The Sound Barrier

In this Lesson

This lesson will focus on the importance of the sound barrier to aviation.  After reviewing this lesson, you will be able to describe and explain the following:

• What the sound barrier is and what supersonic speed entails for an aircraft or spacecraft

• Chuck Yeager and the challenge of breaking the speed of sound

The Sound Barrier

The sound barrier is essentially just a series of waves of compressed air that build up on the leading edge of aircraft surfaces as it approaches the speed of sound.  The speed of sound  is roughly 767 miles per hour at sea level, although it varies based on temperature and humidity and altitude.  This was a challenge for aircraft designers as the vibrations that affect the plane as the pressure waves build up on the wings and fuselage can be quite severe when the design is not optimized for that speed.  Some early pilots approaching this speed (in propeller aircraft while in a dive maneuver during WWII) described the experience as the plane shaking itself apart.  Thus in the early years of aviation, this was seen as an almost insurmountable barrier.

In the image below, you can see an explanation of how the air builds up in waves on the front of the aircraft.  The shock cone pictured at supersonic speeds is what you hear when you hear a sonic boom.  You can also read about it on this site from NASA.

When an aircraft surpasses the speed of sound, it is in supersonic flight.  There is usually a loud sonic boom associated with passing the speed of sound for observers on the ground, although in modern aircraft there is no sound or shaking associated with the event in the aircraft itself.  You can see examples of the sonic boom in this video.  Light travels much faster than sound, so you can see the plane coming towards the observer although the sound itself does not hit our ears until the plane is almost past us.    

Supersonic flight is generally referred to in Mach numbers to reference the speed inside the aircraft.  An aircraft traveling at the speed of sound is going Mach 1, and as it continues to increase in speed it is traveling at Mach 1.1, Mach 1.2, etc.  Similarly the speed can be referenced as 0.5 Mach for example for speeds under supersonic flight.  The SR-71 Blackbird could travel three times the speed of sound, or Mach 3, as another example.  It's simply a different scale to reference speed, just as Fahrenheit and Celsius both measure temperature but have different scales.

Breaking the Sound Barrier

On October 14, 1947, Chuck Yeager broke the sound barrier in the Bell X-1 rocket propelled aircraft you see pictured below (video static image).  The aircraft was specially designed for an attempt at the sound barrier; please watch the 25 min. documentary below to learn more about this process and event.

Summary

Chuck Yeager's flight to break the sound barrier was an important step forward for aviation.  It proved that supersonic flight was possible, and the research and development that went into designing an aerodynamic aircraft to accomplish the goal was critical to both the space industry and for modern jet fighter designers.  Many fighter aircraft have the ability to travel over Mach 1 today, and regularly use that ability to travel respond quickly to events in war and peace time as needed.