howitzer wall thickness

[T Brooks]

Hello Everyone

I have been thinking about making a mountain howitzer in 75%scale.

I have roughly drawn the barrel in SW (picture attached).  I have made sure that I have at least the same wall thickness as the powder area bore.  What I am not sure of is how thick the wall section should be after this zone as the bore transitions up to the full bore.  Can anyone give me some recommendations for wall thickness after the transition?

Initially I used the mountain howitzer website (you know the one with the deer hunt) and a cross section view of the original bronze mountain howitzer as a guide.  I am planning on using low carbon hollow bar with a yield of at least 400MPa or 58000psi.  This is slightly stronger than 1026 HR seamless steel tube.

I can do the bursting strength calculations easy enough but the thing I lack for this is what the internal pressure could rise up to.  Does anyone know what kind of pressure might be seen inside the barrel?  The best thing I have found was a website suggesting that for a rifle the pressure might be up around 15-25kpsi and for a shotgun around 5000psi.

Cheers
Tarquin

[GGaskill]

... but the thing I lack for this is what the internal pressure could rise up to.

This is the eternal problem for doing design by calculation for muzzle loading artillery; no credible pressure values.  I use 20,000 psi in my calculations but that is more a guess than a fact.  15-25Kpsi is also reasonable; the lower with low mass projectiles, the higher with high mass projectiles.

But we really are guessing.

On the full scale US M1836 (or so) mountain howitzer, the wall just forward of the chamber is 1.114" and just behind the muzzle ring is .837".  And it is bronze, not steel.

[Double D]

You would be well served to get a set of drawing for an oprignal gun.  It appears you are coping the  Bux stik gun.  That is a very poor design to build.

[T Brooks]

Thanks Double D

Which aspect(s) do you consider to be poor design?  The welded in breech plug?

I must admit I wondered about that area myself and if the welds only on the end would be acceptable especially with the interface between the plug and barrel not being connected around the whole circumference.

Cheers
Tarquin

[Double D]

Absolutely.  There much  better ways to fasten a breech plug.  You are going to need a top notch welder to get welding done right, and then you are going have heat treat after the welding to anneal the metal.  There is far more weld in that design than needed.



 Take a look at our safe loading and cannon plans and look for the link to the  N-SSA rules and you will find instructions on how to do a breech  plug. Shrink fit and pinned or shrink fit and weld.  But only minimal weld.

Shrink fit stops the migration of fouling between the walls of the tube and the breech plug, which leads to corrosion. 

[sseib]

Tarquin,
 
This is the design I used to build my howitzer. I have been shooting a 4lb  - 3" solid steel ball with 1-1/2 oz FG powder. The sub-chamber only holds about 2-1/2 oz for blank charges. I have also with some success shot 3" dia x 4" long aluminum round bars and some 3" dia x 6" long hardwood plugs.
 

 
I ended up making two plugs, tried .010" press but it only shrank about .007" with liquid nitrogen. I made a new plug with .005" press and it shouldered out without any problems. I work at a machine shop and had a certified welder do the pre-heat, weld and post heat.
 

 
I also had the trunions welded on before boring the barrel.
 

 
If you are interested I can send you my CAD file and you can scale it to the size your wanting.
 
Steve Seib
 
 

[Cat Whisperer]

Good discussion.

Let me emphasise:

Low carbon steel (1018 and somewhat more) = good;
 
press fit = good; 

rounded inside corners (lowers stress risers) = good;

(wall thickness around powder chamber = diameter of powder chamber) = good;

certified welder = good; and

welding that close to the powder chamber?  (first design pix)  NOT good.

[T Brooks]

Thanks for all the replies.

Steve that would be great if you could send me your files.

Double D, I read some of your posts in this, and the practical machinist forums and understand your point better now.

I will modify my design with the improvements discussed.  I will also read up a bit more on the vent liner concept and how best to incorporate one.

I design elevating work platforms for a living our workshop has some very skilled and certified welders that I can get to weld.

So, if I take the assumption that the pressure in the barrel will reach 20,000 psi in at least the chamber area, is it also fair to assume this pressure will exist further down the barrel?  In other words do I need to ensure the entire barrel can withstand 20,000psi?  or will the pressure be less given the rapid expansion in bore from the chamber out to the bigger bore?

Cheers
Tarquin

[GGaskill]

... is it also fair to assume this pressure will exist further down the barrel?

No, the pressure will decrease as the volume increases and heat is lost to the barrel, in addition to that which escapes through the windage around the shot.

[Parrott-Cannon]

If foks are interested I can write a program to make the nessary calculations.  It would take several months.

[Cat Whisperer]

If foks are interested I can write a program to make the nessary calculations.  It would take several months.

That would be most interesting.

My fear (as you've heard/read me say before) is someone not taking into account the need for the material chosen to withstand the repeated hammerings of firing - resulting in harder/more-brittle material - leading to cracking and failure.

That having been said (once again) it would be most interesting to compare the results of the program with some of the vintage designs.

It would be interesting to see the ratio of strengths required of these designs - comparing the 1/3 rule around the powder chamber to the thicknesses employed further forward and the pressures found in each general location.

[T Brooks]

Hello Everyone

I have modified my drawing a bit to take in as many suggestions as I can.

I have also attached the bursting calcs and weld shear stress calcs.  Could someone please give me a sanity check?  I have used Birnie's formula for thick walled pressure vessels without longitudinal forces i.e. open ended tube.

The chamber will start to fail (yield) at around 42000psi and the bore (just after the insert finishes) will start to fail at just over 20000psi.  This is without safety factors added on.  Is this reasonable or would it be preferable to have greater strength.  It would be relatively easy to change to a thicker wall tube.

Cheers
Tarquin

[GGaskill]

This is without safety factors added on.

Are you planning to add safety factors?  Do you have a supply of projectiles that are a good fit for your bore diameter?  Usually we choose bore diameter based on what we can use for projectiles.

[Double D]

Reasearch that chamber shape, I don't think it is correct...the corners look too sharp

[T Brooks]

Hello Everyone

GG, my thinking is that if I assume a likely max chamber pressure of 20kpsi then I would have a factor of safety of 2.1:1 . As we discussed earlier in the thread it is difficult to know what the pressure might get to.  I have been working loosely to the attached image of what I believe is an original howitzer barrel.  The one in the pic does not even come close to 1 bore dia as wall thickness and would likely have been a cast barrel and perhaps bronze.

Do you consider 20kpsi a worst case and unlikely to happen or a likely pressure each time a shot is fired?  Then of course there is the question of what grade of powder to use for this volume and diameter of chamber.

It is a good point about the projectile dia.  At this stage I only intend to make smoke and noise but at some point I will probably want to make a hole in something.  If I step down a size or two of the barrel I could fire a petanque boule.  They are around 78-82mm.  I am not so sure about firing steel solid round as it is quite elastic and could bounce back.  I could make a mould for lead.  Does the lead ball need to be solid?  Can it be filled with something?  I know I read in one of the publications linked in the construction guide that exploding shot or things that might become shrapnel are not permitted, obviously.  Perhaps I just answered my own question?

DD, you are correct I haven't got the chamber shape right at all.  Is the one in the attachment OK, is the one earlier in the thread by "sseib" OK?

I appreciate all the feedback on the design before committing to steel.

Cheers
Tarquin

[Double D]

That's the shape I was thinking off.  That is another original gun that doesn't meet the 1 caliber wall thickness.
 

[armorer77]

I use the same prints . BUT , I have reduced the powder chamber to 1" dia. for 1/2 scale . Ed