BLAST Effect or Bore PRESSURIZATION What propels Mortar Projectiles?

[Cat Whisperer]

I hope these graphs illustrate what you are talking about.

OK.  SO, where in time are:

a) first movement of the projo, and
b) exit from the barrel?

(I have that chart for the M16, but that's not a fair comparison.)

[Victor3]

An air cannon also has blast effect when the compressed air is released.

 Yes, an air cannon relies on a rapid release of compressed air that I suppose one could call a "blast effect." You can't bleed a charge of air out slowly behind the projectile and expect it to get to near the same velocity as when the air is dumped all at once while the projectile is seated at the base of the bore.
 
 That's one reason why almost all Co2 & pre-charged pneumatic pellet rifles employ a poppet valve opened by a striker; very fast opening. Any pressure leaked from the valve after the initial blast that started the pellet moving is largely wasted in the bore space between the valve and pellet.
 
 That said, we're talking about relatively low pressures in air guns/cannons. It would be problematic (and very expensive) to build an air cannon that worked using anything near BP pressures.

[dominick]

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.
 
 

No, not quite.   For lack of a better word we are using the term blast effect to describe the pressure wave that is not restricted into a column by the bore of the mortar.  We could instead use the terms bore pressure and No-bore pressure for a more accurate description but blast effect sounds better.      It just wouldn't sound right saying "we use No-bore pressure to launch that anvil a hundred feet into the sky.   I see this as a simple experiment to determine what effect the bore length of a mortar has on range. No special tools needed.
 
Also, the more I think about it, a Dictator mortar is a poor example of what I described as a blast effect ordinance.  I think the bore is a bit deeper than  1/2 or 1 caliber. I used air cannon as an example of bore pressurization because it relies on the length of the bore and contained pressure within to expel the projectile.  Without its bore length and seal, it wouldn't work well.

[Cat Whisperer]

Some of the writings about the Tannenburg hand gonne spoke about the pressures generated using a powder chamber.

Long thin chambers generated higher peak pressures (focused too) compared to shorter larger diameter powder chambers.

[jamesfrom180]

I still feel a few basic mechanical definitions are not being thought of.  One particular professor keeps coming to mind, he would always draw energy diagrams of flows to try and assure us that the derivation was capturing all inputs. This is not that different as we have already heard about discreet particles in the system.

I am by no means even educated enough to comprehend all the inputs. But so far we have in our system Pressure, friction, gravity, and heat.  The pressure is overcoming the friction of the projectile in the bore and gravity pulling down on the projectile both sides have losses to heat. 

Now that said we also are dealing with how the energy in the system is being transmitted. Initially I was thinking simple SA times pressure = force. Then looking at it we have a wave of particles possibly traveling faster than the speed of sound.  This becomes a problem since PV=nRt will describe the total pressure and also give us an energy input but the efficiency will be governed by the internal heat transfer of the media.  I will admit I had to go to the bane of the academic Wikipedia and look up shock waves.  In a sub category I found Detonation Wave.

I will quote here:
A detonation wave is essentially a shock supported by trailing exothermic reaction. It involves a wave traveling through a highly combustible or chemically unstable medium, such as oxygen-methane mixture or a high explosive.  The chemical reaction of the medium occurs following the shock wave, and the chemical energy of the reaction drives the wave forward. 
A detonation wave follows slightly different rules from an ordinary shock since it is driven by the chemical reaction occurring behind the shock wave front.  In the simplest of theory for detonations, an unsupported, self-propagating detonation wave proceeds at the Chapman-Jouguet velocity.  A detonation will also cause a shock of type 1, above to propagate into the surrounding air due to the overpressure induced by the explosion.
When a shockwave is created by high explosives such as TNT (which has a detonation velocity of 6,900 m/s), it will always travel at high, supersonic velocity from its point of origin.

Now if we contain the waves we can have superposition so I would think that the length of the barrel may in fact cause some harmonics.  At this point I want to throw in the towel and say the only way I see to find out what is going on is to experiment.  I think this system is way to complex to model on paper.   

[Artilleryman]

I hope these graphs illustrate what you are talking about.

These graphs remind me of the pressure curves of normal combustion in an engine cylinder and detonation (knocking).  You get a very high pressure spike during detonation with a rapid drop in the pressure curve.  Regular combustion doesn't get as high of a pressure but it is sustained over a longer period of time resulting in more overall power.  I am not sure if there is any relationship between this and firing mortars. 

