Mortar placement is as crucial to shell performance and safe use as the mortar material and configuration. The following are some general tips that may help a display operator determine what may work best for their particular application.
Remember!
There are a number of variables involved in determining mortar material and placement. The operator must take into account the variables of the type, size, power and fit of the shell as well as the mortar material, configuration and placement, the ignition method, the skill and experience of the operator and crew, the layout of the display site and of course the budget and purpose of the display.
Also NFPA 1123, Code for Fireworks Display has specific requirements for mortar and rack construction and placement, especially for anything larger than 6". Some states such as California and Connecticut and even some local officials may have very specific requirements for mortar type, placement and ignition method too.
Before you choose a mortar, placement method or ignition method, research the regulations and variables you expect to encounter very carefully!
The primary factors in determining the number of mortars and how to place them will be based on whether it is to be fired using manual ignition with a fusee or propane torch, or whether it will be electrically ignited using a firing panel and electric matches. Other factors are whether it will be 100% preloaded (required for electrical) or if it will be reloaded (optional for manual), what the desired rate of fire for the display will be, what the display site environment is, and the resources available (crew size, show budget, etc.).
If manual ignition is used, mortars should be buried 1/2 to 2/3 of their length in the ground or in barrels or troughs. A sand bag or two on the shooter's side of the mortar on top of the ground up to the muzzle provides added protection in the event a shell should malfunction in the mortar if it is manually ignited. This is especially important for larger caliber shells, or more powerful shells like salutes.
If manual ignition is used from traditional wood racks, the racks should be secured to the ground with something that will keep the end boards in place in the event of a shell malfunction, such a heavy steel rebar. The shooter should approach the rack from the end boards and retreat so the endboard is between them and the mortar being fired. This method is sometimes used for a 100% preloaded show and not recommended for a show where mortars are reloaded.
If electrical ignition is used, mortars can be buried in the ground or barrels or troughs, placed in traditional wood racks, in custom trailers or in the newer rail racks systems some operators have developed using tubular steel or stainless steel and parachute cord or U bolts. Some have also built special custom mortar holders for their mortars for firing specific effects, such as clamping a Mighty-Mite at a steep angle for electrically firing aerial comets.
HDPE mortars can be buried like any other mortar and the same principles apply to them as others. When buried they should be braced well underneath, especially in soft sand or dirt. A nominal 2" board or a large flat stone equal to or greater in width than the mortar will help.
Larger mortars like 8", 10" and 12" may need additional support, such as from 2 nominal 2" boards laid in a cross pattern underneath, from 4" x 4" boards, or even heavy pieces of railroad ties to keep them from being driven into the ground. This may make the difference in shell performance or in being able to retrieve the mortar from the ground without having to dig it all the way out by hand.
One can also use ropes or nylon straps around or under the bottom of the mortar to provide easier grips for pulling the mortar out of the ground. HDPE is slippery and sometimes difficult to hold onto while also trying to pull from the ground. Heavy, wet soil can also have a suction effect on mortars and these may help break that suction so mortar retrieval is easier.
Mighty-Mites are one piece HDPE so they do not have a problem with any ground moisture seeping into them, however Pyro-Pipes have a wood plug and a small gap. It is a good idea to put a large plastic garbage bag around the bottom of the mortar when there is wet ground or sand that may allow moisture to seep into the mortar and collect.
Even if the sand in barrels or troughs is reasonably dry, keep an eye open for rain that might come after the mortars have been placed. Some operators use plastic covers for each mortar and these will keep water out from the top, but do little to keep moisture from seeping in between the plug and the mortar wall, which can cause the lift to get wet and cause a malfunction like a flowerpot or low break.
One can also use a shop vacuum to suck water or sand from mortars so they do not have to be pulled and dumped out. Of course this means one needs a power source at the shoot site, something that isn't always available.
HDPE mortars are generally recommended to be fired once per show, primarily because there has not been enough research to come up to establish a rate of fire that indicates when a mortar has been warmed up to a high enough temperature that increases the risks of mortar failure significantly. Many operators however do reload them and fire more than once per show and have had few problems with the mortars becoming weak from the heat buildup.
HDPE is a thermoplastic and as such it will weaken substantially if heat build up inside. Mortars if fired rapidly enough and under the right conditions can reach 212º F (100º C). If it reaches these temperature, it can become too weak to hold the pressure from the lift charge and cause the mortar to bulge or blow out and result in flowerpots or low breaks. The difficulty in determining this point is based on the variables of the air temperature, the mortar placement (buried or above ground), the shell size, weight, amount and type of lift, the rate of reloading and firing and other factors.
Some research on this issue indicates that for mortars in the typical above ground racks, typical commercial ball shells can be fired every 3-4 minutes and typical commercial canister shells fired every 4-5 minutes without significant problems due to the mortar weakening. For buried mortars which won't dissipate the heat as rapidly, the amount of time between firing and reloading would be increased by a factor of 3 to allow the mortar to cool down sufficiently.
