BP Mortar?

[NavyShooter]

Goodday all,

Just curious if anyone out there has put together a Blackpowder Mortar?

I saw this on the web and am interested....something like this would be DAMN cool.

NS

 

[BCRider]

When folks on this forum mention "BP mortar" their assumption is more along the lines of;

I suspect that the charge in the proper mortar shells is formulated to control the pressure in the launch tube. So it's likely very slow burning by black or smokeless powder standards.

For black powder you'd want a much thicker wall since the powder would be loose in the base and it would basically all burn at the same time and generate much higher pressure than such a thin tube would tolerate.

On the other hand if you could figure out how to arrange a suitable charge that burns in a more controlled manner so that it launches the projectile more safely then great. But keep in mind that the form of ignition matters too. A self firing "mortar" projectile takes your BP "cannon" out of the category of not being a firearm or other device.

 

[jpc]

Great looking unit BCRider. Is it yours?

 

[klaus enuff]

I made one and posted a thread on it a while ago.

The timing and location never lined up and thus it remains unfired. I still need to figure out a proof load too.

 

[BCRider]

Great looking unit BCRider. Is it yours?

No, I just yanked it off the interwebz to illustrate the post with an example.

The machine shop is done and I've been doing a few long standing projects already. Got to make a few other things yet like firing pins for a Stevens Visible Loader and a couple of other jobs and then I can look at my planned golf ball mortar. It'll be a sled style mortar and possibly use a solid balk of lumber in a similar manner. I do like the idea of the adjustable elevation though so I'm thinking it would use the other common method of a built up frame. However I do like the look of the iron work with the wood.......

 

[mooncoon]

I suspect that the charge in the proper mortar shells is formulated to control the pressure in the launch tube. So it's likely very slow burning by black or smokeless powder standards.

For black powder you'd want a much thicker wall since the powder would be loose in the base and it would basically all burn at the same time and generate much higher pressure than such a thin tube would tolerate.
.

I think there is a major difference between a modern mortar like the first one posted and a traditional 1800s mortar. I think that modern mortars probably contain a blank cartridge in the base of the projectile and that they projectile more or less contains the pressure of the propelling charge. That would suggest the tube portion probably contains relatively little pressure
The mortars of the 1800s had a powder charge in the base of the barrel and were essentially short barreled muzzle loading cannons and for that reason the tube had to be much thicker. I also suspect the reason for the smaller chamber in the breach of the 1800s mortar may have been more to get a more effective push with less powder than the need for added strength

cheers mooncoon

 

[War Song]

Modern mortars carry their propellant in these little ring-shaped charges that slip into the tuby end of the mortar shell, just behind the fins. A percussion cap on the end sets off the charges, and the mortar man can adjust his range by removing and adding ring charges.

Building a black-powder mortar isn't too hard, most of the ones I seen built are machined from cold-rolled 1026 or 1018 carbon steel stock, which is pretty much low-carbon steel which have been cold-worked to increase its strength. Though you have to watch your barrel length, with the example you posted above, I think in 18th century standards that would had been considered a howitzer or even a short field cannon. Are you sure this isn't just a modern military mortar refurbished for black powder? The longer the barrel, the higher the firing pressure. That being said, one should use the coarsest grain black powder one can find, Fg being the minimum. But with a safe charge and a barrel length less than 15 calibers, an antique artillery piece shouldn't have to expect to take more than 10,000 psi of pressure when firing.

I haven't built one myself, but I done some reading into the subject. Some of the builds out there use seamless DOM tubing for the barrel, so no need to bore out from a solid block of metal. Still need a lathe to cut the threads for the breech plug though. C1026 DOM with would be the minimal steel grade, chrome-moly 4130 or 4140 tubing would be optimal. For a 1.5 inch bore barrel, a C1026 DOM tubing with 1/2 inch wall thickness ought to hold. You can calculate the pressure rating for a cylindrical tube by using Barlow's Formula:

Maximum Working Pressure = (2 x Wall Thickness x Yield Strength) / (Outer Diameter of Tube x [Safety Factor]) <--- For Safety Factor, I would use at least a 2

So for C1026 DOM with a Yield Strength of 60,000 psi, a 1.5 inch ID (2.5" OD) tube with a wall thickness of 0.5 inches would have a maximum working pressure of 24,000 psi ---> 16,000 psi when building with a safety factor of 2.

NOTE: Work pressure would be the maximum pressure the tube can handle before it starts to stretch permanently. The maximum pressure before bursting would be higher, and can be found by substituting yield strength with ultimate tensile strength, which would be around 80,000 psi for C1026 DOM. Of course, you shouldn't build according to this number, since repeated firing over the maximum working pressure would stretch the barrel thinner and thinner until it eventually bursts.

 

[BCRider]

I think there is a major difference between a modern mortar like the first one posted and a traditional 1800s mortar. I think that modern mortars probably contain a blank cartridge in the base of the projectile and that they projectile more or less contains the pressure of the propelling charge. That would suggest the tube portion probably contains relatively little pressure...

I may not have been as clear as this post. I was trying to warn about the same thing in my first reply but mooncoon nailed it with his explanation.

The point is that you do not want to risk using charges of black powder in that thin a wall. At least not unless it's a blank propulsion charge in the base of a proper "dart" like mortar round shape. The design of the fins and base tube on the modern mortar round provides for a lot of expansion room in the modern mortar tube design so the pressures on the thin wall are carefully controlled. In that way it's not really a normal firearm in the way we think about them. If you try to make a similar design thin wall tube that launches the typical fun type ammo like concrete filled beer cans using a rammed charge of black powder then you would likely end up wearing the shrapnel from the tube.

EDITED TO ADD- War Song nailed the rest of it. Nice post it is too.

 

[tiriaq]

A Stokes type mortar with a propellant cartridge in the tail of its projectile operates on the high/low pressure system. The pressure in the propelling cartridge (traditionally a shotgun blank) would rupture the shotgun shell, allowing the propellant gas into the much larger volume barrel tube, through holes provided in the tail of the projectile.
This is very different than what happens in a Coehorn style mortar.
I suppose that a breechplug could be fitted to a piece of seamless Cr-Mo tube, with a reduced diameter powder chamber drilled into it, it one wanted a touchhole fired mortar.
Incidentally, if one of these things is touchhole fired, think about where the blast from the touchhole is going to go.

 

[BCRider]

Incidentally, if one of these things is touchhole fired, think about where the blast from the touchhole is going to go.

Which would explain the little cast in "wall sink" on many of the Coehorn style mortars I've been looking at in pictures. A fuse doesn't need a little cupped out shelf of this sort but a touch hole mortar certainly does.