[Double D]

I guess I just don't under what this blast thing is you guys are talking about...can you explain in simple plain high school level general science terms?

[dominick]

 This is a very simple grade school level experiment to determine the range of mortars with different bore lengths.
 
Here's a brief on what I believe is how M & T are conducting this experiment. There are 5 mortars with 5 identical powder charges placed in 5 identical powder chambers with 5 identical projectiles being fired from 5 DIFFERENT length mortar barrels to determine the effect that barrel length has on the range of the projectiles.
 
"Blast effect" is a loose term I invented to describe a pressure wave initiated by an explosion that is not contained and guided in a certain direction by a cylindrical tube, ie barrel.
 

[The Jeff]

Some of the writings about the Tannenburg hand gonne spoke about the pressures generated using a powder chamber.

Long thin chambers generated higher peak pressures (focused too) compared to shorter larger diameter powder chambers.

Here's an informative website that goes into detail about the long narrow chambers: http://www.musketeer.ch/blackpowder/handgonne.html

I guess I just don't under what this blast thing is you guys are talking about...can you explain in simple plain high school level general science terms?

The way I understand the discussion is that "blast effect" is referring to the "umph" given to the projectile at the moment the gunpowder goes off. Anvil shooting is probably the best example since it's essentially a 0 caliber mortar. Once the projectile anvil moves off the other anvil, there's no barrel to harness the still burning powder and accelerate the anvil. Whatever energy the powder transfers to the anvil as it goes off is all that the anvil ends up with.


On the other hand a cannon has a long barrel which directs the pressure from the still burning gunpowder towards the back of the projectile which accelerates it as it moves down the bore. The projectile can be 5 feet down the bore and still be accelerating while an anvil 5 feet away from the stationary anvil wouldn't.


I think Mike and Tracy want to get some data on these two effects and see how they interact. I expect the longest mortar to outshoot the rest because any little bit of barrel will help direct the expanding gases towards the projectile and transfer more energy. How much it helps is the real question.


I hope that captures the gist of the discussion.

[seacoastartillery]

I guess I just don't under what this blast thing is you guys are talking about...can you explain in simple plain high school level general science terms?   

    DD,    I think both Dominic and The Jeff have explained it better than I did.  See their pertinent comments below:

"Blast effect" is a loose term I invented to describe a pressure wave initiated by an explosion that is not contained and guided in a certain direction by a cylindrical tube, ie barrel.       

   BINGO!

The way I understand the discussion is that "blast effect" is referring to the "umph" given to the projectile at the moment of the explosion of the gunpowder. Anvil shooting is probably the best example since it's essentially a 0 caliber mortar. Once the projectile anvil moves off the other anvil, there's no barrel to harness the still burning powder and accelerate the anvil. Whatever energy the powder transfers to the anvil as it explodes is all that the anvil ends up with.   

      BINGO!


Tracy and Mike

[armorer77]

Another thing that this might reveal . At what point does the projectile start slowing down , when barrel friction starts to overcome acceleration .

[Double D]

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.   
 

Guys that is what I said!!!!

[dominick]

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.   
 

Guys that is what I said!!!!

I knew you understood it.  We just needed to come up with a better way to explain it.   

[Parrott-Cannon]

What Happen When Black Powder is ignited?
Black powder does not explode it burns.  To the human eye or ear it appears to be an explosion
but it occurs over a number of milliseconds and at that time interval it burn.
It burns a predictable rate.  The burning rate increases with pressure.
If we look at a mortar we have a projectile seated over and “sealing” a powder chamber.
When the black powder is ignited it burns releasing gases.
When the gases exceed the pressure excreted by the projectile the projectile
starts its journey down the barrel.
Some of the contributing factors to the velocity obtained by a mortar projectile are:
- Powder granulation, a finer granulation will burn faster generating more gas in the same time.
  Therefore a higher pressure and higher velocity are achieved.
- The length of the barrel.  More powder will be burnt and the gas produced has a
  longer time to act on the projectile and higher velocities will be obtained.
- Load Density – the ratio of the volume of the powder chamber to the volume of the powder charge. 
  As the load density decreases the velocity produced is less than would be expected from the amount of the    powder charge. 
  Everything would seem to be the same a sufficient amount of powder must be burn to exceed the pressure from the weight of the projectile.
  However, if the load density is less than 1 (a full powder chamber) then as the powder burns it pushes the powder column in the empty
  space of the unfilled powder chamber.  This causes a drop in temperature and the addition space between the   black powder particles which reduces the ignition rate of powder particle that are not already burning.
 
