12 replies to this topic

[michaelp]

Hi guys, i have made a two inch mortar tube from a water filter cartridge, its just over a foot long with a 1/4 inch wall thickness, i have built a chamber for the BP by drilling out the centre of a plywood plug / base.

I have been carrying out tests using a ball made of crushed silver foil which weighs 7 grams, i have started the lift charge at 1.2 and progressed up to 10 grams which propels the ball to about twenty feet, my question is what clearance should be between shell and the tube wall, which i presume if tight would increase gas pressure. all advice very welcome.

Edited by michaelp, 29 November 2012 - 08:49 PM.

[Deano 1]

Hi Michaelp, this is my opinion only by some of my own experience. First of all get a heavier ball, I used to make 3" shells and they averaged out at 65 grams,(edit this, I got it wrong, my 3" shells averaged at 160 gram) so I'd say a 2" shell should be around 40ish (edit 100gram), once the projectile leaves the tube, it's the weight that carries it to its hight. As for the fit, you want it as snug as possible, if you have to tap the tube gently on the floor for it to drop in, thats snug, if you have to push it down with a stick, thats too tight and you risk your tube blowing up. I would use around 10% of the weight of the shell for my lift charge, but if your shell is extra light you will need more lift charge to get it up there. You want to be aiming for at least 80 feet.
Hope this helps, stay safe

Edited by Deano 1, 30 November 2012 - 10:14 PM.

[michaelp]

Thanks for that, very informative especially around the snug fit, i will come back and let you know.

[Sparky]

I make my dummy shells by adding rice as the filler. It's almost the perfect weight to replace a break charge and stars.

[michaelp]

All good information, thanks, the tube is possibly HDPE, its a Brita water filter tube with the ends cut off, i plugged the bottom end with a 54 mm deep plywood plug made from three 18 mm cut from a sheet using a hole saw, after making a first hole i drilled all three without a centre drill, then drilled a "chamber" 36 mm deep.

Once glued into place i drilled a 3mm hole 36mm from the bottom so the fuse enters the chamber midway.

My thoughts on this is with the shell sitting on top of the chamber most pressure will be initially created within the chamber, i hope this all makes sense lol


The tube wall is about 1/4" thick

Edited by michaelp, 30 November 2012 - 10:05 AM.

[Sparky]

Hi Michael

Why are you building it so that you have to fuse it like you describe? It would simpler to build the fuse so the lift charge is attached to the shell. Then it's a simple matter of dropping the shell down into the tube but making sure you have a long enough quick match leader once it's all the way down (with some visco too). The combination of lift cup (for your BP) and the nice fit of the shell in the tube should be more than enough confinement to get a good kick from your BP.

I regularly lift my 3 inch shells weighing 160g with 20g of my own BP in this method and they have a reasonable gap between shell and tube which allows the quick match to pass round the side and my BP is definitely not the best. I use old toilet roll tubes for the lift cup but I have also made cones with some heavy kraft paper.

I buy paper mortar tubes at the moment but will probably switch to something more durable when I am more confident about my shells not flower potting. I just don't fancy shards of plastic, fiber glass etc flying about if it all goes horribly wrong

[michaelp]

Hi Sparky,
Fair question, my initial use for it will be new years eve so preparing for inclement weather i did it like that to back feed e-match tails in so i could fire it electrically from inside the house and have champers at the same time, not having done it before i didn't like the idea of wires fowling the shell exit, thinking about it now they probably would not.
Lol i will call it the Mk 1

[michaelp]

once you put the shell all the way to the bottom it don't matter how much wires / quick match even very tight fitting the shell will shoot out of that mortar tube without any problems. a 3" shell will shoot through a sheet of 20mm ply like a bullet.

good luck m8

Thanks, I will drop it down the top, and fill the fuse hole, a few more dummy rounds to go first to check the height, i do sometimes feel a pratt watching some inert object going up in the air, thank god for semi retirement.

[Deano 1]

I've edited my reply, I got my weights wrong, soz

[michaelp]

once you have it just right you will feel so much better, much better than not having it right and using a live shell, (watching it not go up very high.........come down change of underpants moment lol)

Ok continuing to play i have increased the "shell" weight to 47grams using the top of a silicone cartridge (about 2/5 inches length from the nozzle end and filled it with tissue and semi cured silicone, I then wrapped carpet tape to increase the width for a snug fit, with devils horns on my head I i loaded the tube with 5 grams of bp, ....i got a nice little "pop" and about 50 ft height, so next i will try 100grams weight and 10g bp.....good fun