BLAST Effect or Bore PRESSURIZATION What propels Mortar Projectiles?

[seacoastartillery]

     About six months ago Mike and I were talking to Dominic Carpenter about a pretty important topic, we think.  Our phone conversation ran the gamut of artillery topics, but we went back to something he said several times.  Dom said he thought that, after many, many observations, it is most likely the blast effect and NOT acceleration due to pressurization within the bore, which causes a black powder mortar projectile to fly as far as it does.  We agreed that, considering the short stature of most black powder mortars, it is most likely that a larger proportion of the projectile’s propulsion comes from the blast effect rather than gradual pressurization.  We also agreed that it would be interesting to do a series of experiments to see if we could find out if this theory was true or not.

     Yesterday I spent about two hours in our local steel scrap yard to see if some materials were available to prepare some mortars for our experiments.  I looked and looked for low carbon steel tube with an I.D. between 6.256 and 6.356.  I finally found some L.C. tube .750” thick at a few thousandths over 1/40 windage, .154” for a standard steel coffee can of 6.156” Dia.  The bore is 6.325”.  The 3 Lb. coffee cans should hold about 14 Lbs. of concrete.

     In our spare time over the next few months Mike and I hope to prepare 5 coffee can mortars with the following bore lengths, Zero Cal., ˝ Cal., 1 Cal., 1  ˝ Cal., and 2 Cal.  This is basically what we want to show and compare:  With the first mortar, bore length, Zero Cal., we hope to see what chamber blast alone does to boost the coffee can down range.  With the last, at 2 Cals. bore length, we hope to see how much more distance it propels the can and then get a ratio to determine relative effect of Blast V.S. Bore Pressure.
 
     We would love to hear some thoughts from you engineering types out there.  Engineer or not, if you see a better way to do this, please let us know!  Are we being silly here or can we possibly add a little to our artillery knowledge base by these experiments?
 

Mike and Tracy
 
Yes, we see that the third one has been band sawed at the yard a little long.  Probably my arithmatic and NOT their sawing!  Correction on the mill is in it's future.  On the first one on the left, please imagine a welded bottom plate and a chamber piece that is just .125” less than the bore wall height (to give a little lip to keep the coffee can from sliding off the chamber piece) while the piece is at 45 degrees.



 

Thick wall tube with proper windage yields a safety factor that we demand.  We believe a 2 oz. charge should be sufficient to get results for analysis. Too little or too much?

[Parrott-Cannon]

From a velocity standpoint mortars with full powder chamber produce velocity similar to cannons with the same barrel lenght.  The velocity decreases by an expoential amount as the load density decreases in the powder chamber.

[Double D]

What chamber are you going to use?

Are you going to follow up with windage variation?

[RocklockI]

sounds like fun!   after watching your 11" mortar fire a bowling ball weighing 16lbs some god awfull long ways........ ball found in a ditch somewhere east of omaha .it was a nice butterscoth colored ball . it may have circled the earth a time or two .
 
yes i believe it is mostly 'blast'    due to the 'small' chamber it's 'small' powder charge just like anvil shooting  .i like anvil shooting !
 
i do think that a barrel of some lenght ,x2 or abouts will help big time with the flames lighting of a fused shell in combat, due to containment of the fire going up rather than out of all sides.
can't wait  !
gary

[BoomLover]

"Where to find writeup of 02 ...." Gentlemen, this is VERY similar to the plans Double D sent for my SCUBA Tank, Coffee Can Mortar. (The Coffee Cans in question are the Folders, Plastic, 6 3/8" Dia, 6 1/4" tall. They will be filled with Concrete also.) They can be viewed on the post referenced on page 3. The Tanks are 6.5" I.D., with the thickness of the sides being 1/4"  I have the Tank cut, neck cut off, 3" x 3" 1018 mile steel, bored out to 1" x 2". The fuse hole is drilled. The trunnion is 2" x 9", (or 10").  I think the Blast Effect is what propels the projectile, and the Bore Pressurization doesn't provide too much extra "Umphh!" I think the hardest part will be lining up and drilling the outside fuse hole in the side of the tank. (That and cutting out the Base to hold it....)  BoomLover

[Cat Whisperer]

I'm not sure what you mean by 'blast'.  Could it be equivalent to peak pressure?

