A History of Fireworks

The history of fireworks begins with the history of gunpowder in 9^th century China.  Gunpowder was probably invented by accident (the compound is a mixture of only three components) but was soon used at both wars and festivals.  Within about a hundred years of the invention of gunpowder, some form of early fireworks were being sold by Chinese street vendors.

Gunpowder spread to Europe in the 14^th century in the form of cannons and other military weapons, and with the gunpowder came fireworks.  In the 1800s, fireworks manufacturers began tinkering with the fireworks compositions and soon colored fireworks (and other effects) were lighting up the sky.

Anatomy of an Aerial Shell

Aerial fireworks shells are either a sphere or a cylinder, with spheres being more common.  Common sizes range from three inches (slightly bigger than a tennis ball) up to ten inches (about the size of a basketball).  Shells larger than ten inches are extremely expensive to transport, although they can be legally bought and shot.

A spherical aerial shell is composed of five general parts: the casing, the stars, the flash powder, the time fuse, and the lift charge.  As sparks from machines are deadly in fireworks manufacturing, every firework is made by hand.

The casing is two halves of a ball (for spherical shells), made from layers of paper.  Each half is filled with stars and then the halves are (carefully!) pressed together and taped.  To reinforce the weak seam created by the separate halves, the casing is finished with several layers of glue and paper strips.

Stars are the exciting part of the shell: these are what provide color and most of the effects one observes.  Stars can be manufactured in several different ways; rolled stars are most common in commercial fireworks.  Rolled stars are round and range from the size of a pea to the size of a small grape.  Rolled stars allow for color-changing effects: as one layer is burned away, another layer of a different pyrotechnic composition (ie color) is revealed.  The colors are made by burning metals: titanium and aluminum burn white, barium burns green, copper burns blue, etc.  Those experimenting with new pyrotechnic compositions for stars must be careful about which chemicals are mixed together, as some chemicals react poorly with each other and will result in self-ignition.

Flash powder is placed between the two halves of the star-filled casing and is responsible for fireworks going “Boom!”.  Once the flash powder is lit, the reinforced shell casing will initially resist the force of the expanding gases: this allows time for the stars to light.  The internal pressure will eventually overcome the paper casing and the firework will explode with the sound of a cannon to display its burning stars.  ‘Salute’ shells, with a larger-than-average ‘report’ contain more flash powder than ordinary shells.

Time fuse is what lights the flash powder and what determines when the firework should explode after it has been launched.  Holes are pre-drilled in one half of the casing, allowing for one or more fuse(s) to be inserted into the firework.  Time fuse is similar to 3/8” rope, but is manufactured to burn at specific rates: 3sec/inch, 7sec/inch, etc.  The number of seconds needed for the shell to reach peak height is calculated (a shell should explode at the apex of its trajectory), and the needed length of fuse is cut and inserted into the shell.

Finally comes the lift charge and fuse to light the shell (often quick match).  The lift charge is simple: several ounces of gunpowder to get the shell in the air and light the time fuse.  On commercial shells this may be contained in a cone, or in a small cup similar to a Dixie cup.  Quick match is another fuse covered in paper: almost as soon as it ignites, the paper carries the fire and lights the lift charge.

The operator will then load the firework in an appropriately-sized tube (mortar), lift charge down, and either leave the fuse dangling out of the mortar (for hand-fired shows), or attach an electronic match (for electrically-fired shows).

Problems with Shells

Unfortunately, we live in an imperfect world.  And in that imperfect world, shells don’t always go off as planned (or at all).

First is flower pots.  Flower pots never make it out of the mortar, instead throwing their bouquet of stars from inside their tube with a tremendous “Bang!”.

Next is hangfires- arguably the worse of the potential problems.  A hangfire is a shell which has been lit, but has not yet left the mortar for whatever reason.  These are Schrödinger’s bombs: while still in the mortar they are simultaneously misfires and duds and may continue to be so for thirty minutes to an hour.

Misfires can be bad.  Like, this-is-the-reason-you-never-put-any-part-of-your-body-over-a-loaded-mortar bad.  Misfires go randomly: either early (ignited by falling embers) or late (a smoldering misfire decided to light).  If you’ve ever seen one random shell go off several minutes after the show, a hangfire turned into a misfire.

