I think quite a bit about space travel and the speeds required to explore other planets and even solar systems beyond our own (we have a long way to go). I’ve written before that robots will likely need to be used in the near future for this however I’ve often wondered what effects ultra-high speeds have on humans and spacecraft. I thought a good place to start would be the sound barrier since we have a pretty good idea of what this entails. Here’s what I’ve discovered so far.
What is the Sound Barrier?
As airplanes developed through history for faster and safer flight, there appeared to be a barrier that would endanger flight at transonic airspeeds (airspeeds just below, at and a little above the speed of sound). The planes would buffet severely, local shockwaves would form and the aircraft would drastically lose its stability.
Breaking the sound barrier would then interpret into aircraft breaking this barrier of violent buffet and speeding into supersonic flight. Chuck Yeager, in the Bell X-1, was the first one to break the sound barrier (officially credited) on 14th October, 1947.
Speeding Through the Sound Barrier
A plane accelerating through the air, flying straight and level, with the intention of breaking the sound barrier would have the following affects:
- As the airplane moves through the air, it would disturb the molecules of air immediate to it.
- These molecules of air would then transmit the disturbance to other molecules.
- Transmission of vibrational energy through the molecules of air will form a wave of disturbance (pressure wave).
- This pressure wave is much similar to that of ripples formed when a stone is thrown into a pond of stagnant water.
- Since the aircraft is flying at subsonic speeds, the pressure waves it generates in the form of ripples, remain equally spaced and travel in all directions.
- As the aircraft accelerates, this distance between the pressure waves ahead of it, starts to decrease.
- When the aircraft attains the speed of sound, the distance between the pressure waves ahead is so small that they form a single wave referred to as a Mach wave.
- No further waves are now formed ahead of the Mach wave, since the speed at which these pressure waves emanate from the aircraft (speed of sound) is equal to the true airspeed of the aircraft itself.
The Sonic Boom – Supersonic Aircraft
An aircraft flying at subsonic speeds generates pressure waves in all directions. The waves generated ahead of the aircraft travel to the surface of the earth and the observer listens to the sound of the aircraft flying through the air, which apparently gets louder and louder as the aircraft approaches.
However, an aircraft flying at supersonic speeds would generate pressure waves in a pattern much similar to the wake produced by a boat. The water ahead of the boat remains unaffected right up to the moment when the boat directly influences it. These pressure waves trail down, behind the aircraft, in the form of cone-shaped wake (also referred to as the Mach cone).
The sonic boom occurs when the downward-proceeding parabolic edge of the Mach cone strikes the surface of the earth, where the observer hears a loud instantaneous boom, after the aircraft has passed over.
Speed Ranges in Terms of Mach Numbers
Mach number is the ratio of an aircraft’s true airspeed (TAS) to the speed of sound. Hence, an aircraft traveling at a true airspeed equal to the speed of sound, in that specific air mass, would be flying at Mach 1. These speed ranges, as listed in Pilot’s Handbook of Aeronautical Knowledge, are as under:
- Subsonic – Mach numbers below 0.75
- Transonic – Mach numbers from 0.75 to 1.20
- Supersonic – Mach numbers from 1.20 to 5.00
- Hypersonic – Mach numbers above 5.00
In today’s world, the idea of a sound barrier is disappearing as aircraft more suited for flight at transonic and supersonic speeds have been developed. Airplanes capable of cruising at supersonic speeds, now commonly break the sound barrier and not just military aircraft, passenger jets such as the Concorde and the Tupolev Tu – 144, have already flown commercially at supersonic speeds.
Federal Aviation Administration. Pilots Handbook of Aeronautical Knowledge. (2008). Accessed May 22, 2012.
Federal Aviation Administration. Airplane Flying Handbook. (2004). Accessed May 22, 2012.
Encyclopedia Britannica. Sonic Boom. (2012). Accessed May 22, 2012.
The New York Times. Why is there a sound when a plane breaks the sound barrier? (2007). Accessed May 22, 2012.
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