From the assumption that the finer the powder the higher the peak pressure, one would get much more velocity (hence range) from using the finest powder.

My experience is that by adding more powder one gets more range, but changing to a finer powder doesn't add (significantly) to the range, just increases the peak pressure.

AND,  as you will find from the different barrel lengths that for a given powder (granulation and amount) you will get more range from the longer barrel.

By increasing the size of the grain one increases the burn time as the rate is somewhat self-limiting.  That makes the coarser powders more efficient in longer barrels.

[seacoastartillery]

From a velocity standpoint mortars with full powder chamber produce velocity similar to cannons with the same barrel lenght.  The velocity decreases by an expoential amount as the load density decreases in the powder chamber. 

     By load density do you mean amount as in 1/4 full, 1/2 full, etc.??

T&M

What chamber are you going to use?

Are you going to follow up with windage variation?   

     We are using a standard "gas cylinder mortar chamber" only on the inside, on top of the breech plug with the drilled vent bolt, 1/2" dia. coming in from the tube wall over to the chamber and threading in via a 1/2-13 thd.

"Are you going to follow up with windage variation?"   Please explain.


From Rocklock1:

" just like anvil shooting"   Yes, exactly!  Now Cat Whisperer knows what we mean by "BLAST Effect".  Gary, your analogies always surprise me with their descriptive imagery.

Thanks Boomlover, excellent comments and yes we copied your powder chamber!  Another vote for BLAST Effect.  O.K., I hope we are all right on that score, but if we are not, we will publish that fact as well.

T&M

[Artilleryman]

I'm not sure what you mean by 'blast'.  Could it be equivalent to peak pressure?

From the assumption that the finer the powder the higher the peak pressure, one would get much more velocity (hence range) from using the finest powder.

My experience is that by adding more powder one gets more range, but changing to a finer powder doesn't add (significantly) to the range, just increases the peak pressure.

AND,  as you will find from the different barrel lengths that for a given powder (granulation and amount) you will get more range from the longer barrel.

By increasing the size of the grain one increases the burn time as the rate is somewhat self-limiting.  That makes the coarser powders more efficient in longer barrels.

Might be interesting to test a fine grain vs coarse grain powder in the shortest and longest barrels.

[Parrott-Cannon]

From a velocity standpoint mortars with full powder chamber produce velocity similar to cannons with the same barrel lenght.  The velocity decreases by an expoential amount as the load density decreases in the powder chamber. 

     By load density do you mean amount as in 1/4 full, 1/2 full, etc.??

T&M

What chamber are you going to use?

Are you going to follow up with windage variation?   

     We are using a standard "gas cylinder mortar chamber" only on the inside, on top of the breech plug with the drilled vent bolt, 1/2" dia. coming in from the tube wall over to the chamber and threading in via a 1/2-13 thd.

"Are you going to follow up with windage variation?"   Please explain.


From Rocklock1:

" just like anvil shooting"   Yes, exactly!  Now Cat Whisperer knows what we mean by "BLAST Effect".  Gary, your analogies always surprise me with their descriptive imagery.

Thanks Boomlover, excellent comments and yes we copied your powder chamber!  Another vote for BLAST Effect.  O.K., I hope we are all right on that score, but if we are not, we will publish that fact as well.

T&M

By load density do you mean amount as in 1/4 full, 1/2 full, etc.??  That is correct.

 

[seacoastartillery]

I'm not sure what you mean by 'blast'.  Could it be equivalent to peak pressure?

From the assumption that the finer the powder the higher the peak pressure, one would get much more velocity (hence range) from using the finest powder.

My experience is that by adding more powder one gets more range, but changing to a finer powder doesn't add (significantly) to the range, just increases the peak pressure.

AND,  as you will find from the different barrel lengths that for a given powder (granulation and amount) you will get more range from the longer barrel.

By increasing the size of the grain one increases the burn time as the rate is somewhat self-limiting.  That makes the coarser powders more efficient in longer barrels.

Might be interesting to test a fine grain vs coarse grain powder in the shortest and longest barrels.   

 
     I re-read Cat Whisperer's post several times now and I believe I understand what you gentleman are trying to test.  I think we all know here that during the Civil War almost all mortars used a finer grain powder than their big brothers of the same bore size.  It is logical to believe that this was because the various ordnance departments wanted as much of the powder as was possible to be consumed in the mortar and not in the air in front of it! 

