shell pasting

[Tanner808]

how many layers of gummed paper tape should i use to paste a 2 inch spherical aerial shell?

i have been pasting them till they fit in my mortar which was usually 3 to 4 layers

[dan999ification]

how many layers of gummed paper tape should i use to paste a 2 inch spherical aerial shell?

i have been pasting them till they fit in my mortar which was usually 3 to 4 layers

 

 

for ball shells i usually paste 2 layers per inch of diameter.

 

dan.

[Bonny]

I've never made 2" ball shells, but using 35 or 40lb paper I would do at least 3 or 4 layers.

[cogbarry]

I do what you were doing, until it fits the gun. Obviously, the number of layers will vary depending on the weight of the paper, diameter of hemis, etc. I just don't like having to guess when it comes to correct lift charge so I want all my shells to fit in the mortars consistently.

[olac170]

Sorry for the stupid question but what does it mean for a shell to fit into the mortar? How easily should it slide in?

I do what you were doing, until it fits the gun. Obviously, the number of layers will vary depending on the weight of the paper, diameter of hemis, etc. I just don't like having to guess when it comes to correct lift charge so I want all my shells to fit in the mortars consistently.

[Potassiumchlorate]

It's actually not stupid at all. Since the density of a display shell is very low compared to a projectile shot from a firearm or cannon, it should, unlike these, not fit really tightly (bullets in firearms actually have a little bit larger caliber than the barrel) in the mortar. Say a tolerance of 2mm in a 2" and 3" and a tolerance of 10mm in a 6".

 

This way some of the gasses pass by the shell by firing and decrease the atmospheric pressure in the proximity of the shell

Edited July 15, 2012 by Potassiumchlorate

[pyrojig]

I have seen much large tolerances for 3 in shells. Up to 1/4 in , and they fire just fine with a avg. of 15g powder . Depending on the powder, I have been able to lift with as low as 10-12g( but that was pretty hot stuff) .

 

2MM is very tight, and wont allow room for the leader fuse to get around the shell. The amount of layers may depend on the type and # of craft your using. Mostly it comes down to the final dia being able to fit in the gun with room for the fuse.

[Potassiumchlorate]

Dr Shimizu says 71-74mm in a 76mm mortar

[Mumbles]

Well potassiumchlorate, feel free to start using 74mm shells and let us know how that goes for you.

 

Literature is great and is an important resource. However you cannot blindly follow it if you don't have some actual basis in practical applications. I assume the ratios given in that table are the ratio of finish shell diameter and mortar to be fired from. If so, Dr. Shimizu needs to work on his rounding in a couple of cases. The value I see most regularly is for the shell to be 90-95% of the mortar diameter. Even 90% is on the loose side of things. This mostly agrees with the above table. A 3" shell should be between 68.4mm and 72.2mm by this other rule.

 

It's sort of hard to describe the proper fit of a shell. You can always measure which will be good. As you lower the shell, a good fitting shell typically slides down easily under it's own weight, but you can feel a little friction and sliding down the sides of the mortar.

[Potassiumchlorate]

Are we grumpy today?

 

I have actually shot a 71mm shell out of a 73mm mortar, since the pipe wasn't originally intended for this. I don't recommend it, though, but it can be done. It's very hard with a thick quick fuse, though.

[dagabu]

I literally have dropped tens of thousands of shells into mortars over the past five years and IMHO, the 5% rule stands. I do agree with Mum that you "just know" when a shell fits perfectly. The way it acts like a plunger, pushing the air out of its way as it falls to the bottom of the mortar tells you it will be a great lift.

 

Tanner, the question of actual layers is a fools game, each batch of paper differs slightly as do all mortars, go for a MAX of 5% space around the shell and you are golden.

 

-dag

[Potassiumchlorate]

Yes, that's the best, of course. I just said that if you happen to have a tight mortar, it is still possible with a very narrow tolerance.

[cogbarry]

I would agree that the clearance should be a percentage. In the machining world it's usually a percentage of clearance for slip fits and a certain amount of interference for press fits. The shell should slide in to the mortar but there shouldn't be lots of extra slop. Don't forget, your quick match will take some room too. The big thing here is really consistency as varying fits mean varying lift heights. That was my point really. I'll add though, that the idea of counting layers never made much sense to me.

[PyroChem221]

I will point out as stated above that tolerance does matter, however the optimum diameter does depend on also the material the mortar is made from. HDPE mortars (most common in the United States) are all made the same way, extruded. (Except for Mighty Mite Mortars which are formed in a mold) and fiberglass mortars (most common in china, and other countries) vary as much as 2mm ID mortar to mortar. This is because fiberglass mortars are wrapped around a polystyrene shaft and turned on it. Then when it's hard the foam is dissolved out using acetone. (At least that's how I've seen them do it in China) so because of the inconsistency in shrinking as the fiberglass sets they tend to differ tube to tube. Generally in an HDPE mortar at 76mm the shell should be 70mm (that's what size sunny shells are) but as far as layers goes it entirely depends on what casings are used and how thick your paper is. For gummed tape generally 2 layers for each inch of shell diameter. The exception being shells under 2 inch where it's automatically 3 layers. Hope this helps.

[braddsn]

Something else to consider, not all shell hemispheres are the same size. I have built and fired hundreds, if not thousands of 3" shells, and I have had batches of 3" hemispheres that are slightly smaller from time to time. Thus, there is more clearance between the shell and the mortar walls. I simply test fire a couple shells and adjust lift accordingly. You can't really "over-lift" the small shells (2 and 3") so I lean towards using more lift when in doubt. Also as a sidenote, and something to make you laugh a little, I was test firing 4's a few weeks ago and learned a valuable lesson. I was using 1 mortar tube and "reloading". This can be safely done if you are careful. I had 4 shells to test, all 4 of them were boosted color shells (hard breaking). I fired the 1st shell, no problem. Cleared the mortar, checked for smoldering debris, all good. Loaded the next shell and lit it. The shell lifted, but the lift sounded wrong, and the shell only hit about 50 ft. Needless to say, I hit the deck and covered my head. BOOM... stars were hitting all around me, luckily I didn't get hit. I was in a big open field, so no harm, no foul. Hmmmmm....... at first I thought maybe I accidentally didn't give the shell enough lift powder, but I am very methodical when I lift shells, and I can't remember the last time I made a mistake. So then I checked the mortar. The previous shell had blown the plug out of the mortar. The plug was still attached and hanging on, but it had blown it downward enough that it was nowhere near air-tight anymore. The physics of this makes sense, but I honestly had no idea just how important it was to have the bottom of the mortar sealed well. Lesson learned.. I now inspect the plug between every shot!