Pyrotechnics is the science of controlled combustion and explosions. It’s also a craft and an art, as anyone who’s enjoyed a fireworks display can attest. And although fireworks have been developed over centuries, on occasion mistakes will happen. The video shows what’s known as a bottom shot (loud percussive flash charge) that ignites too late and blows the metal roof off a shed.
The video is courtesy of Eldon Hershberger, member of the Pyrotechnics Guild International (PGI). Eldon filmed the entire evening’s display, for later sale to PGI members. He graciously allowed me to use this short segment for educational purposes.
Bottom shots have internal fuses that are ignited by the initial explosion. The length of that fuse determines how long it takes to explode. Fuses are rated for their cm/s burn speed, and quality control is excellent. Of course you also need to know how high up in the air the initial explosion is, the rate at which they will fall, and how high off the ground the bottom shot should detonate. Ideally, you’d like a dramatic pause after the initial explosion with the bottom shot detonating maybe 20 feet above the ground to prevent igniting any foliage. PGI display venues are almost always wide open spaces, but there’s still grasses and shrubs that could burn.
For whatever reason, the fuse on this bottom shot burned a little too long. While viewing from the audience, we all thought the bottom shot had hit the roof of that shed just as it fired off. The explosion pushed the roof downward and it rebounded from the wall, propelling it upward. At least that was the first impression.
As the slow motion sequences clearly show, the bottom shot detonated on the ground in front of the shed. The door of the shed was open, so the shockwave from the detonation (spreading in all directions) entered the shed increasing the internal pressure, and blowing the roof upward from the inside. So what’s the science we can learn from this?
Trenches (and tunnels), like those currently in use in the Middle East and Ukraine, are typically not dug in a straight line. They zigzag to the extent permitted for two solid reasons. First, if an enemy has breached the defense, it will limit the distance down the trench the enemy can shoot. Second, any shockwaves caused by explosives will be attenuated at every turn, whereas a linear trench would actually focus the shockwave and allow it to travel a greater distance.
That’s what happened to the steel shed. Once the shockwave entered the building it was contained by the walls (which are stronger than the roof), so it pushed up on the roof ripping it from the walls. The PGI covered the damage, of course, since they are required to have insurance for events like these. In the many I attended I’ve only seen a few mishaps like this and none led to bodily injury. These “pyros” are skilled pros and usually get it right. If there’s ever a PGI convention near you (schedule is on their website) I highly recommend you check it out. Their displays are WAY better than most public displays.
Next Week in Sky Lights ⇒ Nobel Peace Prize for Nihon Hidankyo