There are many other issues involved in reloading and manually ignited shells, such as whether to clean the mortars out between shots, how and how often, how to segregate shells (salutes from star shells, ball from canister, single break from multiple-break, etc.), how to place and mark the mortars for the reloaders to find the right ones in the dark, how to train the crew to get the timing right for the intricate process involved so the shooter doesn't bump into the reloaders and the ready box is closed when shells are fired, how to deal with hangfires, flowerpots and low breaks, etc.
Many operators prefer to hand fire from high quality schedule 40 steel mortars that are buried and sandbagged, and use HDPE mortars only for preloaded flights and the finale. This method allows more time between the reloading the steel mortars which takes the pressure off the crew, keeps the rate of fire entertaining and yet allows fewer mortars to be used and requires less space than a 100% preloaded setup.
Above ground wooden racks need to be made of a good grade of wood so they are sturdy and rugged. Nominal 2" boards as wide as the outside diameter (OD) of the mortars should be used as the minimum dimensions for the bottom board, two end boards and the middle boards. Nominal 1" boards 3-4" wide are needed for the side boards with at least two side boards per side.
Some racks have no bottom side board only one at the top and middle. The reason is to reduce weight and cost but also because if built well, this will be all that is needed to hold the mortars in the rack. It also reduces the problems with shells that malfunction in the mortars which often will tear the bottom side boards out, causing the rack to come open and spilling the mortars over.
Glued and screwed is preferred over nails or staples to fasten them together, but bolts all the way through the end and middle boards may also be used. This will allow the side boards to grip the mortars just enough so they stay in place, and helps with the Mighty-Mites since they have no plug for a nail or screw to be placed into to hold the mortar in the rack. Wood shims can be used between the side board and the mortar to gently wedge it in place.
It is important the mortars be held snug in the racks to keep them from falling out during handling and in the truck, but more importantly from bouncing in the rack as a group is fired in a sequence, such as in a flight or finale. If one mortar is bouncing up as it is fired, it can drive into the bottom board and crack it.
Treated lumber is not recommended because of its brittleness. It will not hold up well from the rough handling racks are subjected to in loading and unloading from the storage sheds to the trucks to the display site and back and tends to split where the fasteners are installed. The bottom board also tends to crack from the pounding from the mortars and rough handling.
All racks need to be secured squarely to the ground so the bottom board has support from the ground.
All racks should be secured so they will not tipover from normal use. This can be accomplished by adding feet to each end, stakes, or both. Feet should be as long as the rack is high, and in dimensions equal to at least those of the end boards. Sand bags can be placed on them or they can be nailed or screwed to the feet of other racks in a staggered herringbone pattern to provide added support.
Some use an A frame type configuration on the ends to provide bracing instead of feet.
Bracing boards are often used between racks to provide greater stability too. These are usually boards of the same dimensions as the side boards and nailed to the racks so the racks support each other.
Stakes should be attached to the end boards. Large eye bolts and steel rebar equal in height to the rack and driven at least 1/2" their length into the ground make excellent stakes. They can also be placed next to the middle board on both sides of finale racks and racks with larger caliber mortars like 5" and 6" to provide additional support.
Wood stakes can also be used but they are difficult to pound deep enough into the ground. Stakes of any type are useless on hard or frozen ground or concrete streets or parking lots, so the support for racks in these environments must come from feet or bracing boards.
Keep in mind that repeated nailing and screwing of feet, A-frames or bracing boards into the racks can weaken them over time. It also places more fasteners into the racks, which can become dangerous shrapnel in the event of a malfunction.
Racks can also be buried partially in the ground a foot or two to provide additional support.
Finale racks are often placed end to end, but more and more are placing racks so they are independent of each other. While it makes it easier to have them connected to each other in order to chain fuse them, it also makes it harder to stop in the event of a malfunction.
Each finale rack should be placed and secured separately so if there is a shell malfunction that damages one rack, it is less likely to telegraph to other racks. Finales are also more often ignited in rapid sequences from different ignition points rather than a single ignition point for the same reason; control in the event of a malfunction.
The reason is finales which are chain fused in a single chain cannot be turned off and in the event of an errant shell landing in it and igniting it, it cannot be stopped. It also reduces the chances of the finale racks tipping over if one portion of the finale has a shell malfunction in it, which could shoot fireworks directly over or into the audience or nearby property.
All racks should also be placed perpendicular to the audience as well, so if there is a malfunction that causes a rack tip over or fail and mortars spill out, they are not as likely to do so towards the audience. When the audience is 360º around the mortars, then one must consider the distances from the mortars to the audience, the topography of the land and where the majority of the audience will be in order to figure out which angle to be perpendicular to presents the least risks to the most people in the audience.