     Norm, we will make a note to try your experiment, just to see if we get more distance out of the longest tube with the larger grain powder and less distance with the fine grain.  Seems logical, but going back to what was used, it sure sounds like the equivalent to today's 2Fg to us.  Certainly no bigger than 1Fg.  One history buff, pyrotechnician described it as 1.5Fg powder as I recall. 

     Parrott-Cannon,   Thanks for the confirmation.  We will try really hard to decide on a chamber size which is appropriate for all experiments and this will most likely be determined by taking observations of actual pre-experiment testing by making the long Tube mortar first and trying to regulate it's chamber size so that we get range data close to 100 yardsdistance using the 2Fg powder.  We suspect that using maximum density loading techniques with the 2Fg powder, that a wt. of powder between 2 and 3 oz. will be an efficient and effective charge with the 13 to 14 pound coffee can projectile.

Tracy

[Double D]

About the windage.  When I posted yesterday I was indeed thinking the traditional windage.  Thinking more on this particular experiment the only relevance windage will have is if it is not uniform.  As long as it is uniform windage has no relevance. At least as I understand how you are going to conduct the experiments

First you need to establish some definitions.  What is blast and what is bore pressurization?   

Can you share your Hypothesis with us.  What do  you expect to happen and why should it happen?
 
 

[Artilleryman]

     Norm, we will make a note to try your experiment, just to see if we get more distance out of the longest tube with the larger grain powder and less distance with the fine grain.  Seems logical, but going back to what was used, it sure sounds like the equivalent to today's 2Fg to us.  Certainly no bigger than 1Fg.  One history buff, pyrotechnician described it as 1.5Fg powder as I recall. 

Tracy

My experience with an eight inch siege mortar shows that 1fg powder comes very close to the range table at 900 yards.  The same charge of cannon grade was about 125 yards short (with a very nice group).

[Double D]

My I suggest you try one variable at a time. 

For example the first time use one type powder-say Cannon grade with the charge weight powder  you establish as the 100 yard load with each of the 5 barrel lengths.  This will give you a base line.

Second time test different grades of powder (Cannon vs Fg vs FFg) in original test weight for 100 yards in the same barrel length.  Repeat the test in the other four barrel lengths.  This should establish which  powder is optimum in each barrel length.

When you have identified the optimum powder, then use that powder to vary only the load density, say 25%, 50% and 100% in each barrel.

When you have that done it should just in time for the CBCC IV braai and you can present a report to the members.

Gotta go,  Super rugby is on and Brian Habana of  Stormers just scored a try.

 

[seacoastartillery]

About the windage.  When I posted yesterday I was indeed thinking the traditional windage.  Thinking more on this particular experiment the only relevance windage will have is if it is not uniform.  As long as it is uniform windage has no relevance. At least as I understand how you are going to conduct the experiments

First you need to establish some definitions.  What is blast and what is bore pressurization?   

Can you share your Hypothesis with us.  What do  you expect to happen and why should it happen?     

    Wow, you're getting pretty formal in your old age, aren't you, Double D.?  Definitions, Hypothesis, Expectations and Reasons Why or Why Not.  Hey buddy, this old geezer just wants to have some fun!  Without getting really weird, here is the way we see this project.       We consider "BLAST Effect" to be that projectile propulsion component that is limited to the multi-directional blast of powder gasses and particulates that issues forth from the Chamber at the time of ignition.  This "Chamber BLAST" is the high pressure force that overcomes the projectile's "Inertia-at-Rest" and starts the projectile on it's journey down the bore.

     Bore PRESSURIZATION is the effect of increased bore pressure or a maintenance of initial bore pressure due to the release of propulsion gasses from grains of powder which continue to burn OUTSIDE of the Chamber.

     We don't have any hypothesis but this is what we expect to happen:  We expect a larger % of propulsive power to be attributed to Blast effect V.S. Bore pressurization based on relative distances that the projectiles will travel.  In really clear language, we expect the shortest mortar to be able to shoot the concrete-filled coffee can more than 70% as far as the longest mortar with some % of bore pressurization which will provide extra force and extra distance to the long mortar shot.

