Please excuse the long post, but this subject has it's hidden alleyways, oddly enough the more you dig the deeper it gets.....and this stuff is interesting
After doing some more research I have to conclude that the one consistent thing about black powder is that there is no consistency , this is something I should have already been aware of, because black powder more than just about any type of propellant or explosive, has many things that weigh on it's performance,and alter it's state of entropy ( communication theory) a numerical measure of the uncertainty of an outcome.
Windowed combustor measurements yield a low-pressure exponent (0.164) for the burn rate of black powder at pressures greater than 0.3 MPa. At pressures up to 13 MPa there is no evidence of combustion-induced de-consolidation. ( this basically means that it DOES NOT appear to have the ability to fracture it's own grain size half way through the barrel becoming a finer faster burning powder.
IE: C-4 is a high explosive regardless of where you put it,and has a fairly consistent detonation velocity , where black powder on the other hand goes through serious morphs of capability.
Black powder is a low order explosive only when its contained, and it's a propellant when not. In it's free form state it burns at about 500 mpc, but once contained that all changes,and those changes are as different, as the many other variables used in cannons, like barrel length, bore diameter, load size, load compression, particle size, projectile weight, and projectile clearance, all of these things effect the behavior of the powder,and then we have environmental intruders like, temperature, humidity, even barometric pressure has an effect on the burn rate of black powder,as well as the temperature of the cannons metal itself
After reading all of our posts it will probably be easier for some to see why this is very much "alchemy" not chemistry, and not an exact science at all, this is what makes all of your standards and cannoning safety rules about using tried and true examples to create your cannons most valuable,because its the practical experience of all of you, and those before you that lay the groundwork for the rest of us.
In my research on these dynamics I found this writing, I think you'll find interesting.
Ideally the powder grain should burn progressively, with continuously increasing surface, the grain being completely consumed by the time the projectile leaves the bore
In 1860 Gen Thomas J. Rodman of the (U.S. Ordnance Department) began to tailor the powder to the caliber of the gun. The action of ordinary cannon powder was too quick. The whole charge was consumed before the projectile had barely started on it's way,and the strain on the gun was terrific.
Rodman compressed powder in to disks that fitted the bore of the gun being used. The disks were an inch or two thick,and pierced with holes.
With this arrangement, a minimum of powder surface was exposed at the beginning of combustion,but as the fire ate the holes larger,the burning area actually increased,producing a greater volume of gas as the projectile moved forward. Rodman thus laid the foundation for the progressive burning pellets of modern powders.
I think if you read between the lines here you can see the advantages of reduced sized powder chambers, for one it does allow for greater metal thickness in this area giving greater strength, but also having a reduced size powder charge /compared to the bore diameter exposes less powder area to the flame front,and thus slowing the burn, in an attempt to achieve complete ignition, in a proper progressive state as opposed to an "instantaneous flash".= (added barrel stress)
So if we take what has been written all the way back to 1800's till today , it appears that maximum cannon barrel and powder performance, would be achieved by customizing everything to coincide with the actual cannon being used, because the cannon in this equation is about the only thing consistent, and as far as, (bore/projectile clearance) a consistent number for a given cannon should "always" be used, if one doesn't want to keep making powder charge or grain size alterations.
It appears that , these loads, the powder charge and projectile would need to remain the same to achieve consistent results from a specific cannon, this would even include how well stabilized the cannon is in it's mount,as this too has it's effect on the result , I guess this means that in the cannon world Wheels are good ,and as far as mortars go, the bigger bore diameter allows the projectile to leave much quicker not giving too much time for pressure build up,and the reduced bore stress is the only thing that makes it safe to use a no recoil ground mount , like most mortars have.
So I guess to sum up what I'm thinking now on the subject of black powder as it pertains to barrel stress is, it all depends on the barrels length, diameter, what the best charge would be to achieve a complete non explosive burn, and while I'm sure many of us will or have varied powder charge amounts and particle size for a given piece, there would be one that is optimum,and that's why joining forums like this is so important, as I will be able to gain the knowledge of those with actual experience, and have tested many different powders with many different barrels, and have logged those results, it is through those logs that safety is achieved, and I would just like to say thank you to all before me who singed themselves to char in an attempt to achieve consistency.
And I actually thought making the transition from paper mortars to metal ones would be fairly easy, what in the world was I thinking ?
Many of the variables are the same though, use what you know,and fear what you don't, because unlike the real world, in this field it's what you don't know that will kill you... thanks again for the spirited posts I feel like I have landed in the right place for a long and safe enjoyment of cannoning.
Tonight I start my cad drawings for the machinist, I have decided on small bore 1" to 1 1/2" mortar.
My plan is to keep it simple ...making it more to the style of a signal mortar or line throwing mortar, of course keeping it in the pre 1899 replica styles available. I don't think I will make it for using projectiles though, I think I would feel better making this first one for blank loads, to help me get use to the repetition of loading / cleaning and firing....procedure.
I believe I will use a metal that lies between 1018 CR and 4140 and limit its length to between 10" - 12" inches. Fitting the trunnion to the breech end since targeting wont be an issue and I will have no need for adjustments.So if anyone could instruct me on what is required to confine this charge IE: wadding I would greatly appreciate it, and as soon as I have designed the mortar I will post pictures prior to handing them over to the machinist, so those more informed than I can critique them for stability,and safety.....
Thanks again to all of you, for your help, and input, It is greatly appreciated and sorely needed.
