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Consumer Fireworks Display: Mortar Racks, Fusing Techniques




Introduction


This article we'll be looking at the construction of mortar racks from which to fire artillery shells during the show, and some techniques for using various fuses to attach devices together for the display.


Mortar Racks


Mortars are the tubes with plugged ends that fireworks shells, comets or mines are fired from. Mortars can be made of HDPE plastic, fiberglass, paper, or in some special cases, metal.

The mortars need to be secured in an upright and safe position. This can be done by burying the mortars (guns) about 2/3 of their length in the ground. Here are a couple of shots of some of the large guns that were buried for shows and competition at a recent Pyrotechnics Guild International convention.

Fireworks mortar tubes at the Pyrotechnics Guild International convention

Large mortar tubes dug into the ground at the Pyrotechnics Guild International convention
Buried Mortars at a PGI Convention (photo by Mike Hrnciar)

Often, especially with smaller guns, the mortars can be securely held in place in racks, either perpendicularly or at an angle. The racks can be constructed of metal, wood, or a combination of the two.

PGI Convention Mortars, Set Up in Racks (photo by Mike Hrnciar)
PGI Convention Mortars, Set Up in Racks (photo by Mike Hrnciar)

Here are a couple of artillery shell racks made by Brian Paonessa at Skylighter, using Skylighter's PL3182 fiberglass mortar tubes . One is a fan rack, and the other holds the guns straight up and down.
Fanned Fiberglass Mortars in Wooden Racks Perpendicular Fiberglass Mortars in Wooden Racks
Fiberglass Mortars in Wooden Racks, One Fanned and One Perpendicular

Here is a shot showing some of the construction details of the fanned rack. Brian has glued and screwed the rack together.

Angle Rack Construction Details
Angle Rack Construction Details

Below is the angled PL3175 artillery shell mortar rack that Skylighter sells. The swing-out feet hold it in an upright position. When using this rack, I drill holes in the feet and drive spikes through them and into the ground to keep the rack from bouncing and falling over.

Skylighter PL3175 Mortar Rack with HDPE Guns
Skylighter PL3175 Mortar Rack with HDPE Guns

As you can see in the photo of the PGI racks above, wooden racks can also be held upright by attaching them together with lengths of wood 1x3s, or by pieces of plywood attached to both ends of them. In either case, screws or nails are used to keep the whole assembly upright and rigid.

Care must be taken to avoid driving fasteners into the mortars. In pyro this is known as a "bad-thing."

Typically, except in the case of fan-racks, racks are set up so that their ends are perpendicular to the front of the crowd. That way, if a rack happened to come loose and fall down, it would not be firing toward the crowd.

Here is another way to secure wooden racks. Screw-eyes are installed into the rack ends, and rebar pins are used to hold the racks in place. Both ends of the racks are supported in this manner, and racks can be erected end-to-end with only one pin between them.

Wooden Racks Secured with Screw-Eyes and Rebar Pins
Wooden Racks Secured with Screw-Eyes and Rebar Pins

No matter what method is used to erect them, once the racks have been assembled, they ought to be secure enough to withstand a healthy kick with a boot.


Fusing Devices in a Mortar Rack


In this section I'll be referring to and using the various kinds of fuse shown in the photo below. Each one serves its own purpose and has its own unique burn-rate. The burn rate of a roll of any particular kind of fuse can vary. So it's a good idea to cut 10 inches of the fuse off that roll and time it with a stopwatch as it burns to determine its exact burn rate.

Fuse Burn Rates
Fuse Burn Rates

American Visco 2.5 seconds per inch
Chinese Visco 1.7 seconds per inch
Fast Visco 0.25 seconds per inch
Fast Fuse 0.1 - 0.15 seconds per inch
Time Fuse 2.2 - 3 seconds per inch
Quickmatch Instantaneous
Foil-Taped Fast Visco Instantaneous
Foil-Taped Fast Fuse Instantaneous

The foil-taped fast-Visco or fast-fuse may be used as excellent substitutes for quickmatch, which is not shippable. .

