The slow-motion video shows a ping pong ball filled with 15 grams of high speed smokeless gun powder buried in 5 inches of sand. The gunpowder is triggered with a model rocket engine igniter.
within the first 3-4 seconds, the video shows a small explosion that raises the sand, then the sand burps out some gas, and then there’s a much larger explosion that scatters the sand.
If the ping pong ball bomb is detonated in open air or at a depth of less than three inches there’s a single pop (as here), but if it’s more than 3 inches under the sand, this double-burst thing happens. I am told this sequence of events (double burst for the deeply buried bomb) is repeatable.
At a guess, there’s an initial reaction using self-oxidizing chemical content, then a bigger reaction when the sand is pushed out and air (oxy) can get in.
I think the first burst is a shock wave - the second is a rising bubble of gas in the sand (the sand could be acting like a fluid under the shock/vibration of the explosion)
I think it might be acting as a thermobariac bomb, the self oxidizer burns a little bit of the fuel and launches the rest of the fuel into the air and the fine fuel powder auto-ignites from the heat and pressure, making a much larger explosion because it can burn much quicker because of its bigger surface area.
I’d go with this explanation, first the shock of the explosion and then the outgasing, like underwater explosions, first the faster shockwave rises the water above the bang and then the bubble and debris breaches the surface, like so.
The shockwave is from the high explosive, nitro-cellulose. The shockwave is essential to the chain reaction, the nitrocellulose hit by this shockwave explodes… … the N detaches from the cellulose …
But the shockwave does not provide much momentum/KE to the sand, and it does not provide much momentum to the projectile of a gun.
The hot gases expand, and continue burning the cellulose and nitrogen, over a much larger time period, it is the expansion of the hot gases that is providing momentum to the sand, or in a gun, the projectile.
The sand in the frame is pretty loose, and is relatively fluid. What your seeing is the shockwave burp the surface, and then the collapse of the camouflet.
I had a Mk 82 drop from altitude into rocky soil, that I had to BIP*, some of the local engineers were all freaking out for seismic action, or camouflets opening up, yadda yadda (they didn’t understand much). I told them I was more concerned with secondary frag–fragmentation coming from the soil. But it was graded, compacted rock and soil built built up to a level grade in an ammo dump, and the UXO was seventeen feet down. It got a blast, but it was well tamped sideways, and we’d vented it pretty well due to the fact we had to blast down with cratering charges just to get to the thing.