Many racks also have spacing boards between each mortar. These are often nominal 2" boards equal in length to the ID of the mortars and width of the OD of the mortars. The purpose of these is to provide space between the mortars so a malfunction in one mortar does not effect the ones next to it. This seems to help with smaller, low power ball shells but not much with salutes and larger caliber canister shells, since they will generally shred even the most well constructed racks and damage mortars separated by such a short distance.
The main risks with racks is their malfunctions damaging the racks so the mortars spill out of the racks or tipping over on their sides. Whatever rack configuration and supporting methods are used needs to address these risks in relation to the types and sizes of the shells being fired and the display sites these will be used at.
Mortar boxes and troughs need to be made of good quality wood as well. 3/4" to 5/8" CDX flooring is best and one can add 2" x 2" strips to the ends using a floor adhesive and then use bolts through these to hold the box together. With all the holes drilled at the same place, it will make all the boxes uniform
Or a tongue and groove method can be used so no metal fasteners are needed. A cut equal to or just a saw blade wider than the thickness of the board is cut half the height of the box about 3-4" from the end and on each side of each of the 4 boards.
Typically boxes are 24" tall and 48" long. Some may be taller for 5" and 6" mortars or longer with a middle board to help brace it when it is filled with sand.
The advantage of this system is different mortar sizes can be mixed in the same box making it easier to wire up a scrip that bounces from 3" to 5" to 4" to 6" or groups of say 2 - 3" , 2 -4" and a 5" on the same cue. This system also works well for sites where racks cannot be staked to the ground such as parking lots and hard or frozen fields.
Racks and troughs do not need to be painted, but considering the cost of wood these days, it may not be a bad idea to give them a couple of coats of exterior latex from a power sprayer to protect them from the elements. This can also reduce their weight if they get wet and in the case of trough boxes with tongue and groove assembly, keep the wood from swelling to the point they cannot be taken apart.
Other troughs can be plastic or steel 55 or 30 gallon barrels, large commercial dumpsters, plywood boxes, and even barges. Almost anything that will hold a large volume of sand above ground can and has been used to hold mortars.
HDPE mortars can be washed with any dish soap and cold water. They do not need to be washed except when the amount of buildup inside may pose a problem with loading, or to make it easier to grip them to pull the mortars out of the ground after being fired.
HDPE mortars require little maintenance except for an occasional cleaning. Like all mortars they should be inspected before each use for cracks, dents, tears or other damage. Mortars with plugs should be checked for plug damage, shrinkage and to be sure the fasteners have not rusted or otherwise weakened.
With Pyro-Pipes, if the plugs have shrunk significantly the plug end can be soaked in linseed oil and when dry, another coat of polyurethane can be applied to the exposed ends.
Some checking (natural cracks from the wood drying out over time) of the oak and maple plugs is to be expected. Unless a large amount of the plug has cracked and is loose or missing, should not pose a significant problem.
If a plug has been damaged, it can easily be replaced with another plug.
Any dings or dents around the mouth of the mortar can be repaired, usually with just a picket knife. If it is too big a ding or dent, it can be warmed up with a propane torch until it is more pliable and worked back into place.
HDPE mortars are so light crews have a tendency to toss them around, even in racks. They also will bounce or slam into each other and other equipment in the trucks and unless secured.
Most of these dents and dings are no problem but on occasion one may be right at the mortar mouth and also be significant enough that it could make it hard to load them. Or worse, a ding or dent at the lip could cause a shell to have a screwball flight when fired or damage it as it come out of the mortar.
Reasonable care needs to be taken when handling them and transporting them to reduce this type of damage.
The operator and crew also needs to inspect them and repair any damage prior to loading them with fireworks. Repairs can easily be made with a small sledge hammer or pocket knife. Simply pound the ding out or smooth with the knife.
HDPE mortars should be stored inside away from the sun. Prolonged exposure (10 years or more) to ultraviolet light can break down the plastic. The best way to store them is on pallets on their bottoms, top up.
The wood plugs or racks should not be placed on uncoated concrete floors so they do not absorb moisture. They also should not be stored outside in the rain and snow for the same reason. Dry rot of the plug as well as rust of the fasteners are the primary reasons for protecting them from the elements.
If they are outside or in a shed with a leaky roof, a tarp can be placed over them so they don't fill up with water. Keep in mind though tarps leak through holes and tears, and they need to be tied and weighted down so the wind doesn't rip them up. Also any water they catch is heavy so when removing them later, it can take some manipulating to keep them from tearing or dumping the water into the mortars.
One can also put a plastic cap over them too. One brand, Caplugs from Buffalo, NY, has a popular series SC that is widely used. These also serve to protect the mortars and the shells from moisture when the show is setup.
The mortars and racks can be stacked on their sides but this will often cause them to sag somewhat as the pull of gravity works on them, especially the larger mortars and in hot climates. If this happens, simply turn the mortars on the other side or place upright in the sun for a while, and they will return to their rounder selves.
© April 1996, Mighty-Mite Marketing, Inc., Green Bay, WI USA. All rights reserved worldwide.
Revised June 13, 1998