Happy New Year Pyro’s!
Has anyone experimented with length of tube when building mortar racks? From what I can tell, a “standard” tube is 12” with a 1.5” plug of wood or cement and most consumer mortars are fused appropriately for that length of tube. I’ve heard rumor that longer tubes, produce better height. I’m preparing to build racks from DR11 I picked up today to start preparing for the 4th. Yes, I know it’s early, but I’m fairly sure I’m not alone preparing for the next shoot (whenever that may be).
Is there a considerable difference or benefit between 12” and 14” or longer?
Certainly, the longer the tube, the more velocity is generated and the higher the shell will rise.
However, you reach a point of diminishing returns rather quickly. For example, I did some testing with tube lengths of 18", 24" and 30" for 4" professional shells. My main purpose was to determine if the 30" offered a significant increase over the 24" and I happened to have some 18" tubes that are supposed to be used for mines, so I compared those as well, just for grins.
The results: Firing identical shells out of each tube, the 24" tube yielded a break height probably at least 25% higher than the 18" tube. However, the 30" tube, while noticeable when fired side by side at the same time was not significantly higher than the 24" tube, less than a 10% increase, for sure. In short, it’s not worth the extra expense and storage space required to use 30" guns for 4" shells.
So, there is a sweet spot in tube length for performance and manufacturers/builders have pretty well figured this out for each sized shell. For a 4" professional shell, it’s 24".
For 1 3/4" consumer shells, like that which you are dealing with, it’s around 12". I do see some 15" tubes sold for 1 3/4" shells, but my guess is the height gain will be negligible.
My recommendation, just go with 12" tubes, save a little cost and space and also, as you mentioned, your fuses will reach the top of the tube. Besides, why do they need to go higher, anyway? The lower the break, the bigger it will appear. Safety is the main concern and a 12" tube will get 1 3/4" shells to a proper and safe height.
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The issue with longer motor tubes is chamber pressure? Ie don’t put to big of a launch charge or it will explode