Cardboard tube mortars

[ivars21]

Hi,

I just got 4" (100mm) ID cardboard mortar tubes for free (7mm wall), I was wondering what I can do to cheaply strengthen these mortars? I know, that I can waterproof them with Sodium silicate, but what to do to strengthen them?

I could use fiberglass but its quite expensive (because I have 10 of them).

 

These are spiral rolled from carpet rolls.

 

Thank you

Edited September 29, 2012 by ivars21

[Peret]

I would be very wary of using spiral cardboard tubes as large as 4 inches for mortars. A 4 inch shell generally needs around 40 grams of lift, which is a considerable amount of BP that produces a lot of pressure, hundreds or even thousands of PSI. Remember that pressure is per square inch, then consider that a large diameter tube has a lot of square inches, and pretty soon you see that it's asking the tube to support the weight of an elephant. Think about blowing up a balloon - it's hard to get it started, but once it starts it gets easier and easier as the pressure acts over a larger and larger surface. I don't think any practical strengthening can make these tubes safe.

 

You could perhaps make awesome fountains, or cut them into short lengths and use them for star mines.

[ivars21]

Thank you for your answer, but I have fired from unprotected 4" spiral cardboard mortars like 3 times with a success. I just wanted to ask is there anything else I could do. Edited September 29, 2012 by ivars21

[sparky]

Maybe put them in sand something taller and wider than a 5gal bucket lot of pissin around,just get good mortars and be all set and no worries.

[Peret]

Thank you for your answer, but I have fired from unprotected 4" spiral cardboard mortars like 3 times with a success. I just wanted to ask is there anything else I could do.

Well, it's your funeral. Burying them in sand would definitely give them some necessary support. How do you close the end?

[mikeee]

Fiberglass would be your most cost effective way to strengthen your cardboard tubes.

You would need to use a modified epoxy resin which will not shatter like the standard resin.

Several layers applied one at a time, and make sure you get all the air bubbles rolled out.

Try to find a fibgerglass shop in your area, they can sell you the fiberglass matting, resin & catalyst.

You should also roughen the surface up to improve bounding of the resin to the cardboard.

You could also use the resin to help bond and seal the wood plug in the bottom of the tube along with screws.

 

Mikeee

[Arthur]

Real Chinese mortars are so cheap in the UK that it's hardly worth buying anything else. Yes you could wrap your tubes with resin and glass (or carbon fibre) but the point of failure may well be the inside layer of paper simply coming unstuck each time you fire, getting bigger each time

[ivars21]

I use wood plugs, screwed in the bottom of tube + some glue. The thing is that I have already tried one of them and shot 3 canister up with a success, the only thing is that inside the tube one layer of paper has wrapped of. So I am asking advice what to do to straighten/make them long lasting tubes. I will go shopping today so I will let you know what I will find. And I have already 4" fiberglass mortar, the problem is that you can fire only one firework at time, it's quite difficult if you are preparing home-show.

 

Thank you for your advices.

[taiwanluthiers]

Is it that a fiberglass mortar tube needs to cool down? Not sure how its different from paper tubes too, however if its a number thing (as in you need a tube for each shell) then its all the same. Fiberglass tubes are cheaper from what I understand.

[bob]

it's just that he has only on fiber glass mortar and so needs more mortars and no fiber glass is not cheaper the paper just think about it

bob

[Mumbles]

Treating the tubes with both sodium silicate and calcium chloride is generally considered to be one of the best ways to treat cardboard tubes for longevity. From the old timers who have told me about it the calcium and sodium combination treatment is more resilient than just sodium silicate. It will prevent any lift residue from soaking into the paper, which is the cause for delamination sometimes.

 

http://www.pyropage..../cardboard.html

 

It will depend on the quality of your tubes, but I've seen some 4" spiral tubes that seemed to be quite strong. At 7mm, they should be able to hold up to normal shell usage, even if they have spiral walls. Trust me, I've tried to get away with much less, not that it's at all recommended. While not ideal, for 4" guns you should be alright with spiral tubes. Burying the guns will certainly help enhance the strength. If you can swing it, you may want to save the cardboard for single break shells and mines and stuff and the fiberglass for the heavier shells. I've fired shells upwards of 10lbs from fiberglass guns before, though they were well buried.

[ivars21]

Thank you Mumbles, I was wondering, can I use Ca(OH)2 instead of CaCl2? In reaction it still should perform CaSiO3 (if I am not wrong). Because Ca(OH)2 is more available in my country.

What I am going to do - I will treat my tubes with waterglass + Ca(OH)2 to perform CaSiO3 on walls, than I will plug my mortar and test the strength. Will try various weights of canister dummy shell (like 500g, 1000g, 1500g) and look where the breaking point of tube is.

I will take some videos as well.

Thank you guys again.

Edited October 2, 2012 by ivars21

[Mumbles]

Calcium hydroxide has very poor solubility in water. You clearly understand that the idea is that you impregnate the walls with calcium, and the sodium silicate precipitates it out to make a resilient layer. I don't think you'd get enough calcium into the walls to make much of a layer. You could use any soluble form I'd imagine, calcium chloride is just usually the most available. Can you get calcium nitrate? Another option would be to dissolve calcium hydroxide in vinegar or something to make the acetate, which is also soluble.

[ivars21]

Today bought Ca(NO3)2, so I treated my mortar tubes with waterglass and Ca(NO3)2. After few days I will post my results about their strength. Red bath is water + Ca(NO3)2 and white - waterglass + water

Some photos:

Edited October 2, 2012 by ivars21

[ivars21]

Today made few tests with my hardened mortar tubes -

500g - success

700g - success

1000g - failure

So I guess my 300g ball shells will go up without problems.

Might be that 3th test failed because all three tests were made with few minutes break.

[Mumbles]

How did the tube fail when the 1000g load was set off?

[ivars21]

It just broke in peaces. One big peace approximately 1/3 of the mortar (the bottom with plug) which stayed on the ground, then a big peace around 20x30cm which blew apart and then few small peaces. The good thing is, that dummy shell still fired up quite high.

And all peaces were in ~5-7m radius so I guess it is quite OK.

 

Video of 3th test:

http://www.youtube.com/watch?v=ax-hVwLWFto&feature=youtu.be

Edited October 8, 2012 by ivars21

[808goboom]

Yo.i personally wouldn't use cardboard tubes for shells bigger than 3". But my suggestion to strengthen them is to soak the tubes in wood hardener and then once dry, wrap the tubes in duct tape or gorilla tape. This ensures that if or when the tubes blow up it will stop shrapnel from flying everywhere and contains most of the pressure from the lift gas.

[mikeee]

You can buy fiberglass mortar tubes.

If you have cardboard tubes you can use a fiberglass resin and add a layer or two of glass cloth to the outside of the

cardboard tube and brush the resin onto the fiberglass cloth. The resin will bond with the cardboard fibers and the glass

cloth equally. This will make a very strong mortar tube when it is cured out. If you can get a modified epoxy resin that is the

best type of resin to use, it will not shatter under impact, it will stretch a little.