Aerial
shells are the most commonly used fireworks in public displays today.
They operate by being launched with a black powder charge from a pipe (a
mortar) made of fiberglass or HDPE. Upon the charge igniting, a fuse is
lit that leads to the center of the shell. Within the shell is another
charge of black powder surrounded by the visual effects - These ignite
when the fuse burns through and bursts the shell. There are two common
designs that shells use. The most common, ball or spherical shells, will
be covered here. The other commonly made type of shell is a canister
shell, and is described on its own page.
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Other
Projects:
Aerial
Shells
Ball
Shells
Ball shells
originated in China and are now the most commonly used shells in the
USA. They burst in a spherical pattern which, depending on the size of
the shell, can be made of several layers. In the shell burst to the
right an outer layer of blue stars was used, and an inner layer of
larger silver stars (comets). The design of a basic ball shell is as
follows:
Ball
Shell Design
The drawing
on the right has been numbered to allow for simple description.
A ball shell is made up of two hemispheres (1). Each half is filled and then they are quickly snapped together. A layer of pasted strips of paper makes up the outermost layer of the shell (2). This layer provides additional confinement to the contents as well as extra protection from the high pressures when the lift (7) is ignited.
On the bottom of the shell is what is called a lift cup (3), which holds the black powder lift (7) to the bottom of the shell. Upon igniting, the lift both pushes the shell out of the mortar at high speed as well as passing fire to the time fuse (8). Time fuse is made to burn from one end to the other without allowing fire to burn through the sides. This is crucial for proper timing of a shell. To easily ignite the time fuse, the end in contact with the lift is split and a piece of black match is inserted and tied in place (9). This process is called crossmatching.
When the time fuse (8) burns to the end, it reaches what is called a passfire (10). Passfire can mean any number of things that quickly moves an ignition point from one place to another. In this case, the passfire moves the flame from the end of the time fuse into the center of the shell. It is made of a thin paper tube with several strands of black match within it. Black match will burn very quickly when enclosed, and allows the contents of the shell to ignite as soon as the time fuse burns through. It is important that a ball shell be ignited in the very center for a nice round break to be possible.
Inside the shell, the first thing to be ignited from the passfire is the break (11). This fills all empty space within the shell after adding the effects. The break is composed of black powder, usually coated onto a filler material such as rice hulls. A filler material is used because less black powder is needed than what would be able to fit inside the shell. The filler saves weight and powder.
Outside of the break and right up against the inside of the hemispheres (1) are the effects of the shell (12). The shell in the drawing is filled with rolled stars - The most common effect in ball shells. In very large shells, instead of stars the space could be filled with smaller shells or other complex effects.
The last detail of a ball shell to be discussed is the external fusing (5 & 6). A very fast burning fuse called quickmatch (6) is run down the side of the shell into the lift (7). It is made long enough to reach to the top of the mortar. Quickmatch is made similarly to the passfire (10) inside of the shell. Since it burns so quickly, a delay fuse (5) is taped to the top end. For many shells the quickmatch is run through a loop of string (4) that is attached to the shells pasting. This loop allows the shell to be lowered into the mortar by the quickmatch.
A ball shell is made up of two hemispheres (1). Each half is filled and then they are quickly snapped together. A layer of pasted strips of paper makes up the outermost layer of the shell (2). This layer provides additional confinement to the contents as well as extra protection from the high pressures when the lift (7) is ignited.
On the bottom of the shell is what is called a lift cup (3), which holds the black powder lift (7) to the bottom of the shell. Upon igniting, the lift both pushes the shell out of the mortar at high speed as well as passing fire to the time fuse (8). Time fuse is made to burn from one end to the other without allowing fire to burn through the sides. This is crucial for proper timing of a shell. To easily ignite the time fuse, the end in contact with the lift is split and a piece of black match is inserted and tied in place (9). This process is called crossmatching.
When the time fuse (8) burns to the end, it reaches what is called a passfire (10). Passfire can mean any number of things that quickly moves an ignition point from one place to another. In this case, the passfire moves the flame from the end of the time fuse into the center of the shell. It is made of a thin paper tube with several strands of black match within it. Black match will burn very quickly when enclosed, and allows the contents of the shell to ignite as soon as the time fuse burns through. It is important that a ball shell be ignited in the very center for a nice round break to be possible.
Inside the shell, the first thing to be ignited from the passfire is the break (11). This fills all empty space within the shell after adding the effects. The break is composed of black powder, usually coated onto a filler material such as rice hulls. A filler material is used because less black powder is needed than what would be able to fit inside the shell. The filler saves weight and powder.
Outside of the break and right up against the inside of the hemispheres (1) are the effects of the shell (12). The shell in the drawing is filled with rolled stars - The most common effect in ball shells. In very large shells, instead of stars the space could be filled with smaller shells or other complex effects.
The last detail of a ball shell to be discussed is the external fusing (5 & 6). A very fast burning fuse called quickmatch (6) is run down the side of the shell into the lift (7). It is made long enough to reach to the top of the mortar. Quickmatch is made similarly to the passfire (10) inside of the shell. Since it burns so quickly, a delay fuse (5) is taped to the top end. For many shells the quickmatch is run through a loop of string (4) that is attached to the shells pasting. This loop allows the shell to be lowered into the mortar by the quickmatch.
