Potassiumchlorate Posted November 9, 2012 Posted November 9, 2012 (edited) I was testing my BP the other day. I used 50 grams of 0.5 to 1.5mm grain size, (i.e. something like 4FA) to lift a 5" shell, which was 8.5% of the shell weight. The lift seemed powerful, but both the mortar and the shell made it fine. The question now is: is this because the BP is still - relatively - "weak" in performance? I have said before that although made with pretty good care, my BP doesn't seem to perform really good. At another occasion Mumbles said that with grains this fine and a 5" 5% of the shell weight should be sufficient. How do I know? Will this for instance be too powerful for a 6", even if I then do go down to 5% of the shell weight? I realize that these fine grains are at least far too powerful for anything bigger than 6". I don't want a flowerpot. Your opinions about this? Edited November 9, 2012 by Potassiumchlorate
marks265 Posted November 9, 2012 Posted November 9, 2012 I know a guy who lifts shells up to 12" with 4FA
Potassiumchlorate Posted November 9, 2012 Author Posted November 9, 2012 Hm, that sounds good. What kind of mortars does he use? This is made of something called PP, similar to HDPE but not really as strong. I have bought some fiberglass mortars, though, and I don't want them to blow up.
marks265 Posted November 9, 2012 Posted November 9, 2012 Ball shells at our club are all shot out of HDPE or fiberglass.
AirCowPeacock Posted November 9, 2012 Posted November 9, 2012 I figure HDPE is used because it doesnt shatter when it breaks, and fiberglass is used because it does not break
Algenco Posted November 9, 2012 Posted November 9, 2012 I figure HDPE is used because it doesnt shatter when it breaks, and fiberglass is used because it does not break fiberglass is lighter but strong, large HDPE guns are much heavier. Both beat wrestling with steel when it isn't nessecary.
marks265 Posted November 9, 2012 Posted November 9, 2012 They all break, it is just a matter of who, what , when , where, and why
AirCowPeacock Posted November 9, 2012 Posted November 9, 2012 Obviously. And HDPE could shatter. But the point is fiberglass is really strong compaired to HDPE, and the risk of it breaking is significantly less. I imageine fiberglass would create dangerous shrapnal if it broke.
Mumbles Posted November 9, 2012 Posted November 9, 2012 If you're using it in normal temperatures, HDPE does not shatter. Even if salutes go off inside of it, it largely remains in one solid piece. It mostly ruptures and stretches out. The weakest point is actually the wooden end plug, which can be prone to being torn out the bottom with too heavy of shells. Most of the fiberglass failures I've seen aren't that bad either. They tend to break into a number of pieces in the worst case, so in a sense they do shatter. However they tend to not fly all that far due to the light weight and typically being kind of feathery. At club shoots I tend to see more fiberglass guns fail. I don't know if that is a frequency of usage thing though. Being less elastic than HDPE, it might just be more prone to slowly building up stresses and faults.
marks265 Posted November 9, 2012 Posted November 9, 2012 OK let me try this with you: ALWAYS PLAN TO FAIL! HDPE and fiberglass are customary for use as acceptable mortar tubes. Always leave plenty of safety fuse for retreating or fire electrically from a distance. If you're worried about which to use buy one of each and try it and let us know how it works for you. "How do I know?" you ask. Start a little light on the powder and work your way up. Usually with the size of shells you speak of won't matter that much when comparing say 2Fa to 4FA. Meaning either one will and has worked. Unless you start doing peanut shells or cylinder shells. This is why it is critical to stick with what you know. If you know your powder, it is one less think to worry about. Any decent powder in these grain sizes can be used at recommend amounts usually published somewhere like Skylight for example. You can adjust up or down from there based on your observations and your goals. It's hard to mess up that bad if you follow the guidelines of printed info. If you make crap powder you will have a low break is all. Its all part of the game. Just fine tune it to what you would like.
Nessalco Posted November 9, 2012 Posted November 9, 2012 Obviously. And HDPE could shatter. But the point is fiberglass is really strong compaired to HDPE, and the risk of it breaking is significantly less. I imageine fiberglass would create dangerous shrapnal if it broke. Sure does. A worker on one of our hand fired shows was caught by a 3" titanium salute going off in a fiberglass gun. The explosion destroyed the rack and 2 adjoining it, and engulfed the shooter in an enormous fireball. He was wearing bunker gear, coat and bibs, and received 14 pieces of fiberglass and plywood shrapnel in the muscles in the back of his legs and butt. It was a couple months before he was fully mobile. Kevin
Potassiumchlorate Posted November 10, 2012 Author Posted November 10, 2012 Interesting answers. BP isn't very expensive to make, but it requires a good deal of work, so I don't want to waste it. This fine BP also gives very little residue. But at the same time I don't want to risk a flowerpot. To start with I will use coarser grains for anything above 6".
