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Animated diagram of time-delay fuse

Time-Delay Fuse

As the firework shoots through the air, the time-delay fuse continues to burn. When the shell is close to its apex, the fuse has burned low enough to ignite the black powder in the first break (or compartment). Colored stars ignite in every direction. But the show isn't over yet. The fuse keeps burning, making its way toward the stashes of black powder in the second and third breaks.

Timing is critical. In a three-break firework, the middle break needs to ignite at the highest point in the shell's trajectory—the first break should blow a little before and the third break, a little after. If the timing is off, the firework might detonate too close to the ground. Great care is used in designing the fuses and calculating their lengths.

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