16 replies to this topic

[Green_as_my_powder]

Fellow pyros,

I've found that the smaller the difference between shell o.d. and mortar tube i.d., the higher the shell will go, all else being equal. Weight of the shell doesn't seem to matter nearly as much. I imagine that at some point, safety concerns are increased (e.g. launch tubes bursting...). The consumer mortar shells seem to have a "loose fit". I've also noticed that some companies will sell a 3" plastic shell, with an actual o.d. of 2 1/2". It seems like this shell isn't going to go very high. I don't try to reinforce my plastic shells...

Is there an ideal tube i.d. for, say a 2 1/2" o.d. shell? The club that I'm with has 3" launching tubes, what is the best final o.d. of a shell for this size tube?

Thanks for sharing...

[Mortartube]

I have used commercial shells that needed to be pushed down into the tube with a stick, (a sensible amount of force naturally) and have had some with an apparently large gap. Snug fit is best IMHO.

If you have a good mortar it shouldn't burst, the shell should always move.

A friend of mine (many years ago), accidently once left a large metal "Arrow Brand" stapler lying on its side, on a 3" rack and the rack got fired. The stapler was bent into a semi-circle.

I think given the amount of force as illustrated by the stapler, the shell would most likely flower pot if bottom fused rather than stick inside a good mortar and burst the tube.

[mike_au]

I forget who said it but I remember reading that "as long as you don't have to jump on it you are all good".

Ideally the shell should fall *slowly* to the bottom of the mortar as the air forces its way around the edge. If you have to encourage it a little bit, that is OK too (be careful not to put your hands over the mortar). If it drops quickly to the bottom then you will lose height as the lift gases will escape around the shell.

As long as the shell does move relatively smoothly (and your mortars are in good condition) you shouldn't have any trouble with them bursting.

[knackers]

OH dear,, my so called 3" shells are only 2.5" OD... maybe some serious pasting is in order ! ( perhaps with thick heavy duty cardboard ) or maybe bumping up my lift big time ? like 50% even,

what would be the safest way to rectify this problem.... ( is it a problem ) ?

[Mortartube]

Are you completely sure that you don't have 2.5" shells sold to you as 3"? Do they have any size marking on the label?

You can wrap card (pizza box type seems to work well and brown tape around the perimeter of the shell to mke it a better fit. I have done this in the past. It's a pain but it seems to work (Tape it tightly) and is a lot more reliable than playing with lift ratios etc.

Edited by Mortartube, 11 March 2009 - 10:10 AM.

[yorkie]

Are you completely sure that you don't have 2.5" shells sold to you as 3"? Do they have any size marking on the label?

You can wrap card (pizza box type seems to work well and brown tape around the perimeter of the shell to mke it a better fit. I have done this in the past. It's a pain but it seems to work (Tape it tightly) and is a lot more reliable than playing with lift ratios etc.

Why not just buy some 2.5 inch mortars?

[knackers]

Are you completely sure that you don't have 2.5" shells sold to you as 3"? Do they have any size marking on the label?

You can wrap card (pizza box type seems to work well and brown tape around the perimeter of the shell to mke it a better fit. I have done this in the past. It's a pain but it seems to work (Tape it tightly) and is a lot more reliable than playing with lift ratios etc.

yes yes yes yes Nooo, not completely sure, the actual size is 2 5/8", i ordered 4'' and 3" and thats what i got, they were in a clear plastic bag with no description, i don't know if the id of a 2" mortar is exactly 2" or a nominal bore size,, i got a handfull of cardboard tubes from a local factory, they are the inside core of that industrial pallet wrapper plastic.. 12 mm wall, 20 " long and 75mm id.. will that 100 mph tape ( grey pvc duct tape) wrapped around it be ok, ? or will that just muck up the symetry of the burst pattern,. i don't have that many of them and can't really be bothered hunting around for 2.5" mortars if i don't have to.

Edited by phill 63, 11 March 2009 - 10:46 AM.

[cooperman435]

Ok Phill IM not sure where you got these "un labled" shells but they sound bad news mate.

Id advise firstly ensuring the tubes your intending on firing these from are up to the job. Mortar tubes are not just cardboard tubes though I will add that pallet wrap tubes have been used many many times in the past to good effect.

2 5/8" shells will fire perfectly from a 3" tube and I've personally fired worse fitting shells than that without problem in the past. Id advise strongly against extra wrapping of anything or increasing the lift as these are commercial shells.

[Bonny]

When loading the commercial shells at shows I've worked, the 2" shells almost always need to be pushed down into the mortars, (some needed quite a push) where the 3" and larger drop freely to the bottom.
My homemade 3" shells usually finish with an OD of about 2-3/4" and shoot fine from my cardboard mortars that measure as large as 3-1/8".
I've also fired some 5" shells that had close to 1/4" gap all the way around and they were fine.

[Fred]

Is there an ideal tube i.d. for, say a 2 1/2" o.d. shell? The club that I'm with has 3" launching tubes, what is the best final o.d. of a shell for this size tube?

Takeo Shimizu discusses the size of the gap between shell and mortar in his book „Fireworks from a Physical Standpoint“, first edition 1976 in german language „Feuerwerk vom physikalischen Standpunkt aus“: Chapter 12 „Die Ballistik der Feuerwerksbombe“ (12.3 Aussenballistik, part 5 at page 186).

Shimizu gives a formula:
If „diameter of shell²/diameter of mortar² ≤0,5“, the gap size is too much.

Table:


Shimizu used to shoot shells with ratio between 0,83 und 0,90 (page 188 table 35).

Edited by Fred, 11 March 2009 - 08:43 PM.

[mike_au]

When you say you got unmarked shells in a plain plastic bag, do you mean shells or hemis?

I had assumed you were talking about home made shells using bought hemis, but I think cooperman has assumed bought shells.

[jeffrocket55]

I forget who said it but I remember reading that "as long as you don't have to jump on it you are all good".

Ideally the shell should fall *slowly* to the bottom of the mortar as the air forces its way around the edge. If you have to encourage it a little bit, that is OK too (be careful not to put your hands over the mortar). If it drops quickly to the bottom then you will lose height as the lift gases will escape around the shell.

As long as the shell does move relatively smoothly (and your mortars are in good condition) you shouldn't have any trouble with them bursting.

unless you use to much benzolift

[knackers]

When you say you got unmarked shells in a plain plastic bag, do you mean shells or hemis?

I had assumed you were talking about home made shells using bought hemis, but I think cooperman has assumed bought shells.

thanks mike ,for identifying my bad description.... i did mean hemis, empty plastic hemis,

sorry phill cooperman for a misleading post,........ i won't make that mistake again

regards phill

[Mortartube]

If you have a few hemis. Just make an inert one filled with sand. Spray it with bright orange paint (helps recovery) and do some test firings. You could put the different lifts in plastic bags and just push them to the bottom of the tube with the inert shell on top., and fire.

[knackers]

If you have a few hemis. Just make an inert one filled with sand. Spray it with bright orange paint (helps recovery) and do some test firings. You could put the different lifts in plastic bags and just push them to the bottom of the tube with the inert shell on top., and fire.

i'll do that this weekend, thanks