Tube Lengths?

[Pyromarty]

is there some kinda formula or a rule of thumb for how long the launch tube should be, in regards to the shell size?

[SoonerKid]

Per PGI guidlines.

Mortars up to 6 inches in diameter shall have an inside length (excluding the plug) of at least five times their inside diameter.

But this guidline isn't for doubles or above plus i would use the rule of thumb of (excluding the plug) six times the inside diameter for single ball & cannister shells.

[Pyromarty]

Thanks! sooner. now i hope they'll go up more than 20' then blow:D

[noel]

Normal Rule is 6" per shell inch


3" = 18"
4" = 24"
etc

[Marty]

I think in an earlier disscussion here someone posted some test results on consumer mortar size performance?
IIRC, the fit of the shell affected lift performance more than small variances in tube length (12" vs. 18").
Basically the poster was proving the longer tubes in kits was a marketing ploy not a technical need.
I couldn't find the thread, anybody remember it?

I do know it holds true for some 6" tubes I tested last year. Using 2 tubes the same length but one with a loose fit, the loose tube had markedly lower performance. After making another loose tube 6" longer, there wasn't much of an improvement.

For mines, varied tube lengths changes the look & height of the effect quite a bit.

[Don F]

You can go too long on tube length
there comes a point where the expanding gasses reach equalization in a long tube beyond that point . The long tube will actually cause a lower break .
Because the shell exiting will cause a suction in the tube.
Its basic balistics .
In shell building one can reduce the amount of lift powder needed by saboting the shell ( a semi soft crushable disc between the lift and the shell) This expands outward when crushed improving the "seal" between the shell and mortar . Its the difference betweem a dry paper wad and a spit ball ;)
Granted I have never seen this used in commercial shells either 1.4 or 1.3
but some hobbiests have used the technic

[JoeRatman]

The NFPA 1123 has this table of recommended mortar lengths (A.2.3.9) based on the inner diameter (ID) of the mortar tube

ID___1-Break___2-Break___4-Break
03"____15"______18"_______21"
04"____20"______23"_______27"
05"____24"______28"_______32"
06"____28"______32"_______37"
08"____34"______40"_______46"
10"____40"______46"_______54"
12"____46"______52"_______62"