Tracy and Mike



   

[RocklockI]

Once the ignition begins it is restrianed by the walls of the chamber .
 
The instant the expanding gasses are basicly 'let loose' by entering the bore ,and pressure drops ...alot .

[Double D]

I have been working on taxes for three days.  I had to check the box that says Katherine is deceased. Telling IRS your wife is dead gives a finality to it...and does not inspire good polite conversation...so instead of expressing my self in the word of my emotions, I settled for more formal wording...

Your goal is optimistic....70% wow! 

So when is range day lets do it!

[The Jeff]

Thanks for undertaking this experiment and I look forward to seeing the results.


Here's an interesting 1/2 caliber, 5.5" mortar with an effective range of 550 yards with 9 ounces of powder: http://www.gboreloaded.com/forums/index.php/topic,206625.msg1099086298.html#msg1099086298


That's a little less than half of a 24 pounder Coehorn's 1,200 yard range with half a pound of powder. Not quite an even match up and possibly I'm comparing effective and maximum range, so who knows.

[GGaskill]

This is going to be a long one so I am going to make an initial post and then add to it as I compose.  Feel free to post contrary information at any time.

It's been 45 years or so since I sat, and sometimes slept, through the PV=nRT chemistry lectures and although I don't remember everything about them, I do remember that they do not completely constrain the results of an action; that is, you can say in the case of black powder combustion, that the pressure and temperature decrease as the volume increases but you can't say in what ratio.  So virtually all of the exercises were like the mathematical exercises involving projectile trajectories, that is some simplifying assumptions were made.  In the case of trajectories, you were to ignore air resistance (even though air resistance is 50-100 times as great as gravity); in the chemistry exercises, you were to hold one of the factors constant.  This made for easy calculation but detached the results from reality.

One other thing you can say in a real case like firing a muzzle loading mortar, is that n decreases since you have leakage around the projectile and through the vent.  The only thing that doesn't change is R, the gas constant (and it really changes a little bit but orders of magnitude less than the other factors.)

Another thing to consider is that heat is lost from the gas to the barrel and the projectile since both are significantly lower in temperature than the powder gasses.

So let's think about the first step, the bore of negligible length.  Once the shot lifts off the chamber the gas is free to divert to the side without transferring any energy to the shot.  So whatever energy is imparted to the shot will come from momentum transfer from the gas particles to the shot particles.  Assuming a flat can bottom (not entirely true if this is a typical tin can), this will be about a one-to-one transfer.  If the bottom of the can were shaped to reverse the direction of the gasses, it would be closer to two-to-one.

But consider that all the powder will not have burned by the time the shot is lifted away from the chamber.  At the beginning, the shot will be pushed by the unburned powder farthest away from the ignition point.  Once it is out of the chamber unburned, maybe as much as half of the powder will not burn or will burn in a place that will not add to the propulsion of the shot.

[Cat Whisperer]

I did build a bunch of removeable breach golf-ball caliber mortars of various lengths.

The shortest was the 1-caliber long tube, built to launch the golf ball with a firecracker.

The ball did get out of the tube with a firecracker.  With 3F I could get about 8' high and 8' range, max.

I think we're in agreement - theory & practice.

[KABAR2]

I like the idea behind this.........
.

May I be so bold as to sugest the name of "Operation Calliope" for these tests?

[shred]

As a control, once you gather the data from the separate tubes, you can saw down the longest one to match the others bit by bit...

[seacoastartillery]

 Thanks KABAR2, that is a good name for it.  From now on we will call you the "Jester". 
 
 A real Steam Calliope onboard the passenger ferry, Mini-Ha-Ha.
 

 
 
 Can you remember the first "Where are These Cannons located?" Contest?  The following is a picture of the vessel where the Calliope is installed.  This vessel was featured as a clue as to the cannon's Empire State location in our very first,  Where are these cannon located? Contest.
 
 The following question greeted contestants on August 9, 2007 along with the photo we took while visiting the fort the spring of 2007.
 
 Can you name the Fort in the "Empire State" where this Long 12 Pdr. is located?  Name the lake too.
 
