Sonic booms

(also known as sonic bangs...)

Some of the long range artillery used in WWII (both land and sea) had supersonic muzzle velocities. Some anti-aircraft and anti-tank projectiles were also supersonic.

Aside from the V-2, I have never seen reference to hearing the sonic boom caused by the passage of a supersonic projectile. (OK, I do remember mention of high velocity sniper fire...)

It occurs to me that observers below a projectile's trajectory should have been able to hear a distinctive 'crack' or 'bang,' separate from the firing of the piece, or the impact on the target. I realize that battles tend to be noisy enough that picking out SBs among the din might be 'difficult,' but let's say you were under the trajectory of 'Anzio Annie' (28cm railway cannon) on an otherwise quiet evening...

Also, might airmen hear AAA projectiles 'passing by?'

Can anyone cite references? Better yet, do any of you 'who were there' remember hearing any such, and if so, did you realize at the time what was the cause?

Actually *all* WWII era artillery (which is to say not mortars, but including all guns and howitzers) fired rounds at supersonic speeds. Supersonic projectiles were a novelty in the Civil War era.

Carey Sublette

A couple of points. First, for high angle fire I've always assumed that even a shell that has a supersonic muzzle velocity falls below supersonic pretty quickly and covers most of its trajectory at subsonic velocities. But I admit that I have never looked deeply into that issue.

Secondly, I've seen many references to the sharp crack made by tank and anti-tank rounds and I assume those are what you are referencing.

Again, by the time such a projectile reaches the airplane's operating altitude, it would most likely have shed enough velocity to be subsonic. And airplanes, especially in the piston era, are pretty noisy places to work anyway. Even a supersonic shell would have to pass pretty closely by to be heard inside the cockpit and through the helmet and earphones, I would imagine.

A bullet going past your head makes a SNAP! (rather nicely simulated in the video Band of Brothers). That is a sonic boomlet.

Indeed, I've read that it was precisely because we knew that a bullet could go supersonic without harm to itself (at least until it ran into something that we designed the first supersonic aircraft in the shape of a bullet.

all the best

Indeed. The Bell X-1 was shaped like the standard WW2 50-cal round, which was not only supersonic but had an excellent reputation for stability in flight. A good example of how advances in military technology (and technology in general) feeds off previous experience.

My memory, Dan, noted long ago, was a phut sound - maybe the bullets used were slightly different shape. Here is the memory... At one stage during the war I was associated with an army unit on an operation behind the lines. During the training period in India I had a a discussion one day with a Gurkha officer on this topic and we decided to try a test that would, we thought, resolve it. We already had some shallow trenches dug - I think, for exercises in which the enemy were using mortar - and so we each in turn took on the role of target. When I settled down on my right side into the trench, additionally protected by my own rifle butt on the rim the edge above, my left ear was about six inches below likely bullet level He fired three shots low across the trench from some sixty yards away, and then I did the same with him in the listening role. Results were not conclusive but each of us, on at least one shot, heard a double sound which we reported at the time as a phut-bang, the first a sound like a cork coming out of a bottle, the second the normal loud rifle-shot you would expect to recognise from that distance. We decided we had the answer, that you could indeed hear the passage of a supersonic bullet - and thareby released ourselves from the need to undertake any more such tests.

A nice story, thank you, and I yield to your experience:

PHUT! it shall be henceforth.

all the best

60 yds is only about 160ms for the muzzle report. That's a lot of overlap between the sound of the passing bullet and the report, even assuming infinite bullet speed.

As are nearly all rifle bullets.

The boom is going to be relative to the mass of the projectile... and how streamlined it is... So, a rifle shot going past your head may very well give you a loud 'crack'... but it won't be heard much past that....

Realize also that an artillery round is firing on a ballistic arc, so at the top of its path, it is traveling the slowest it is going to go...

I have been under 155mm artillery rounds as they passed overhead, if weather conditions are right, you can see their vapor trail... However, most are going pretty damned slow...

This is peacetime stuff, so they may have been using non-war shots, so they may have had lower muzzle velocities...
http://www.csun.edu/~btk29323/artiller.htm

The above is for modern stuff, and it shows most artillery rounds traveling at about 500 m/s... about mach 1.5....

It was... but it was believed that due to the density of a bullet (solid), it didn't fail... an aircraft would not be solid...

This is precisely the point most people don't understand. Shape plays a large part in the size of the boom. NASA has been testing a retrofitted F-5 recently that significantly reduced the size of the normal signature for the airframe http://www.dfrc.nasa.gov/Gallery/Photo/SSBD/index.html

Supersonic speeds does not equal sonic boom. There are other factors that come into play. The point about the projectile slowing down to subsonic speeds is also very valid.