Not too long ago, we presented an article entitled The Lighter Side of Explosions, in which we examined how engineers and designers have taken something potentially dangerous, analyzed it, harnessed it and use it for a variety of applications. Considering that Independence Day will soon be celebrated across the United States on July 4th and building upon a similar theme, we figured it would be worthwhile (and timely) to delve deeper into fireworks, a very specialized form of explosion used for the sole purpose of entertainment.
But as with anything entertaining, there is a lot that goes on behind the scenes just to make that 15 to 60 minute extravaganza; a lot of planning and many lessons learned by pyrotechnic engineers, many of which start with some basic engineering principles:
Ballistics
Fireworks just aren’t fireworks without being up in the sky, far enough away from people to be safe, but close enough to be seen and heard, so a way up must be engineered. Fortunately, we need only look no farther than Newton’s Third Law and the building blocks of rocketry, where for every action there is an equal and opposite reaction.
In this case, the firework itself is packed into a “shell” and placed into a cardboard tube known as a mortar. The shell has a fuse, that such when lit, ignites propellant in the “lift charge” which provides significant thrust to lift the shell to its desired height, up to as much as 1,200 feet.
Depending upon the nature of the show, pyrotechnics engineers will have to coordinate the launching of anywhere between 40 shells for the smaller community displays to as many as 50,000 for the city-wide spectaculars, such as New York’s Macys-sponsored Display on the Fourth of July. In smaller venues, the fuses are lit by hand, but in the larger scale events, they are coordinated by sophisticated computer software.
Pyrotechnics
Once the shells achieves their desired height, they explode. This requires precision timing via a time-release “bursting charge” fuse that is lit at the same time the lifting charge is ignited and determines the timing that the shell will soar before bursting.
For each shell’s unique structured geometric burst, they are packed with any number of pyrotechnic pellets known as “stars”. Stars can vary in size, shape and color, producing a myriad of effects. Varying the mass of a star will cause the material to burn for a shorter or longer period of time.
Varying the shape, including the angle of the stars’ fuses will determine whether they fly outward when bursting, crisscross or spiral away. Each shell can also have up to eight different colors.
Depending upon the effect, from the traditional spherical monochromatic “peony” to rings in varying shapes to multi-colored, multi-break shells with cascading bursts. Additional special effects may include confetti, glitter, tails and animation.
Chemistry
To produce both lift and burst charges fireworks manufacturers must have knowledge of combustion and the chemistry behind it. There are typically 30 different ingredients that go into a standard shell and any miscalculations can have less than desirable effects.
The basic components of any firework shell consists of a fuel, an oxidizer, a binder, a color and a chlorine donor. The fuel allows the stars to burn, while the oxidizers support their combustion. Typically, this consists of a black powder mixture consisting of potassium nitrate, charcoal and sulfur. The binder holds the pellet together. Varying chemicals produce different colored effects, from the vibrant reds of strontium or lithium to the deep greens and blues of barium and copper. Lastly, the chlorine donor helps provide more vibrancy and intensity to the colors.
Fireworks have become so ingrained in our summertime culture. So this Fourth of July, while you’re out enjoying an evening pyrotechnics display, celebrating Independence and the heroes who made it all possible, think as well of those heroes working behind the scenes who have put countless hours of thought and attention into ensuring that you and your family have memories that will last a lifetime.
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