In the rest of this article, I will refer to quickmatch, and you'll know you can make substitutes for it with the fast-Visco or fast-fuse as described above.


Hand Firing


So, I have filled 6 tubes in my rack with an artillery shell, comet, or a mine. If they are to be hand-fired, the shell-leaders (fuses) can simply be left hanging out of the guns, ready to be lit one at a time with a propane torch.

These shell leaders are fast-Visco fuse, and I'd expect a burn rate of about 4 inches per second, which will produce about a 3 second delay between lighting the fuse and the shell launch.

A shell of this size will take about 3-4 seconds to rise in the sky and display its starburst. So if I light the next fireworks fuse immediately after the first shell has launched, and so on, I'll get a nicely paced series of bursts that lasts a total of 18-20 seconds.

If the shell fuse leaders are a bit on the short side and threaten to drop down into the mortars, they can be held in place with a little masking tape. Be sure the shells are all the way on the bottom of the guns, though, to insure proper height when they are launched. A shell that's not seated solidly on the bottom of its mortar can become a "low break," which, in turn, can cause fires or injuries.

Fireworks Shells Loaded in a Rack and Ready to be Manually Fired
Fireworks Shells Loaded in a Rack and Ready to be Manually Fired


Fast Chain Fusing


But, let's say I want all of these shells to launch at the same time at some point during the show or at the end of it (the "finale"). In that case I'll chain them all together with a length of quickmatch. Chaining shells simply means attaching their fuse leaders together in a series. If the shells are chained together with quickmatch, and then the end of the quickmatch is lit using a piece of Visco or an electric match, once the flame hits the quickmatch the shells will all ascend skyward in quick succession.

This is done as follows:

  • Cut a length of quickmatch as long as the run of mortar tubes containing the shells, plus about a foot. Always use a razor blade or anvil cutters to cut fuse, never scissors.
Red, Waterproof Quickmatch, GN3001
Red, Waterproof Quickmatch, GN3001

  • Pierce the quickmatch wall with an awl where each shell leader comes out of the top of the mortar, making sure that all the layers of match pipe are pierced and you can see the black match inside.

Punch a Hole All the Way Through to the Black Match
Punch a Hole All the Way Through to the Black Match

  • Put a fresh diagonal cut on the end of the shell leaders with a razor blade in order to expose the powder inside the leader.
  • Insert the shell leader into the quickmatch for an inch or so.
  • Use masking tape or aluminum foil tape to secure the shell leader into the quickmatch. I really like the aluminum foil duct tape with the peel-off paper backing. The stuff sticks like crazy, will not gradually come loose over time, and is fireproof.
Cutting and Inserting Shell Leader into Quickmatch
Cutting and Inserting Shell Leader into Quickmatch

  • Use string to tie the fuse chain down to the rack between each mortar. I like waxed string for this purpose. It makes "threading the needle" with it a breeze. This prevents the first shell from yanking the chain as it is launched, which might pull the rest of the leaders loose from the chain.
Tie Fuse Chain to Rack at Each Mortar
Tie Fuse Chain to Rack at Each Mortar

Warning: In the past, some folks have used a staple gun to staple quickmatch chains to the tops of wooden racks. More than once, the stapler has created a spark which has ignited the chain and instantly sent shells skyward. This has killed or seriously injured some people. Don't use a staple gun to secure flammable fuse, nor use one anywhere near pyrotechnic compositions.

The nifty thing about this fusing method, and the following ones, is that they can be applied to fusing rockets set side-by-side in launch tubes, or to fusing cakes laid out in a field or on a piece of plywood. A whole show can be laid out, fused together with a combination of these methods, and fired by lighting one fuse or firing one electric match.


Delayed Chain Fusing


But Wait, There's More! Maybe I want that nice 3-4 second delay between the shells' firing that I spoke about earlier. Maybe I want a different delay time, but I want to fire the shells in a chain as in the section above. How can I build that delay in between each shell in the chain?