AirCowPeacock Posted November 10, 2012 Posted November 10, 2012 (edited) Thats interesting Mumbles, considering good fiberglass is way, way stronger (higher tensile strength) then HDPE (by like >100 fold.) My fiberglass morters are only about 1/10 as thick as my HDPE morters, meaning my fiberglass morters should be atleast 10 times more resistant to fracture (right?) Edited November 10, 2012 by AirCowPeacock
marks265 Posted November 10, 2012 Posted November 10, 2012 I was thinking about this and I think I thought what I should have thunk was what I first thought and then I started thinking about what I first thought and what I really meant to think and it is what I first thought when I thunk it. 1
Algenco Posted November 10, 2012 Posted November 10, 2012 I was thinking about this and I think I thought what I should have thunk was what I first thought and then I started thinking about what I first thought and what I really meant to think and it is what I first thought when I thunk it. Brilliant!!
Mumbles Posted November 10, 2012 Posted November 10, 2012 I'm not trying to make any claims that my experience is typical, just passing along what I've seen. Under normal use, both HDPE and fiberglass are quite suitable and will last several years with minimal maintenance. My observations at club shoots is probably just from fiberglass being used more often. Fiberglass is generally somewhat less expensive, and quite a bit lighter, so it's probably favored when having to replace damaged guns and if you have to dig one into the ground. Thus in the event of a failure (usually resulting from a shell malfunction), it just happens to be more likely to be from fiberglass. The one HDPE malfunction that comes to mind was a result of a plug being blown out/cracked from a relatively heavy shell. I do agree that fiberglass probably had more overall strength. There are certain shells that I make that I would not trust out of HDPE, but would gladly shoot from fiberglass. I feel that the single piece construction is a definite advantage in this regard. I really don't shoot enough with my personal guns to say if there is any difference in long term durability. I own a mixture of fiberglass, HDPE, cardboard, and steel mortars. I've never had one fail, but they're used at most a few times a year.
AirCowPeacock Posted November 11, 2012 Posted November 11, 2012 Ive never had a failure ever either. For example, some of my small cardboard morters Ive used year after year with no problem. My fiberglass morters are less frequently used, and I recently bought my first HDPE morters and have yet to use them. I knew you weren't suggesting this is the norm, I just thought it was interesting because it defied conventional logic-but of coure that could be explained by many things.
asilentbob Posted November 12, 2012 Posted November 12, 2012 (edited) Tensile strength is static strength, not dynamic strength. The load of pressure/force is applied slowly, it is not a shock loading. As it might as well be considered when a shell is fired from a mortar or especially when an aerial salute fails to leave the gun. A good analogy is found in climbing ropes, ie rock climbing, canyoneering, caving, etc... Climbing ropes where shock loading (falls) can occur like in rock climbing are dynamic with a good deal of stretch, something around 30% is common at a certain pounds of force most have printed with them. Ropes for canyoneering and caving where your rappelling and ascending the rope with mechanical ascenders, prusik knots, etc... and there isn't much of a chance for a fall are static with much less stretch at the same pounds of force, probably closer to 5%. ... Now... we all know materials like kevlar and dyneema/spectra (ultra high molecular weight polyethylene) are incredibily strong... and they are. However, that is still static strength, not dynamic, they have very low elongation, they don't stretch significantly, so they don't draw out the loading force over time. Dyneema is still used in some climbing equipment like woven slings or a strap between two carabiners (dogbone, or quickdraw including the carabiners)... however it is used with the understanding that a dynamic rope will still be used in the system, and the rope will draw the force of a fall out over time. This is critical, otherwise even the aluminum climbing carabiners would fail with the force of a fall. My point is fiberglass... while being very strong as a composite... doesn't take shock loading as well as we would like for it to... and over time there is a good chance that it is only going to become more brittle. IE flexing from lifts breaking the glass strands, weathering and UV messing with the plastic or epoxy resin, etc... In pyrotechnic use we put our mortars in a strange situation. We want them to take the shock of a lifting shell no problem... but we don't want them to stretch when lifting a large salami either, where its a lot of force, and it must go to lifting the shell, if the mortar expanded enough for a significant amount of the lift gasses to escape those salamis may take a short trip in the air and back down to earth... and when a extra spicy large salute goes in a tube we want to minimize shrapnel... Thats a lot of requirements to try to cope with. HDPE and fiberglass work very well with smaller single and double break shells, but forget trying to use one for some massive salami. Burried steel guns work very well for all sizes, especially the extra large and heavy... but a detonation is still a detonation, and the gun will shrap. EDIT: Also, relating to powder size, for smaller shells finer powder is fine. Keep in mind the shock loading though, especially if you have any chlorate stars or somewhat sensitive burst powder. If you move up to larger shells, especially multibreak you may be able to get away with fine BP for lift, but a more gradual lift with a courser powder will be much kinder on the shell and mortar... Same can be said for powder for loading into shells as burst/filler... on very large shells, often very course green or nearly green BP is used for break, sometimes you must, sometimes otherwise the stars would be blown blind. Edited November 12, 2012 by asilentbob
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