 
 
 
      GGaskill,  Well, we read your scholarly tome on the chemistry and physics of our proposed experiments and after the third reading I believe we pretty much understand what you have written.  There is one section that is very intriguing to me, however.  It's the one which is covered by this quote from your dissertation:
 
     "So let's think about the first step, the bore of negligible length.  Once the shot lifts off the chamber the gas is free to divert to the side without transferring any energy to the shot.  So whatever energy is imparted to the shot will come from momentum transfer from the gas particles to the shot particles. Assuming a flat can bottom (not entirely true if this is a typical tin can), this will be about a one-to-one transfer.  If the bottom of the can were shaped to reverse the direction of the gasses, it would be closer to two-to-one."
 
      Let's see if I understand this concept properly, If the can has a flat bottom, as our coffee can projectiles do, then the momentum transfer will be one to one.  If the can were to have a concave surface approximating a hemispherical surface, then the momentum transfer would be approx. two to one.  Now please consider this:  If the projectile was a typical cannon shell shape, presenting a lesser surface to which the gas particles can transfer momentum, then would the transfer ratio be one to two?
 
     Shred, let's not prime the pump too much just yet.  We will take your suggestion under advisement.
 
     Jeff, the pleasure is ours and the completion date for this project is July 4th 2012 unless we are still on the road delivering Brooke seacoast rifles around the country.
 
     DD,   We are always optimistic when we take on a special project.  70%,  Ha, no problem.  We will carefully consider your suggestions too and discuss them at a later date after we have done some construction.
 
     CW,   Theory and Practice, I like that. 
 
 Tracy

[RocklockI]

I agree with shred ,cut it down bit by bit and even though it will require a trip back to the shop for each cut .....removing 3/8" at a time would really be difinitive ! I can not wait for more trips to the lone prarrie this summer .
 
BTW Mike and I went shooting 'out there' today it was a nice day but windy . I was crying the blues over my group size at 100 yds . When I got home I measured the groups and none were over an inch ,    and one was .625" ct to ct 'Mikes hand loads' . I was shooting my new CZ 527 .223 varmit.
 
She needs some work but she'll come around after a bit of TLC.....
 
Mike took his Browning 1886 in 45-70 ...and what a CANNON it is !
 
Mike shot a group at 100 yds with it. They were in the black and about 3" ct to ct and maybe 2-3 " at 9 oclock from the 10X ring I have a picture to prove it !
 
But good grief what I have to listen to! He shoots the 3 shots   ..... , ....   stood up from the bench and muttered 'this is no bench rest gun whaayyaa '  .
On the way to the target ...'I can't even see the front sight any more OH  boo hoo' I was almost feeling bad for the guy.
 
Then we get to the target and there's this 2 1/2" to 3" group a at 100 yards ......Then it was all   ' Oh yea that's how we do it in the bush'  .... or some such blather I had to listen to for the 100 yd walk back . I almost though he was going to start telling time by the sun  .
 
Life is hard
Gary  
 

[GGaskill]

Now please consider this:  If the projectile was a typical cannon shell shape, presenting a lesser surface to which the gas particles can transfer momentum, then would the transfer ratio be one to two?

To the extent I remember this stuff, if the forward velocity component of the deflected gas particles is zero, then it would be 1:1.  To the extent the forward component is greater than zero, that much of the momentum is not transferred to the shot.  The most central particles will transfer the most, and the others will transfer basically as the sine of the angle between the "bore axis" and the tangent of the impact point.  So on a 10" shell, the sine of tangent of the central point is 1; at 1" radius, the angle is 78.46° and the sine is .9798; at 2" radius, the angle is 66.42° and the sine is .9165; at 3" radius, the angle is 53.13° and the sine is .800; and at 4" radius, the angle is 36.87° and the sine is .600.  So for an 8" circle (area=50.265 in²) that represents 64% of the area of the 10" shot, you would get at least 60% of the forward momentum (in my younger days, I could have done the calculus to figure this out completely but not any more.)  If your chamber were only 2" in diameter, you should get over 98%.

Now how you calculate the momentum of the exploding powder is another question.  The classical formula is p=mv where m is mass and v is velocity.  We can determine the mass of the charge easily but not the velocity so we are kind of at a standstill from the mathematical modelling perspective, at least based on my knowledge.  However, consider that all of the proposed tests will start from this condition and add the expansion of the gasses pushing on the cross section of the shot at decreasing pressures for increasing lengths.

