2,5" pasting layer thickness problem

[klachner]

Hello everybody,

 

I have a big problem with pasting 2,5" shells, because after pasting 5 layers they are still too small for my mortar. The 2,5" hemis I bought are 54mm in outer diameter and my 2,5" mortar has 64mm inner diameter. After pasting the 5 layers the shell is still only about 58mm in diameter, so the shell has too much tolerance in the mortar, about 6mm. I read 2mm tolerance for this caliber is normal, and it sounds much more reasonable to me than 6mm. But pasting until the shell has only 2mm tolerance would mean 10 layers for a 2,5" shell? Do I use too thin gumtape oder should I paste more layers?

Hope you can help me

Edited March 8, 2016 by klachner

[Bobby]

I believe that you are fine with the 6mm clearance. I would think that 2mm clearance refers to the radius, not diameter. 6mm clearance for the diameter is 3mm for the radius.

 

I also don't think you need to paste 10 layers to get to 2mm tolerance. A couple more should do.

 

I am sure more experienced members will chime in.

[Mumbles]

Do you have the space or capabilities to fire a dummy shell? That'd be the most effective way to judge your clearance and any effects it may have. Filling a shell with cat litter actually pretty closely approximates the mass of a shell. 6mm is a little larger than I'd personally prefer. I'd rather be in the 3-4mm range, but you're not far off either. This really isn't a question of if it will function or not. It'll work no matter what. It's more of a question of being able to safely get it to the correct height. This may require more lift, or more pasting, and probably a little faith.

 

What are you using for a burst? In general, I think you're getting to the sweet spot of pasting. 5-7 layers should probably do it. I'm just trying to get an idea of how prone this shell would be to over pasting. Some bursts, like BP aren't too bad in this regard. Some bursts like KP are more sensitive to confinement and can go from great to salute in a matter of layers.

[Arthur]

If it were my shell, I'd try to add one layer, then try it then try another layer. I'd be looking for 6 - 9 layers.

 

Ultimately it's your shell and your design. more pasting makes the burst harder, bigger, and probably rounder, but too much pasting makes it very loud with the risk of unlit stars making the break look patchy. Add into all that, the fact that a loose shell will rise less than a well fitting one.

 

I doubt that there is a single correct answer, it's really all down to you, and for you to find whether you like the look with 6, 7. 8, or 9

 

My start point is 3 layers per diameter inch, start there and make your choices.

Edited March 8, 2016 by Arthur

[mikeee]

Has anyone ever tried installing an inner sleeve inside a mortar to take up the space.

If you had a round mandrel the size needed for your shell, you could wrap layers of paper

around it until it matches the inside diameter of your mortar, tape the outer paper to keep it

from unraveling. Remove the sleeve from the mandrel and tape the inner splice of the paper

to keep it from unraveling. Wet the inner and outer layers of paper with a resin and let it dry

this would hold the layers of paper together and allow you to drop the sleeve into the mortar

and shoot your undersized shells.

 

A polymer sleeve could also be turned on a lathe to the correct wall thickness that could be dropped

into mortar to take up the space.

[MadMat]

how about creating a soft sabot of sorts with paper... the loose wrapped paper falls away from the shell as it leaves the gun and.... viola! The only problem with this idea is the required cleanup afterwards (you don't want to be a littler bug) Also, you would want to make the paper fireproof to avoid starting a ground fire, treating the paper with water glass (sodium silicate solution) would accomplish this. I imagine there are other ways of making paper fireproof as well (some may be a bit cheaper as well)

Edited March 11, 2016 by MadMat

[Mumbles]

Ammonium salts tend to be good at preventing paper from lighting on fire or smouldering. This should be obvious, but ammonium nitrate and perchlorate are probably not good choices. Sulfates or phosphates are the most common choices. Wouldn't waterglass treated paper get stiff? I've never used it on anything I intended to move again.

 

A spanish style ball shell dry wrap might help to seal off the bore and generate more back pressure.

[mikeee]

How about wrap a little aluminum foil around the ball shell to add some dimension to the shell,

you would not have to worry about burning paper falling to the ground.

[MadMat]

Hmm I never even considered aluminum foil. I imagine that would work and be a bit more trouble-free than my paper idea. As far as water glass making the paper stiff, yes, a bit, depending on the concentration of the solution. But I thought the stiffness would be an asset in creating the sabot.