Duds are, as their name suggests, inert.  The firework may have gotten wet, it may not have been lit, the time fuse didn’t catch… generally something went wrong and the firework didn’t go off.  A dud may be found either in its mortar, or in the fallout zone around the fireworks area (this is the main reason why fireworks crews bar entry to what otherwise seems like the perfect place to watch fireworks: a grapefruit hurtling towards earth at terminal velocity).  Duds are rendered completely inert with water, and then transported back to the fireworks supplier for proper disposal.

Regulating Bodies

Fireworks are explosives and so are potentially subjected to federal, state, county, and city regulations.  On the federal level, the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) covers firework requirements when the fireworks are not in transit and the Department of Transportation (DoT) rules when fireworks are in transit. 

On the state level, most states have adopted various versions of the National Fire Protection Agency’s (NFPA) 1123 or 1126 standard (depending on the type of firework).  Fireworks operator’s are licensed on the state level and applicants generally have to provide evidence of working on shows as well as passing a written test based on state law.

The local level is where the Authority Having Jurisdiction (AHJ) usually resides, as well as local laws about prohibited fireworks.

Further Reading

Wikipedia’s “Fireworks”

Wikipedia’s “Gunpowder”

ATF’s “Fireworks”

DoT’s “Fireworks”

America Pyrotechnics Association’s “State Law Directory” for Consumer Fireworks

Researching fire stunts last week got me into a ‘stunt-y’ sort of mood, and what is one of the next logical stunts to examine?  Falling from height!

My initial research only brought up people who had performed incredible high fall stunts.  In the Assassin’s Creed (2016) movie, stunt performer Damien Walters jumped from 125ft in the film’s Leap of Faith stunt.  In a Twitter tweet, the Assassin’s Creed team touted the stunt as the highest free-fall for a film performed in 35 years.  The movie that the tweet referenced was likely Sharky’s Machine (1981) where stunt performer Dar Robinson leapt 220 feet from Atlanta’s Hyatt Regency Hotel.  In the movie, however, only the beginning of the stunt is shown.

While not involving art in any way, Luke Aikins 2016 free fall from 25,000 feet deserves an honorable mention.  The daredevil jumped out of a plane without either a parachute or wingsuit and, after a two-minute free-fall, landed in a custom net.

At the end of the fall, all that truly needs to happen is to decelerate the jumper until they are moving with zero velocity (ie stopped moving).  Over the years the way this deceleration has occurred has changed.

In film’s Western era, stopping falls relied quite a bit on breaking wood.  Stunt teams would set up sawhorses around a crash site and delicately place a few pine boards on the sawhorses.  Cardboard boxes would be placed under the boards, and some mattresses on top.  When the stunt performer hit the mattresses, the boards would be put under stress and would begin to bend.  After a few inches of bend (and some reduction in velocity for the performer), the boards would break, and the performer would continue their slowed fall into the cardboard boxes.

High falls performed in this way were limited to below 50 feet, as that was all the stunt performers could handle.  Additionally, this way of performing the stunt was highly wasteful: the boards were broken, and the crushed cardboard boxes would not be reused for another fall.

Cardboard boxes continue to be used for lower falls today, as can be seen in this clip from an old episode of Fear Factor, when contestants were asked to drive a car off a building and into an (enormous) stack of boxes.

Nets, as demonstrated above, can be used for high falls, as well as for lower falls common in circus activities.

Bungee jumping has also become somewhat popular in films, as a stunt performer can be filmed from above without a need to erase their crash pad during post production.

The most accepted way of decelerating a stunt performer at the end of a high fall, however, is with an inflatable stunt air bag.

Inflatable Stunt Bags

The beginnings of inflatable air bags started in 1959 by John Scurlock as he attempted to create an easy-to-store, easy-to-deploy tennis court cover.  As he tinkered, he noticed that his son enjoyed jumping on the inflatable mattress, and soon inflatable “moonwalks” were invented, followed eventually by the forerunners of bounce houses.

It did not take long for others to connect the child’s plaything with the fire nets firefighters were currently using to catch people trapped in burning buildings.  The trick then, as is now, is how to cradle the jumper so that they don’t hit the inflatable pad and immediately bounce off it.

Inventors soon realized that the trick was to vent the air the jumper displaced.