Near the end of the Pyrotechnica XI article, Traditional Cylinder Shell Construction, Part II, "Finale and Flight Chaining" is addressed. This is a fascinating explanation of "old-time" chaining methods using quickmatch, paper buckets (rolled tubes of kraft paper), string, spolettes and regular time fuse. It's a valuable addition to my pyro library. In the photo above, there are about 3 inches between the center of each mortar. If I run one of the Visco fuses down the line instead of the quickmatch, and attach my shell leaders every 3 inches, then I will get 3 inches of delay between shots.

3 inches of the American Visco will give me a delay of 7.5 seconds between shells. That's more than I want, but that might work in some cases. 3 inches of the Chinese Visco will give a delay of 5.1 seconds between shots. That's more like it. I could go with that, although it's a bit more of a delay than I really want.

To use Visco for a chain, simply tape the end of each shell leader alongside the Visco fuse as it runs along the tops of the mortars. The two fuses must be parallel to and touching each other for at least an inch of tape. Then tie the chain down to the mortar rack as shown above. Don't try to run the shell leaders into the Visco chain at a right-angle. You'll get poor or failed ignition that way.

Tape Shell Leaders Side by Side to Visco Fuse Chain
Tape Shell Leaders Side by Side to Visco Fuse Chain

There is another, more precise, way of incorporating delays into a chain of shell leaders, though. It incorporates sections of cross-matched time fuse, or hand-rammed spolette fuses.

The roll of 1/4 inch time fuse that I have burns at a rate of 2.2 seconds per inch. If I use 1-1/2 inches of it between each shell in the rack, I'll get a 3.3 second delay between the firing of each shell. This is done as follows.

I want 1.5 inches of time fuse delay, and I'm going to split each end of the fuse 1/2 inch for cross-matching. So I cut five, 2-1/2 inch sections of the time fuse. I split each end 1/2 inch with my razor blade, insert three 2 inch pieces of the thin black match that can be found in the fast-fuse or quickmatch, and I tie each end of the time fuse closed with a clove hitch and overhand knot to secure each knot.

Splitting and Cross-Matching Time Fuse
Splitting and Cross-Matching Time Fuse

Then I make "buckets" out of 3-1/2 inch x 3-1/2 inch pieces of kraft paper, rolled around a 1/2 inch wood dowel, with the edge of the paper glued down. I then tie a bucket on each end of the cross-matched time-fuse pieces, with the knots just to the inside of the pieces of cross-match. Tie the knots very tightly so that hot gasses cannot escape the bucket and transfer over to the next one before the time fuse has burned through.

Making Buckets and Tying Them onto Cross-Matched Time Fuse
Making Buckets and Tying Them onto Cross-Matched Time Fuse

Now it's just a matter of making a chain of these bucket time-delays, in similar fashion to the chains that were made above. The first bucket in the chain has a piece of quickmatch coming into it from the ignition source, and a piece of quickmatch coming out of it into which the first shell's leader is tied or taped. I don't want a delay before this first shell's fuse is ignited. This first bucket also lights the first time-fuse delay element.

Inserting Quickmatch Into First Chain Bucket
Inserting Quickmatch Into First Chain Bucket

I bare the black match in the quickmatch for 3/4 inch before inserting it into the buckets. It's easy enough to clip the buckets a bit shorter with scissors as necessary. It's just important to avoid cutting into the cross-match with the scissors, and to leave enough bucket so that the knot can be tied without any blackmatch protruding beyond it.

During the chain assembly, it can help to tie each delay down to the rack before assembling the next link in the chain. This helps to insure that the quickmatch pieces leading to the shells are long enough, and are routed away from each other and away from the mouth of a previous mortar, which would lead to a premature ignition.

6 Chained Shells with Time Delays Between Each One
6 Chained Shells with Time Delays Between Each One

The chain shown above is designed to be ignited from the left end, to have 3.3 second delays between each shell, and to pass fire from the right end to the next device in the line if desired.

This same type of chaining using time fuse, can be used to link box-cakes to each other. Let's say I start with the ignition of a cake that has a 30 second burn time, and I want to overlap the next box 5 seconds into the first cake's time. I'll put a 25 second delay time fuse and buckets at the ignition point of that second cake. On and on, this type of show can be assembled.

Happy Fourth,

Ned

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