Breaking the Sound Barrier

The expression “sound barrier” refers to the speed of sound in a given medium.  In the field of aerospace, which studies motion through air and space, the medium in question is air.  Space is a vacuum, which is no medium at all.  Sound can only travel if there is a medium to carry its information from one neighbouring molecule to the next.  As such, sound does not travel outside our atmosphere, contrary to what silly movies like “Armageddon” would lead one to believe.

What is the speed of sound in air?  It is dependent on two factors associated with the air itself: its density and its bulk elasticity.  For standard atmospheric conditions, the speed of sound in air is about 340 m/s.

When an object travels through the air below this speed, its motion is said to be subsonic.  The term Mach number (M) refers to the ratio of a vehicle’s speed to the speed of sound of the medium it travels through.  A typical commercial airplane may travel around 170 m/s, or M = 0.5.  Jets travelling beyond 340 m/s are supersonic.  When an aeronautical engineer hears about Mach 4, he or she thinks of supersonic air travel, and not a Gillette razor. 

When a meteorite hits our atmosphere, it is travelling at supersonic speed.  So, supersonic flight predates man.  However, a fun piece of trivia is to determine man’s first creation that broke the sound barrier?  You do not need to look to the skies for the answer.  Just think, “Indiana Jones”.

Indeed, the bullwhip represents man’s first foray into supersonic speeds.  Due to the chain reaction involved in the mechanics of snapping a whip, the very tip of it will travel at speeds in excess of 340 m/s.  When an object accelerates from subsonic to supersonic speed, the achievement is accompanied by what is known as a sonic boom.  Invariably, the loud snapping noise of the whip is due to this phenomenon.  As it turns out, the speed of sound is no barrier at all; it is, however, a very critical speed.

The moment that an object transitions from subsonic to supersonic speed, the governing aerodynamic principles that affect it alter drastically.  The result of supersonic travel is that the object arrives without warning.  We are accustomed to receiving the sound of an oncoming vehicle before it arrives.  A car’s noisy engine serves as a useful warning for a pedestrian walking on a narrow the street.  As it turns out, the oncoming air that a supersonic jet whooshes through appreciates the advanced warning as well.

For subsonic flight, the air molecules that the airplane displaces have time to prepare for the displacement.  They organize themselves because the sound of the oncoming plane conveyed the following information: “Get out of my way!”

In supersonic flight, the plane is travelling faster than the sound it emits, so air has no time to prepare for the disruption.  The airfoil of the air molecules no longer forms an organized parade around the wing.  Instead, the parade seems to have been organized by drunken molecules (imagine if the St. Patrick’s Day parade took place in the evening instead of the morning...).  The effect of supersonic flow is that a shock wave is emitted through the air; shock is an appropriate term, because the air never saw it coming.

The cone-like shape that forms around an in-flight jet is due to the Mach angle of the jet.  The cone border divides the section of air that received sound wave warning from the jet from the section of air that had no idea it was coming.  So, jets travelling at Mach 4 create much steeper cones than do those travelling at Mach 2.  This angle determines the direction in which the shock wave will travel, and is the reason why a jet passes over your head quietly, but sends a sonic boom once it is a fair distance away (as if to say, “I was here.”).

As the medium of air behaves completely different for M < 1 than for M > 1, the wing design for subsonic planes and supersonic jets is quite different.  The rounded edges of a commercial airliner’s cross-section of wing complement the rounded path of the organized air molecules.  A supersonic jet’s wings will have a sharp diamond-like cross-section to optimize the pressure gradients that manifest around them.

When it comes to aerospace, flying through space is completely different than flying through air.  Whether moving slower or faster than sound, the air allows the plane to generate lift, but also deters its motion with drag.  A rocket may require vast amounts of energy to reach an Earth orbit, but once there, they require no fuel to maintain that high velocity.

To see some beautiful shock waves from jets, and even a rocket ship leaving the atmosphere, click on this link (and make sure to watch it to the end, as the rocket creates a truly astonishing sight).  Hopefully with this very introductory information in supersonic flight, the shock waves that you see will come to you as less of a shock.