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Challenge: Make Fireworks

Updated Monday 28th January 2008

The team of scientists who took the Rough Science trip to Carriacou were asked to create explosions in the sky

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Working with gunpowder is a potentially dangerous activity because of its explosive and propellant properties. Would we be able to produce a colourful - and controlled - firework display on the Rough Science island? fireworks Copyrighted image Icon Copyright: Used with permission

Gunpowder consists of an oxidiser, fuel, and binders. When it burns, it quickly releases a large volume of hot gases. In firework displays, gunpowder is made to perform in different ways such as rockets, bangers, Roman candles, Catherine wheels.

How are these colours produced?

To understand the science of colours, we must first know something about the nature of light. Light is a form of energy that behaves like a wave. Visible light of different wavelengths is detected by our eyes as a range of colours. Of the light that we can see, violet has the shortest wavelength and red has the longest.

wavelengths of light corresponding to different colours Copyrighted image Icon Copyright: Used with permission

Fireworks generate light and colour due to the physical and chemical attributes of specific compounds. When these compounds are heated and combusted they give out energy, some of which may coincide with the wavelengths in the visual region of the spectrum.

What range of colours is available?

The main colours produced by fireworks are yellow, orange, red, green and blue. Yellow is produced via atomic emission. The other colours in fireworks are produced by a combination of atomic emissions and molecular emissions.

animation showing atomic and molecular emissions Copyrighted image Icon Copyright: Used with permission

When you heat a substance, the heat energy can go into the electrons of the atoms or molecules. The heat raises the energy of the electrons. As they fall back to a lower energy they give off photons. The energy of these photons, and hence their wavelengths, differs between substances and so we see a range of colours.

Molecules have another way of absorbing or radiating energy. Energy can be stored by the vibrations of the different parts of the molecule. Whole atoms, or groups of atoms, can vibrate relative to each other. The overall colour that we see from an excited substance is a combination of the atomic and molecular energy changes.

Red is the lowest-energy visible light, so in a red-hot object the atoms are just getting enough energy to begin emitting light that we can see. If you apply enough heat energy the electrons will generate all the colours and appear white.

The charcoal in the gunpowder mixture behaves like a black body. As the charcoal is heated to sufficiently high temperatures, it glows with a faint red colour. As it is heated further, the colours change from dark red to bright red to orange to yellow to white. By then it is radiating photons with all the wavelengths that our eyes can see. For materials like charcoal incandescence occurs from about 480oC to about 1400oC.

animation heating a piece of charcoal Copyrighted image Icon Copyright: Used with permission

Other metallic fuels, such as aluminium (3500oC), magnesium (3200oC) and titanium (2900oC), achieve high levels of brightness when burned, so can be used to make the bright sparks that are given out by many fireworks.

Now that we have lots of different colours to play with how is the actual firework made?

There are two parts to a firework: the propellant and the effect.

How can we make a rocket?

The basic principle behind a rocket is Newton's Second Law: "To every action there is an equal and opposite reaction".

A rocket firework throws mass in one direction and benefits from the reaction that occurs in the other direction.

rocket motion Copyrighted image Icon Copyright: Used with permission

The mass comes from the weight of the black powder that the rocket engine burns. The burning process accelerates the mass of fuel so that it comes out of the rocket nozzle at high speed. The fact that the fuel turns from a solid into a gas when it burns does not change its mass, because all the oxygen needed for combustion is contained within the rocket's fuel.

In other kinds of combustion the mass of products is greater than the mass of fuel because the fuel reacts with oxygen from the air.

How do we propel the rocket?

One of the crucial things about making a rocket is establishing how much force the rocket needs to take off and fly far enough to explode safely. For the rocket to be propelled, we need to generate a force greater than gravity.

First, we need to understand Velocity, Acceleration and Force.

To find out what the rocket's velocity is, this equation is used:
v = s/t
where v is velocity, s is distance, t is time

Acceleration from a stationary launch would be as follows:
a = v/t
where a is acceleration, v is velocity, t is time

The force would then be:
F = m x a
where F is force, m is mass, a is acceleration

We can work out how far the rocket travels above the spectators' heads based on the following information? The rocket generates a force (F) of 20N, its mass (m) is 0.5kg and it travels for 2 seconds (t).

To work out the distance travelled, we need to calculate the velocity and in order to do that we must first determine the acceleration.

F = ma
20 = 0.5 x a
a = 20/0.5
a = 40ms-2

Now that we know the acceleration, let's work out the velocity:

a = v/t
40 = v/2
v = 40 x 2
v = 80ms-1

Finally, we can use this information to work out the distance travelled:

v = s/t
80 = s/2
s = 80 x 2
s = 160m

So the distance travelled by the rocket is 160m - safely above the spectators' heads!

We know how a rocket works, but how are Roman candles made?

"Roman candle" is the traditional name for a firework that has been around for centuries. In its simplest form, it's just a cardboard tube with a star sitting inside it. The fuse runs into the tube and ignites a lifting charge, popping the shell out of the open top and into the air where it explodes.

The lifting charge is black powder which when ignited produces a lot of gas pushing the bundle of stars upwards.

The best known Roman candle is the air-bomb which has stars that explode with a loud bang when they reach the right height.

Some Roman candles have more than one shell inside, stacked on top of each other, separated by a lifting charge. The fuse runs down the inside of the tube, igniting each charge in turn. Thus the shells are launched one after the other, with a pause between each one. This is known as a 'multi-shot' candle. The effects can be varied depending on the types of shell inside the tube.

How are bangers made?

A banger is probably one of the most powerful fireworks because the whole contents explode at once. Whereas the contents of most other fireworks go off bit by bit, in a banger the whole lot is packed into a single cardboard tube.

The fuse ignites this and the explosion shoots out of the end of the tube and into the air. The effects are not only powerful, they are sudden, and occur from ground level upwards.

How are Catherine wheels made?

In the 4th century, St. Catherine of Alexandria was tortured on a wheel (they used all sorts of worrying implements in those days) giving rise to the traditional name "Catherine wheel" for this rotating device.

A Catherine wheel consists of a number of rocket-like motors mounted on a wheel. Each one burns to provide both sparks and thrust with the exhaust gases, spinning the wheel around. The fast motion of the wheel adds to the effect, throwing out sparks as it spins.

The Rough Science pyrotechnic team managed to put on a successful firework display of rockets, bangers and Catherine wheels all from a safe distance thanks to Jonathan's detonating device.

Web Links

The BBC and the Open University are not responsible for the content of external websites.

UK Firework Review - for UK firework enthusiasts with firework reviews, pictures and video.

How Rocket Engines Work - from the How Stuff Works site - explains how a rocket engine works.

Radical Backyard Science Ideas - on the How to Build a Water Rocket site - links to sites giving information about water rockets.

Model and High Powered Rocketry - by Don Irving on the Irving Family website - pages about all types of rockets, from the smallest to the largest built by hobbyists.

The Chemistry of Firework Colours - from the About.com site - explains about how the colours of fireworks are created - a marriage of art and science.

Books

Advanced Physics by Tom Duncan, John Murray

Handbook of Chemistry and Physics by David R. Lide PhD, CRC Press

The Chemistry of Fireworks by M. S. Russell, The Royal Society of Chemistry

Chemistry of Pyrotechnics and Explosives by John A. Conkling, Marcel Dekker

 

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