A 1973 US patent (filed by the same John Scurlock) called for two air cushions stacked on top of each other: the top six feet in height, the bottom only three feet.  As the jumper falls into the bag, the pressure build-up due to a decrease in bag volume eventually opens “breathers” on the side of the bag.  With six feet of cushioned air to fall through, Scurlock calculated that a jump of 100 feet would be totally (and safely) stopped by the top cushion, but added the second, non-venting pad as a backup safety feature.

Scurlock also included equations in his patent, showing that the difficulty is in successfully balancing an energy equation.

The Equations

This is a reproduction of the equations filed in John Scurlock’s 1973 patent and changing technology may have rendered these equations useless.  If you are trying to figure out how to safely perform a stunt, I applaud your effort, but suggest that you find someone with previous experience to work with you.

1).   A_a = 8(1+3d- ((2d²)/h))

A_a is the affected area of depression

d is the depression

h is the height, or thickness, of the air cushion

I have no idea where this equation came from.

2).   V_loss = 8(d+(0.9d²-((0.6d³)/h))

V_loss is the volume of the air cushion lost due to depression

d and h continue to be the depression and height of the air cushion, respectively

As velocity is the mathematical integral of acceleration, this equation is (mostly) the integral of equation (1) with respect to d.  It is stated that this equation was found after realizing that the depression takes the approximate shape of a cone.

3).   P_t= 15(V_loss/V_i) + P_g

P_t is the pressure at any particular depression

V_loss is the volume lost

P_g is the initial pressure in the cushion (typically 0.5 psi in 1973)

4).   Or by combining equations (1) and (2),

P_t= (120/(A_i*h))*(d+0.9d² – ((0.6d³)/h)) + P_g

P_t, h, d, and P_g remain defined as above

A_i is the total surface area of the cushion

5).   F = 1000P_t(1+3d-((2d²)/h))

F is the upward force of the air cushion on the falling body.  This is essentially the same force that a chair exerts on you as you sit on it.

This equation comes from Force=Pressure*Area, with 1000 likely as a conversion factor.

6).  F_max = 1000k(1+3d-((2d²)/h))

Where k is likely a modifying factor based on the height of the air cushion (?).  The patent contains a table which is not well-described.

7).   Work = F*D = F ʃd = 1000P_avg(1+3d-((2d²)/h))Δd

P_avg is the average pressure in the air cushion

Work is defined as force occurring over distance: mathematically one does the same work running a 5k as one does walking it.  An integral is just mathematical shorthand for “find the (obnoxious) summation of the area under this line”

8).   P_m = 3.3/(1+d)

P_m is the maximum pressure allowed in the air cushion

No idea where this formula came from, but it is likely something that a manufacturer will list.

9).   V_a = 350(P)^1/2

V_a is the velocity of the escaping air (ft/sec)

P is air pressure (psi)

I suspect this is also something for the manufacturer to concern themselves with, and inform the client of.

Other Useful Equations

10).   Potential Energy = weight*height

This is a physics formula, for the potential energy of the falling performer.  This is equal to the performer’s kinetic energy:

11). Kinetic Energy = ½ mass*velocity²

Essentially, any person high in the air has potential energy: if the floor suddenly disappeared, they would begin moving.  As a performer has chosen to convert all of their potential energy to kinetic energy, potential energy equals kinetic energy.  From this equation, one can calculate the performer’s velocity the moment before they hit the bag:

12).   Velocity = (2*g*height)^1/2

Where g is the universal gravitational constant (32.2 ft/sec²).

So there it is: as the performer falls, they will reach the inflatable air bag with some velocity depending on their weight and height they’re falling from.  The moment they impact the bag, it begins to depress with affected area A_a and after a few moments to allow the internal air pressure to build, the air will vent out of the bag through “breathers”.  At any point, the air bag is acting on the performer with a force F which is dependent on the pressure in the bag and the affected area A_a.

For further information (including a sample calculation and location of breathers, I would highly recommend looking at the patent).

Further Reading

United States Patent

GamesRadar’s Assassin’s Creed Jump

Polygon’s Assassin’s Creed Jump

Wikipedia’s “Dar Robinson”

USA Today’s Skydiver Stunt

Wired’s Physics Behind Skydiver Stunt

NPR’s “Hollywood ‘Stuntman’! Reveals Tricks of Trade”

Wikipedia’s “Inflatable Castle”

One of the main reasons that I chose to go to school in Middle-of-Nowhere, Missouri is that my school offers pyrotechnics classes down at the “on-campus” experimental mine.  It was during one of these classes (the very first day, actually) when I got a second-degree burn by wearing pants with too much spandex in the blend.

