Can multiple-break shells be used in HDPE mortars?
The answer is yes, but what is meant by multiple break? 2 break? 3 break? 4 break? 10 break? the number of breaks is only a nominal measure of the size and power of a shell and the pressures it generates on the mortar. There are many other variables in- volved, such as the weight of the shell, the tightness of the fit, the amount and type of lift powder, the amount of space between the bottom of the shell and the mortar, and the type of shell (canister vs. ball). As those change, so do the pressures generated, no matter how many breaks there are.
Typically, multiple-break shells are canister shells, although shells of shells and sun & planets are usually round and could be classified as multiple break. A canister shell generates roughly four times the pressure of the same caliber ball shell. The higher pres- sures for canister shells have to do with the cubic inches of volume available for the lift gasses to fill, as well as the generally high weights of canister shells because they have more volume than a ball shell.
NFPA 1123 (1995) A-2-3.6 recommends thicker walls for steel or cardboard mortars to be used for 2-break cylindrical, but cited "Data not currently available" for HDPE. The field experience h a s been added to, but not yet updated in the Code.
Many operators fire 2- and sometimes 3- break color or color 85 report shells from 3" and 4" HDPE mortars and have not reported experiencing any significant problems. Some do so every day at theme parks and the mortars have generally held up well for years, including some of the original lighter gauge molded bottom, one-piece HDPE introduced in 1984. The few operators left who use 5" and 6" crossettes, color 8B tourbillions or ser- pents have not reported any real problems either using HDPE, but typically those shells are lifted with a coarser, slower burning lift powder than their smaller counterparts.
Very few operators fire multiple break or canister shells above 6" from HDPE, so the amount of information available for 8", 10" and 12" is very small. Most operators tend to shoot only ball shells in these calibers from HDPE, and use steel mortars buried in the ground for bigger, more powerful, or heavy.
HDPE can take the pressures in part because such pressures are of short duration and be- cause of the flexibility of the HDPE. There are other problems to consider though, such as the type of fasteners used to hold in the plug in the bottom, and what condition they are in. Rusty old staples or a couple of nails or screws may not be able to take the pressure. Some use bolts all the way through the plug with washers that provide a more even dis- tribution of the pressure and a more secure hold, especially over time.
Some other factors are whether you are at sea level or in the mountains, what is the air temperature, how is the mortar placed (wooden racks above ground or buried in the ground), what kind of bracing you have un- derneath to support the bottom, etc.
Once in a great while one might see a split or tear in the HDPE or a weakening in the plug fasteners if staples, nails or screws are used (more likely in the 5" and 6"), or if a shell is particularly heavy or tight, or both. If this is a concern, use a small, light cardboard tube, like a paper towel core, on the top of the plug for the shell to rest on. By giving it an inch or two space it should reduce the hammering effect of the lift gasses on the HDPE, as well as the fasteners, without seriously degrading performance.
For heavy shells or salami type multiple breaks, schedule 40, or better yet, schedule 80 steel with a good, heavy bottom plate and recessed welding outside, buried in the ground with sandbags and remotely fired, are usually the preferred method. These types tend to generate very high peak pressures at lift and need to have as much height as pos- sible for proper functioning. CPW
My mortar update article generated a number of comments from people all over the coun- try. Many were favorable but there were many questions too. Here is some more de- tailed follow up.
We've always used cardboard in racks to shoot our shows and have never had a prob- lem. Why shouldn't it be used for hand firing? Let's take a quick look at the process of hand firing a show. Typically in a hand fired show each mortar will be fired 10 to 20 or more times. That means a lot of reloads into a "hot" mortar every time, except at the begin- ning. Usually the aerial portions are sand- wiched between ground effects and that means quick firing in a short period of time.
Cardboard will burn and begin to unravel a little bit at a time. Some cardboard will last a very long time while some will not, even from the same lot. In the dark, it is difficult to tell and you may not find out how bad it has come apart until there is a problem. With more firings and shorter cooling off periods it will come apart faster and that makes reload- ing successively riskier with each shot.
