft must be uniquely modified compared to general aircraft. The sharp nose and swept wings help to decrease the surface area in the front of the plane as much as possible, and provide places for the air to be deflected, so the plane can more easily break through the “wall” of pressure.

As the sound barrier fades, so does the sonic boom. NASA is currently testing supersonic aircraft that does not make a sonic boom. The designs show even more exaggerated designs using sharp noses and swept wings. Here are a couple concepts below:

 

 

 

 

Today I learned a lot more than before about the effects of concentration of waves from objects in motion. I would say that I have advanced my knowledge in the history of supersonic travel and I have learned much more about the behavior of the sound barrier. That was not one of my learning objectives at first, but it is an important part of how sonic booms work.

Sources:

http://www.uasvision.com/2012/04/04/nasa-tests-supersonic-aircraft-without-boom/

http://www.acs.psu.edu/drussell/Demos/doppler/doppler.html

http://hyperphysics.phy-astr.gsu.edu/hbase/sound/soubar.html

Java Applet for the Sound Barrier:

http://beltoforion.de/java_applets/machnumber/machnumber_en.html