After you have fired your first shot in the zero caliber mortar and determined an idea of how far the shot will go, fire your next shot straight up if it isn't going very far.  You will have a more symmetrical launch and an easier calculation of energy since the work of lifting the shot (and also the potential energy at the top of flight) will equal the kinetic energy at beginning and end (less air resistance losses which should be low if the velocity is low.)  W=Fd (force times distance), the force is the weight of the shot, the distance is the height of the shot.  Easy.

[Victor3]

I was shooting my new CZ 527 .223 varmit.
 
She needs some work but she'll come around after a bit of TLC.....
 
Gary  
 

 Great minds think alike, Gary...
 
 I gots me somethin' kinda like yours (but it needs no TLC 'cept regular polishing    )
 

 
 Gots me a varmint too, but in 17 HMR...
 

[Artilleryman]

We seem to be getting away from the topic of the thread.

[dominick]

Mike & Tracy,  It's close to what I'm thinking about.  The results of your experiment would be interesting. Thanks for taking the time to do this.   
 
      I occasionally receive a request to build a mortar with a longer barrel and the reason stipulated is to increase the range.  Because of these requests, I,ve been pondering this question for some time now.
 
  Which is a more effective method for increasing range, longer bore OR larger or no powder chamber? 
 
I'm using the word effective, not efficient as in cost per shot or anything like that.  Here are some examples of what I think Tracy and I mean by short or no barrel that uses "blast effect".  Claymore mine, Anvil launcher, 13" Seacoast Dictator mortar.  These use primarily the force of the blast to propel the projectile down range.  The Dictator mortar is capable of sending a 218 lb. projectile 4200 yards down range using 20 lbs. of powder.
 
 
Here are examples of "bore pressurization" [Pumkin Chunkin] air cannon, spud gun or stokes mortar. 
 
When I was an 81mm mortarman in the U.S. Army [1974 - 77] we would adjust the range with either the elevation or amount of charge or both.  The 81mm round had 7 small burlap bags of powder tied to the base of it near the stablizing fins.  The assistant gunner or ammo bearer [I don't remember which one] would tear off a number of these bags to adjust the range. The projectile also had a ring seal similar to a car piston that would expand when the round was fired. The projectile would exit the bore with more of a "thump" than a boom.  This lighter charge and bore seal to send a projectile far down range is what I think Tracy and I mean by"bore pressuration".   
 
 
 
So here's my question..
 
Could the shortest barrel mortar with a very large or no powder chamber achieve the same range results as the longest barrel mortar with the smaller chamber?

[Double D]

Allen, Allen, Allen...Boeing-17 targets, Calliopes....have you been sniffing the ozone coming off your microprocessors again?  Remember what your Mother told you about that!

[Parrott-Cannon]

I hope these graphs illustrate what you are talking about.

[Double D]

Dom and Tracy,

Now I see what you guys getting at.  I thought you guys had discovered some new property of expanding gases.  You are saying there is principal called blast effect generated by chemical  generation of gases and second principal called bore pressurization. I think you saying that there is no blast effect in an air cannon only bore pressure.

I don't agree. They are not separate, but one is part of the other. Blast effect as I see could best be defined as the sudden unconstrained release of energy in the form of an expanding gas.  Bore pressurization is simply the focusing of that expanding gas up a bore.  An air cannon also has blast effect when the compressed air is released.

There isn't a bit of difference in principal between releasing compressed air  from a pressurized tank into atmosphere and releasing a a gas by combustion of a chemical into the atmosphere. Both gases expand and diminish in pressure until they reach atmospheric pressure.

What I think you guys might perceive as a blast effect is the more violent expansion of a chemically generated gas. 

The only difference is that a chemical can generate a greater volume of gas at a higher velocity than compressed air, thus the violence.

If you really want to test this theory then you must test the effects of air pressure on an equal basis with chemically generated pressure.

Determine the pressure using blackpowder to launch the coffee can from the 2 caliber barrel 100 yards.

Attach a compressed air tank to the 2 caliber barrel.  Run the pressure up in the tank to the same pressure as the pressure of the black powder charge.  Release the pressurized air into the barrel.  If the can goes less than 100 yards then there is a blast effect.

Those pressure curves Parrot-Cannon posted will occur with or with out a barrel.