BP Granulairity Question

[User64]

A question that has been eating at me the last few days:

 

What is the purpose for using different granularities of Black Powder ?

 

I've seen lift charges of rather large grain BP almost shiny coated and smaller grain flat black BP. Also other charges of fine grained BP which I'm guessing is to help as a booster for ignition of a stage.

 

What I'm wondering is what is the purpose of using different granularities in each stage ?

Edited July 26, 2013 by User64

[Mumbles]

The shiny stuff is coated is graphite, and is almost totally restrained to purchased BP used in reloading firearms.

 

The purpose of using different sizes of BP is to control burn rate. A finer grain will burn more rapidly. Very fast deflagrating BP is useful for lifting and breaking smaller shells. There is a limited amount of space, so you want to get the most out of it. Larger shells typically are lifted and broken by a larger grain BP. This is to temper the burn rate some. It's not difficult to overpressurize a large mortar and cause it to rupture or damage the shell. A more gentle lift does the job without the danger. The same can be true for inside the shell when it breaks. A sharp explosion, as you'd get from a fine grain BP, can cause the shell to break unevenly or prematurely. A slightly slower burst charge allows the entirety of the shell to become full of fire and pressure and break more evenly. This is one of the big reasons that flash broken shells are hard to make look as nice as traditionally broken shells.

[pyroman2498]

i just learned something new, thanks mumbles

I thought you could lift bigger and smaller shells with 2fa? Correct Me if i'm wrong here.

[Nessalco]

I thought you could lift bigger and smaller shells with 2fa? Correct Me if i'm wrong here.

 

I've used 2FA to lift smaller shells (3" for example) but you need more powder for the same effect. For 2" guns and smaller, I've always used 4FA, and have never really tried 2FA.

 

Kevin

[Mumbles]

You can lift most sizes of shells with 2FA. As Nessalco said, you just need more of it. I've also had sort of mixed results using it in shells smaller than 4". It almost seems like there is a critical mass or critical pressure with regard to BP. The larger the grain, the larger that mass or more work it takes to get to that pressure. I liken it to something like self-confinement as seen in flash powders. You can burn a small amount on the ground, and it will just go poof. If you burn too much you get a bang. I've noticed a similar phenomenon in BP. If you don't use enough, the shell can kind of anemically flop out of the mortar. A couple grams more, and it's being lifted to a safe height. This effect seems to be less of an issue with smaller grains, as they can get up to pressure faster.

[User64]

Thank you for the explanations, very helpful.

 

Much more involved than working up the perfect charge for a rifle or pistol. Right now I'm at the stage of wondering how one even goes about picking a certain granularity and charge weight for the lift or break. Interesting science.