A reloaded shell may not fall smoothly to the bottom properly, blocked by the core coming unraveled or a build-up of paper shell cas- ings or both; the mortar itself may be smol- dering and cause any loose lift powder to ignite during reloading; the difficulty pre- sented in cleaning out any debris without causing any damage to the mortar itself; and the most common method of having card- board mortars above ground in racks, pres- ents a shrapnel problem should a flowerpot or detonation burst the cardboard and send chunks of wood with nails or screws, not to mention stars, flying towards the crew.
HDPE really doesn't present those problems since the tests done so far show the sides will just bulge out on detonation, but it has some of its own. HDPE is hard to cut and most are roughsawn and that means a jagged edge. Unless smoothed it may cause the match or shell casing with the lift charge to break or
tear during reloading, or force the assistants to place a part of their body over the mortar to avoid this problem. And there are concerns from some that successive hand firing will cause the plastic to heat up and become pli- able, causing problems again with reloading properly.
HDPE and cardboard though, present a common problem with hand firing; the plugs. Except for some very neat new 3" and 4" HDPE that are molded into one piece with NO need for a plug, cardboard and HDPE are usually fitted with wooden plugs. With each successive shot the pressure on the fasteners increases. That is especially true as the mortars get older and the fasteners become weaker from rusting away. Visual inspection is next to impossible so there is no way to determine how many shots can be fired pru- dently by hand firing.
Any significant weakening or plug failure may cause shells to burst lower or send the whole mortar and shell up together. Since the shooter and reloader will be within a few feet, he or she is at the greatest risk of all, even when the mortar is buried. And if the mortar is above ground, a burst mortar may send a deadly chunk of wood plug with nails or screws, flying even greater distances into the crew or audience.
Hand-firing a SINGLE shell during a show from a BURIED cardboard or HDPE mortar isn't as great a problem since it will not be reloaded and is in the ground. This is actu- ally recommended for firing salutes.
Also, when a shell fails to fire from a finale rack or due to an electronic or match mal- function from a rack during an electronically fired show AND still h a s match hanging from the mortar, most operators will hand fire that shell rather than risk reloading it.
Why shouldn't salutes be fired from steel mor- tars? It's stronger!
Yes, it is stronger, but salutes are also more likely to burst the steel than most other types
MORE ON MORTARS
of shells. While that is not likely to happen, it represents a deadlier risk to the crew and audience when it does happen. A cardboard mortar that is buried will definitely blow apart if a salute detonates, but is not likely to be as life threatening.
When should steel be used?
Anytime a show is hand fired or for any ex- perimental, multi-break or large caliber shells, it should always be buried and sand- bagged. In order to prevent low breaks, these shells are often lifted a bit more and the welded steel plug supposedly helps contain the gasses to get it up higher for the breaks to work where they should.
We've always had people who want to get in as close as possible to the finale. Why should we keep them back to 300'?
Finales are most often fired from racks of cardboard mortars and are usually made up of 3" and 4" shells that are chain fused. Any shell detonation in a mortar or muzzle break will send debris of the racks, plugs and card- board some distance, and although rare, it is much riskier the closer the crowd gets.
Using most muzzle loading and shotgun ammunition tables as a basic guide for killing effect, 300' or 100 yards is about average for most loads and types. Considering the sizes and weights of shrapnel from steel mortars and the screws, nails and wood used in plugs and racks, it is a fair comparison. Buckshot and deer slugs are potent at greater than 100 yards while birdshot loads are potent at 50 yards. Fireworks debris though is not fired parallel to the ground like guns, so trajectory angles may increase deadly range depending on size, shape, weight, type, turnover, wind, and velocity.
What happens more often though, is a rack or mortar may tip over causing any remain- ing unfired mortars, which can't be stopped, to shoot directly over or into the crowd. Since 3" shells can reach heights of 200' to 300' and then spread open another 200', you need at least 300' of space plus any fallout as a minimum distance. Since many stars will
easily burn through clothing and flesh, it's simply a matter of keeping people away enough to minimize the risks. It also reduces the chances of a dud landing where someone can get to it before you do.
