Black Powder Firecracker - Double Explosion?

[bananamber]

So I had some used tubes from cheap-o fountain fireworks. I got the idea of seeing if I could make a firecracker out of the used tubes. So I took the biggest tube I had, about 8" long and 1.25" diameter. The top had a small hole slightly larger than cannon fuse. So I filled her up to the top with triple fine black powder, stuck a fuse in and did NOT seal the end.

From not sealing the end, I expected it would really just be a big smoke bomb. I couldn't have been more wrong. I lit it and get behind protection, and it detonates... Twice. A fraction of a second after it exploded there's a second explosion.

I can't figure out why there were two bangs. As far as I know, black powder is a low-order explosive, it just burns. Obviously the small hole allowed enough pressure to detonate, but I can't figure what the deal was with the second bang. I'd imagine after the first explosion, there would be nothing left of the tube and hence no pressure buildup to cause a second explosion if the powder only partially burned.

 

Thanks!

[mikeee]

Are you sure you did not hear an echo of the first detonation from sound waves bouncing off of buildings around you?

[bananamber]

Are you sure you did not hear an echo of the first detonation from sound waves bouncing off of buildings around you?

 

I'm pretty sure it wasn't an echo. At first I did think it was, but later I found one of my old tubes I made myself out of drywall paper and epoxy which was quite a bit smaller. I filled that up and set it off and it was significantly louder but I didn't even hear an echo on that one.

 

Perhaps the the second one being significantly louder dampened the echo or messed up my ears enough to not hear the echo, but I'm almost certain the the second explosion I heard wasn't an echo.

[OldMarine]

Sorry, this is my stock answer to such questions: