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Author Topic: What would happen if an object moved at exactly the speed of sound?  (Read 2376 times)

Offline thedoc

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Louis Clement-Harris  asked the Naked Scientists:
   
What would happen if an object moved at exactly the speed of sound?

Objects accelerating past the sound barrier produce a sonic boom, but if an object stayed at that speed would the sound waves just build up indefinitely?

Thank you!

What do you think?
« Last Edit: 09/04/2013 15:30:01 by _system »


 

Offline yor_on

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I'm not sure but I think the aircrafts wings etc would be severely stressed as the sound waves constantly trying to build up to that single sonic shock wave it would present after passing the sound barrier. In the end the metal should be so stressed that it would get fractures, microscopic in the beginning but fast growing. You won't get a sonic boom as you never pass the sound barrier, but the sound/air waves should still interact with each other.
 

Offline yor_on

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Offline evan_au

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You would burn a lot of power trying to travel at exactly the speed of sound.

Drag is at a maximum around the speed of sound - there is much less drag just below the speed of sound, and somewhat less drag above the speed of sound. So supersonic planes try to spend a minimum amount of time near the speed of sound - for example, by making a bit of a dive as they accelerate through the sound barrier.

Not all parts of the plane reach the speed of sound at the same vehicle speed, because air has to accelerate more to go over some parts of the plane. But it's tough on the plane! Subsonic planes (or propellers/rotor blades) which accidentally exceed the speed of sound tend to shake themselves to pieces.

See graph at: http://en.wikipedia.org/wiki/Air_resistance#Wave_drag_in_transonic_and_supersonic_flow
 

Offline yor_on

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Nice link Evan.
 

Offline evan_au

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Another thing that would happen if you tried to travel at exactly the speed of sound is that your engines would stop working.

Conventional air-breathing engines (jets and piston engines) rely on thoroughly mixing outside air with a liquid fuel, igniting it, and then extracting energy from the expanding gases.
  • Mixing occurs slower than the speed of sound, so the fuel would leave the engine before it could mix properly
  • If the fuel is well-mixed, burning can travel faster than sound, but otherwise burning is slower than sound. Again, the fuel-air mixture would burn incompletely, and only once it had left the engine
  • This energy is used to drive a propeller or turbine providing most of the thrust. If the plane is traveling at the speed of sound, the tips of the propeller or fan blades would be traveling faster than sound, and suffer considerable drag and vibration.

A supersonic airplane has an air intake which is designed to create a supersonic shock wave inside the air intake, slowing the air down below the speed of sound, so the engine can still operate.

The SR71, which holds records far the fastest aeroplane, has an elaborate conic engine intake, which moves in and out so that the shock wave just covers the engine intake, and no more.

This is why rockets are commonly used for supersonic flight- they suffer a weight penalty because they must carry their own oxygen with them, but they don't have to slow down the air before they can use the oxygen in the air.
« Last Edit: 12/04/2013 23:22:08 by evan_au »
 

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