It’s Full of Stars!

Fireworks Monolith.png

Wow, these fireworks shows are getting trippier every year!

Since the Fourth of July is pretty much synonymous with fireworks for me and my United States readers, it seemed like the perfect time to research something I was curious about myself—how do fireworks displays work?

What I’m talking about here goes beyond the basics of fireworks themselves. That’s pretty simple to explain, and it uses pretty much the same old black powder that was invented by alchemists in China all those centuries ago. You fill something with black powder (which combines fuel and oxidizer), you light it on fire, it goes boom.

The displays I have in mind are the complicated ones that you see in cities and theme parks, which may involve ten or fifteen minutes of exquisitely timed and spaced fireworks in a dazzling array of shapes and colors, often set to music. That’s a lot of elements to control, and it’s really impressive how seldom it goes wrong, in my opinion.

To understand what makes these displays so great, let’s take a look at the aerial fireworks themselves, from the moment they launch to the moment they explode into crowd-pleasing concatenations of color and cacophony. First, how do they get off the ground?

For any good-sized fireworks show, the answer is almost certainly “under computer control.” This makes sense when you consider the precise timing and synchronization involved, which would be incredibly difficult for even a group of humans to handle. There’s no competing with machines for millisecond-level timing.

Things can still go wrong with computer control, of course. San Diego found this out in 2012 when the annual Big Bay Boom encountered technical difficulties, resulting in all of its fireworks going off within 30 seconds. Most of the time, though, computer-controlled displays are more ambitious, better executed, and safer.

At the proper times, the computer sends electrical current through a series of electric matches. These aren’t like friction matches, but instead use a coil or loop of wire that heats up until the flammable material surrounding it (the pyrogen) catches on fire. This lights a fuse that eventually reaches and explodes the black powder that provides the lifting charge. The explosion propels the firework shell, which is made of thick paper, cardboard, or plastic, into the air from a mortar tube at speeds of hundreds of miles per hour.

Fireworks Mortar Parts

In this process, a second fuse is lit inside the shell, controlling the timing of its midair explosion. Most aerial shells seem to have about the same fuse length, giving similar delays between being fired out of their mortar tubes and actually exploding. The larger the shell, the higher it goes—ranging from about 100 to more than 1,000 feet—which implies that the larger it is, the faster its launch speed. I wanted to follow this up in more detail, but had trouble finding specifics for launch speeds (other than this infographic, which doesn’t list sources in a very helpful form, so it’s hard to verify their numbers).

As you might expect, shell size affects the size of the actual firework burst as well as its height. Sources agree with the general rule that you get 45 feet of blast radius for every inch of shell radius. For consumer use, the largest legal shell diameter in the United States is 1.75 inches, giving you a blast almost 40 feet in radius, or nearly 80 feet across.

To understand how the explosion ends up as a breathtaking starburst, let’s look inside the shell. The time-delay fuse touches off a burst charge that detonates the black powder that fills the shell, blowing it apart. Along with black powder, the shell contains stars, the small bits that actually create the colored bursts we associate with aerial fireworks.

Fireworks Shell Parts 1

The stars contain even more black powder, as well as carefully measured amounts of chosen metals. These elements absorb energy from the explosion and then release it as light with wavelengths that are unique to each element or combination of elements, resulting in different colors. Here’s a handy guide to some of the metals that are commonly used for specific colors:

Firework Colors

Historically, the hardest color to produce for fireworks has been blue, since copper’s energy emission only produces a strong color within a relatively narrow temperature range. Multicolored bursts come from stars with concentric layers of different materials, which burn from the outside in and change colors as the layers are consumed.

In addition to the colors, I was curious about how to get different shapes out of a plain old fireworks shell. It turns out that this is actually pretty simple, at least in theory. If you want a two-dimensional shape, you just take a piece of cardboard and glue stars to it in the shape you want. When the blast charge goes off, the stars are driven apart and explode in your desired shape. (This Popular Mechanics slideshow has a real example of patterned stars laid out in a shell.)

Fireworks Shell Parts.png

You might say it’s a whole fleet of stars!

This is definitely one of those processes that’s simple but not necessarily easy. It can take a lot of testing to get the right spacing, especially if you want a more complex or three-dimensional shape. For example, letters are still a big challenge, since relatively small changes in spacing can make them illegible, or at least different letters than the ones you want.

Letters also illustrate the problem of high-speed upward propulsion, which makes it very difficult to control the exact orientation of the firework shell when it explodes. Experts are still working on safe ways to weight shells so their orientation is more predictable, but even so, your audience may end up seeing your painstakingly-designed shape as just a line of stars if it’s at the wrong angle. To avoid this tragedy, displays usually launch multiple shells of the same design simultaneously. This gives viewers a good chance of seeing at least one of your shapes as it was intended, but is a weird way to handle spelling out words or numbers, so there are still improvements to be made.

All of this adds up to a pretty expensive package for professional displays, which often have music or other timing considerations along with the design of the shells themselves. I had trouble finding a cost breakdown by types of shell, as most professionals apparently prefer to charge for the whole display based on all the factors involved.

The estimates I found agree that a good display will cost a few thousand dollars per minute, probably more if you want a really great show. You’re not just paying for the components and the programming, but also the artistry that goes into designing the show. Given the amazing way that chemistry and physics combine to create such incredible beauty, I’m inclined to say it’s worth the cost.

You know, for science!

[You can read more about the science of fireworks with this resource from the American Chemical Society or this guide from geology.com. Improve your vocabulary and learn how to identify your favorite types of aerial fireworks with this short list of terms or this one with videos, or check out this medium-length list and this longer glossary! Serpentines, flying fish, and crossettes are some of my favorites.

It’s hard to find a single video that exemplifies the awe of a great fireworks display. For classic Fourth of July examples, look up fireworks shows from Boston, Nashville, or New York City. For a smaller-scale version of the launch setup, check out this Raspberry Pi-controlled launcher!]

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