Stuff First Sergeant told me

On Saturday, I met a gentleman who had an extensive military career. We got on the subject of explosives, and he was telling us a story about how he’d been in a course that had done demonstration terrorist sizes explosions (car, suitcase, and so on) so the students of the course could see/feel what those were like.

“You may survive the shockwave coming from the explosion,” he explained, “but if you’re standing with a wall behind you, the reflection of the explosion may kill you because it accelerates the pressure.”

Aroo? What? “Why?” I asked, like you do.

“I didn’t ask. There was a lot of guys who had no interest in the physics of it and me delaying the class to ask that would have been a bad idea.”

Good point, that.

But I have the internet, and found this in the Federal Emergency Management database.  Chapter 4, Explosive Blast.

When the incident pressure wave impinges on a structure that is not parallel to the direction of the wave’s travel, it is reflected and reinforced, producing what is known as reflected pressure. The reflected pressure is always greater than the incident pressure at the same distance from the explosion… This figure shows that reflected pressures for explosive detonations can be almost 13 times greater than peak incident pressures and, for all explosions, the reflected pressure coefficients are significantly greater closer to the explosion.

So, standing in front of a wall, the blast wave is going to be reflected back at 13 times whatever it is at the time.

Fascinating stuff. I’ll have to take a look at those stats on ballistics. I might learn something useful.

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