Tubing to make a cannon tube

[powder2burn]

Some more questions:  Why wouldn't this work?

1. Lets say one has tubing that is rated equal to or 50% greater than Switlik's highest ave. pressure of the tests he did some while ago 26,300psi (16 oz. of Pyrodex CTG in 3" Ord. Rifle)

2.The properly affixed breach plug is 1 1/2  or 2x's the bore. ( to the above tubing)

3. That tubing is placed inside a larger tubing,( again with a psi rating of equal to or 50% greater than 26,300 psi,) pressed fit or the larger tubing split and the smaller placed inside it and the larger tubing is then welded together, with it's own breach plug properly affixed.  The 2 pieces are then welded together at  the muzzle  and the ends of the breach.

Switlik's pressure tests, for the recommend load according to the 2oz per in of bore or cannon grade BP or 3 oz of FA, BP, shooting a 4lb 8oz, projectile highest ave. 8'500 psi, 8 oz of BP, cannon grade, firing a 11lb 6oz projectile, 10,500 psi, using FG sporting, BP, 19,700 psi.

Why wouldn't this be safe at it exceeds Switlik's highest pressures?

Same construction but tubing is rated at 26,30 psi

[Rickk]

Get your hands on Lyman's Black Powder Handbook.

Lyman used pressure transducers when they worked up their load tables.

If you flip thru it, you will see some strange things that do not seem to be intuitive.

One thing I saw in the 44 caliber section was that 36 grains of 3F produced significantly higher pressures than 36 grains of 4F - same barrel, same projectile.

Having said that... unless you are going to have a pressure transducer in your barrel, how can you tell if your pressures are less than Switlik's?

An old rule of thumb for engineers (I am one of those) is "if in doubt, make it bigger... preferably much, much bigger".

[Will Bison]

Rick brings up an interesting point that I noted many years ago in the Lyman book. That is that the 4f produced less pressure than the 3f. ??

Some things, as Rick indicates, are not always intuitive at least as we percieve them. So when in doubt overbuild.

Bill

[Cat Whisperer]

I'd overbuild anyway.  Why set the criteria as doubt?  Knowing that it's a bomb or a cannon, overbuild and settle the issue.

[CU_Cannon]

It is important to note that when you put two tubes together the strength is not doubled.  The strength is increased but it is difficult to say how much.  The interactions between the two tubes are complex.  It is very rare to see a pressure vessel made up of two layers.  This is just not done in engineering.  The thickness required for one layer is less than it would be for two layers put together. 

In my opinion building up layers to create strength is a poor design technique.  Once you start adding layers you introduce more variables and more places to run into problems.  I would go with something full thickness from the start and design from there. 

[powder2burn]

Perhaps I was a little premature in making this post as I have not looked up psi's of the different diameters of tubing, wall thickness and metal type.

I know one tube welded over another doesn't double the strength, only adds to it. I guess what I want to know if one piece of tubing, for instance which one of the popular cannon manufactures, uses a  liner of 1/2 steel with a 3" bore, rated at 60,000 psi, then gave another psi figure and an elongation figure. I dont have the catolog in front of me at this time. It seems to me that the liner in of its self would bed enough to make a cannon out of.

The adding of extra tubing over it would be for two reasons, 1. to add to the strength, added safety factor, 2, to add layers to bring cannon dimensions closer to CW era dimensions. Rings, trunnions, castables can be welded on and ground smooth.

[GGaskill]

I guess what I want to know if one piece of tubing, for instance which one of the popular cannon manufactures, uses a  liner of 1/2 steel with a 3" bore, rated at 60,000 psi, then gave another psi figure and an elongation figure. I dont have the catalog in front of me at this time. It seems to me that the liner in of its self would be enough to make a cannon out of.

I will say again that chamber pressure and material strength/stress, even though given in the same units, are not the same thing.  Taking your 1/2" wall by 3" bore tubing as an example, 10,500 psi chamber pressure will induce 37,500 psi stress in the tubing; 26,300 psi chamber pressure will induce almost 94,000 psi stress in the tubing (clearly more than the 60,000 psi strength rating.)  You have to know the math and do the math to use the numbers in a meaningful way.

Depending on the details, an extra layer could more than double the strength, although that would probably be assuming the use of too small of a central tube.  The Parrott design clearly demonstrates that a second layer can make a given design viable.  But in that case, the second layer is shrink-fitted to the first, not just slid over it.

Remember, you only need the wall thickness to equal the chamber diameter in the breech area.  The rest of the tube can be tapered.