From staring at the dime-sized burn, it was only natural to wonder: “How do I avoid this happening next time?” and then it was a hop, skip, and a jump to, “How do stunt people set themselves on fire without injury?” And so I present:

Full Burn Film Stunts

To start off, this is one of the more dangerous stunts that professionals do: you should not try this at home.

With that said, a full body burn is all about the set up.

Before the Stunt

First, an undershirt, long underwear, and a hood is soaked in a specially-formulated gel.  This gel is the key part of what protects the performer and is made with a (trade-secret) compound which absorbs a ton of water.  The water is held in the gel as tiny droplets, with millions of droplets in a glob of gel.

Water has a high latent heat– it takes a lot of energy for water to change from ice to liquid, or from liquid to vapor.  Like ice cream on a hot day, as long as you eat it quickly, you can enjoy your frozen treat.  In stunts, as long as the burn is performed quickly (before the water has had time to boil off), movie-goers can enjoy the finished effect.

The underclothes are no ordinary material either.  They are usually made from Novex or Kevlar which are fabrics that firefighters wear.  If heat is applied to Kevlar for more than a few seconds, it will start to burn.  If that heat is taken away, the flame will extinguish itself within a few seconds.  Novex doesn’t burn at all but will simply char and flake away.  As both are impregnated with the no-burn gel, they are highly unlikely to catch on fire.

The next trick is that all parts of the stunt- clothing, gel, and performer- are kept as cold as possible, for as long as possible.  This is to prevent the water droplets in the gel from boiling off early, either due to energy from the sun or heat from the performer’s body.  Clothing and gel are kept on ice, and the performer is shooed into the shade.

The performer next slides into the gel-soaked clothes and proceeds to slap no-burn gel on all exposed skin.  Any skin not covered with the thick gel risks being burnt.  A costume made of only natural fibers is then worn on top of the Nomex and gel.

Meanwhile, other crew members have prepared the set- putting down a fire blanket or wet furniture blanket where the performer will be ending their stunt, and gathering and testing fire extinguishers.  CO₂ fire extinguishers are often used and, as they put out the fire by starving it of oxygen instead of removing the heat, a water hose is often nearby for if skin was accidently left un-gelled and heat needs to be quickly removed (the high latent heat of water in use again!).

During the Stunt

Finally the time has come, the fire extinguishers have been manned, and the performer gets into position.  As the performer has just spent a fair amount of time becoming inflammable, burn gel is now applied to them so that something is able to catch on fire.  Burn gel (or burn paste or any other mixtures used) have one main trick: they burn at a lower temperature than fires usually experienced in life.  This allows the no-burn gel to last longer and for stunts to be more elaborate.

The burn gel is applied, the performer is lit, and the rehearsed movements are acted.  One less-obvious safety consideration is practiced here: the performer is almost always moving forward.  This keeps both flames and (more importantly) hazardous smoke out of their face.  If the performer is asked to stand still for a shot, they hold their breath, and the length that the stunt can be safely performed is greatly reduced.

After the Stunt

At the end of the take (or if the performer is feeling unsafe), they go to the prepared blanket and drop to their knees.  This is the signal to those manning the fire extinguishers, and the flames on the performer are quickly smothered.

The director examines the footage that was just shot as the stunt coordinator checks in with the performer, with medics hovering nearby.  If the performer is fine and the director asks for the shot to be done again, the performer applies more gel and re-does the action.  Once the director is happy with the footage, the performer retires from scene to go take a long hot shower.

Want to read more?  Check these out!

NPR’s “Trial by Fire- Literally- in ‘The Full Burn’”

Wired’s “We Watched an Artist Set Fire to Themselves On Stage”

Popular Science’s “Gray Matter: In Which I Set Myself on Fire”

How Stuff Works “How Stuntman Work”

StackExchange’s “How do they shoot flaming human scenes?”

Wikipedia’s “Fire Retardant Gel”

Explain That Stuff’s “Nomex”

Action Factory’s “Fire Resistant Suits and More”

TBF Pyrotech’s “Fire Paste and Fire Gel”

Roger George Rental’s “Pyro Gel”