Barrel wall thickness for mortars?

[jamesbeat]

For cannon, we say that the barrel wall thickness (at least at the chamber) should be one caliber.
On mortars, the 'barrel' is the chamber itself, so this should obey the 1/3 rule.
What about the actual barrel, ie. the short tube that holds and guides the projectile?
I see that, for example, bowling ball mortars have very thin barrel walls, but understand that this doesn't matter because of the pressure drop.

There must still be some pressure though, so what is the rule of thumb for calculating the wall thickness?

What about howitzers?

[GGaskill]

The continuing problem of design by calculation is the virtually total absence of reliable pressure figures.  The calculations themselves are pretty easy and there is even a website with a javascript calculator.  But without good pressure values, the calculations are only as good as the pressure value guesses.

[Double D]

George has given the best answer. 

If you are concerned about the problem, you can use the original drawings and build to scale.

[Cat Whisperer]

Good question.  Not an easy one.

It's obvious that the pressure levels are different.

There are several different styles of powder chambers as well.

Time may have something to do with the equations as well.  With a mortar the time in the barrel is very short compared to the time a cannon ball spends traveling a much longer length.

Loading (type and amount of powder), size and shape of the powder chamber (affecting location of pressure [wider or more narrow], and the mass of the projectile as well as the windage (clearance) all have an effect.

So, where does one start?

Start with what we know - the rule of thumb of wall thickness equal to diameter of the powder chamber.  There are other nominal diameters that folks have observed about the minimum thicknesses towards the muzzle end of cannons as well.

So let's say we come up with a reasonable guess of the pressure.  The strength-pressure formulas that are out there are generally for constant pressure - might be a good idea to crank in a factor for the repeated hammering - that relates to design of the tube, materials used and how they are processed.

With golf-balls there is very little pressure.

[Cat Whisperer]

That's the short answer - build to scale of a SUCCESSFUL design.

[GGaskill]

Several years ago when we were in the midst of an active discussion of "handgonnes," I purchased one of the Oehler pressure measuring chronographs but I have not yet made one pressure measurement with it.

[jamesbeat]

I'm not building or designing anything, it just occurred to me that I'd never come across a rule of thumb for this so I asked out of idle curiosity
I've also never heard of any mortars going kaboom either, so I guess you can get away with a pretty thin wall.