So here's a question for ya...

[SkinNFluff]

We all know that air behaves like a fluid. We also know that when an object passes through air (or water) at a high velocity, the air or water is forced around the object - Well, we know that a sonic crack or boom is caused when an object travels faster than the speed of sound - Ok, so that crack is caused by the object moving so quickly through the air, and throwing the air away from itself so quickly that the object is long gone before the air even has a chance to move back to fill the void left behind (thus the crack) - So here's the question - What's in that space behind the bullet as it passes? Can it be measured?

 

[Woobie]

My barstool uneducated guess is it is cavitation, which might also partially explain the noise. In other words, almost a vacuum, but really a low pressure area.

 

[Bennettjh]

Good question. Someone smarter than me should have the answer.

 

[bulletsmith]

What's there is the same thing that is in our atmosphere everywhere else, just less of it. It should be measurable, at least in theory. There is a similar phenomena that Bernoulli discovered that is used to measure flow of a liquid. It's the whole idea of how pressure drops as velocity increases. So as the air is parted around the bullet, the area of lowest pressure is right behind it. In flow this region is referred to as Vena Contracta. It's basically how planes fly.

 

[Dirtebiker]

I just wonder why you would even think about this?

 

[bluewraith]

Hmm.. if only we had a resident nerd..

There's a bit of a vacuum behind the bullet, which incidently creates a bit of extra drag. Not enough to really be concerned about with small arms, but artillery can gain a some range by using a base bleed. A bit of gas is generated and occupies the low pressure area behind the shell, reducing drag.

 

[bulletsmith]

Hmm.. if only we had a resident nerd..

There's a bit of a vacuum behind the bullet, which incidently creates a bit of extra drag. Not enough to really be concerned about with small arms, but artillery can gain a some range by using a base bleed. A bit of gas is generated and occupies the low pressure area behind the shell, reducing drag.

That's an interesting approach. No boat tails in big artillery eh?

 

[bluewraith]

That's an interesting approach. No boat tails in big artillery eh?

No idea. Nobody let's me play with the big toys. Just stuff I gleaned from a google session.

 

[Stang51d]

There are boat tails in the big ones. The 2,700 lb 16" rounds are boat tailed. The 2,200 lb ones are not.

 

[foszoe]

Just plug some values into Navier-Stokes equations for compressible flow and you will find the answer.

 

[bulletsmith]

There are boat tails in the big ones. The 2,700 lb 16" rounds are boat tailed. The 2,200 lb ones are not.

Wait, 2700 lb projectile? I had no idea! Just wow!

 

[BugI02]

The 'crack' is not caused by air rushing back into the low pressure zone (ala thunder) its a mini-sonic boom. The shock wave generated by the bullets supersonic flight is a strong pressure gradient across a couple of molecules of width. And what you hear as sound is any pressure disturbance sufficiently strong enough to be above the threshold of hearing. This particular type of disturbance is well above that threshold.

 

[BugI02]

Wait, 2700 lb projectile? I had no idea! Just wow!

Yep. The recoil heels the boat if the guns are trained out. Can't ripple fire without allowing time for the effect to damp out.

 

[bulletsmith]

Yep. The recoil heels the boat if the guns are trained out. Can't ripple fire without allowing time for the effect to damp out.

That's just effin impressive.
There are a number of good explanations that show up in a google search that show the stacking of pressure waves and an explanation of the double boom that is often heard. Physics is fun!