Thus if the wind is blowing to the north, any crowd to the south should be at least 300' away and any crowd to the north 300' plus a fallout zone based on the largest shell being fired in the show and wind velocity. Keep in mind though, wind at setup time may not be the same as at show time and moving thou- sands of people back in the dark or moving all those racks and sandbags is harder to do than setting up a perimeter farther away in the first place.
It is too much work to bury the big 8", 10" and 12" mortars. We put them in garbage cans and fill them with sand and it's worked fine. Why
is this bad?
It's a common practice to use garbage cans or 55 gallon drums, but is extremely risky. While unlikely to happen, a large shell that detonates and does burst a mortar or blow out a plug or weld, whether steel or card- board or HDPE, the debris is extremely deadly. Many of the fatalities and serious injuries involving fireworks displays have come from above ground placement of large mortars.
This method adds even more metal that is potential shrapnel. It may seem all that sand piled around should be able to contain any blast but don't be fooled; it can't. It will barely slow down those chunks of jagged steel or wads of cardboard or wooden plugs, and* that's not enough to protect you or your audience. This risk is increased as more big shells are being fired and the more powerful these shells become.
By placing the mortar in the ground and sandbagging it, you reduce the risk of shrap- nel from such a blast mixing in with people, but you don't eliminate it entirely. The big mortars, even when buried, need to be given plenty of space from the rest of the show in the event you have a problem such as a hangfire, muzzle burst or flowerpot.
Keep more ready boxes and keep them far- ther away from each other, than the usual 25' and separate by size of shell so an 8" doesn't get loaded into a 10" by mistake. They also need extra time to cool down be- tween shots and if you believe in cleaning them more often, keep your crew closely syn- chronized. You also don't want to have one being fired while a ready box is open or an- other battery is being reloaded, as the spread of these shells can be 600' to 1,200' and a flowerpot or low break will easily send large
If you are considering designing or repairing the wood lining and "lattice" ventilation lining in your small hobby magazine, try what I did. I'm pretty lazy, and I do not care much for things that require a lot of maintenance. So I set out to find a way of lining my Type 2 magazines where I would not have to paint steel screw and nail heads constantly or reset them back into the wood when they work themselves out. I must face the fact also, that the interior of a magazine takes a beat- ing considering the characteristics of the wood lining.
It can be very humid in the summer, and the wood will absorb water from the air and swell significantly. The next day it could be dry and sunny with temperatures easily over 100° inside the magazine. The wood can lose its moisture quickly and shrink, especially across the gain. Several of these cycles will pull out countersunk heads with ease and then you have a spark hazard. So why not use brass or aluminum fasteners? A little more expensive, but safer.
I ended up with a plan that worked quite well for me. I drilled small holes in the half-inch plate floor for drainage of any standing water, then lined the floor with sheets of plywood. Next, I cut the sheets for the ceiling and held
amounts of stars where they shouldn't be.
Also, always have at least two mortars of any size in the event you have a problem. The more big shells you fire, the more mortars you 11 need so you can shift from one to an- other to allow more time to cool down or switch off, if one is unable to be used for some reason. That means you 11 need more space, distance and fallout area, more crew and stricter management of it all. CPW
them in place with temporary 2x4s. For the walls I cut plywood to fit but before I put the sheet in place I screwed 1-inch wide boards (that were previously ripped, three from a 2x4) from the back to the inside. I used steel drywall style screws but because they are screwed in from behind, they can never be exposed to the interior surface.
These "lattice" boards provide the necessary ventilation along the walls. They are fun ver- tically. Then these panels will hold the ceiling and floor boards in place by driving a wooden wedge at the ceiling line. Wedges can be cut from waste pieces of hardwood of a size and taper to meet your gaps. After a hot summer I found my wood shrunk considerably. Wide wedges were cut and installed. As they loosen all that required is a good rap with a block of wood for a hammer.
This method requires no holes be drilled into the sides or ceiling. Should the panels need to be removed to repaint the inside steel or clean up spilled materials or repair a leak, I simply knock out the wedges and the interior wood can be completely and quickly removed, virtually without tools. Please consult your state and federal laws, keep your magazine safe, and spend more time enjoying the hobby. CD