The acceleration of a projectile is directly related to pressure and the base area of the projectile and inversely
proportional to the mass of the projectile.  Velocity is simply the acceleration times the time and is
accumulative during the bore transit.
 

[seacoastartillery]

I don't have 4 hrs. to post 2 paragraphs.  So which is it; are the GBO servers desperately slow tonight or is my computer crapping out?

Tracy



 

[Cat Whisperer]

The Jeff -

That's one of the two articles that best describes it.  The other describes various shapes/sizes of powder chambers.  I think both are in the reference sticky.



In a modern rifle, the bullet either hasn't moved or has moved only an inch or so when the peak occurs.  What is the blackpowder mortar/cannon timing?

[Cat Whisperer]

I don't have 4 hrs. to post 2 paragraphs.  So which is it; are the GBO servers desperately slow tonight or is my computer crapping out?

Tracy

 

On my end it's usually ME.
 

[Double D]

Based solely on the topic title and the follow on OP there are two thing to test; blast effect and bore pressurization.

We all understand what bore pressurization is and understand testing it  via varying barrel lengths.

But how are you going to test blast effect? The zero barrel length won't test it because the powder is confined and when ignited the pressure wave is focused out the mouth of the chamber.   The Chamber is a barrel

You could attach the 100 yard charge to the bottom of a coffee can and suspend it in the air then ignite it.  That would test blast effect.

[Artilleryman]

You could attach the 100 yard charge to the bottom of a coffee can and suspend it in the air then ignite it.  That would test blast effect.

If you are going to do that then it might be better to put the can on a flat surface.  Two problems though, it introduces a variables (no powder chamber vs powder chambers for the the rest) and if you are measuring distance how do you get the 45 degrees that the rest of the barrels will have?

[Parrott-Cannon]

A question was raised about peak pressures and time to peak pressure.  The following information was calculated using the validated internal ballistics program VP-Sim:
12 pound Nepolean:
     Powder Charge: 17500 grains
     Peak pressure psi: 18316
     Time to peak pressure (ms): 1.65
     Powder Burn at Peak Pressure:  9788 grains   
     Bore Travel Time (ms):  5.86
Golf Ball Parrott:
          Powder Charge: 400 grains
          Peak pressure psi: 514
         Time to peak pressure (ms): 1.18
         Powder Burn at Peak Pressure:  11.7 grains   
         Bore Travel Time (ms):  5.36
24 pound Coehorn:
          Powder Charge: 3500 grains
          Peak pressure psi: 4043
          Time to peak pressure (ms): 1.70
           Powder Burn at Peak Pressure:  959 grains   
           Bore Travel Time (ms):  3.37
Golf Ball Mortar:
          Powder Charge: 120 grains
          Peak pressure psi: 326
          Time to peak pressure (ms): 0.69
          Powder Burn at Peak Pressure:  2.35 grains   
          Bore Travel Time (ms):  2.35
The program VP-SIM correctly calculates the velocities for these weapons.  It also produces  pressures estimates that are consistent with published data.

[Double D]

You could attach the 100 yard charge to the bottom of a coffee can and suspend it in the air then ignite it.  That would test blast effect.

If you are going to do that then it might be better to put the can on a flat surface.  Two problems though, it introduces a variables (no powder chamber vs powder chambers for the the rest) and if you are measuring distance how do you get the 45 degrees that the rest of the barrels will have?

I thought about his Norm.  My first thought was a flat surface also.  But even a flat surface make this a focused blast.   Even attaching the charge on the bottom of the can makes this a focused charge.  In the 40 minutes or so since I posted I thought about how to do this and here is what I came up with. 

 The coffee can is laid in a small trough made only large enough to just hold the can at 45 degrees. The charge is suspended in the air close to but away from and centered on the 45 degree base line of the can.

[GGaskill]

I don't think you will get an effective burn of the powder without confining it in a chamber or something resembling one.  Besides, aren't we talking about this in the context of artillery?

[1Southpaw]

Been thinking ..... that's usually the first step to a nap    :>)
But .... Depending on the free  bore  there has to be be some advantage to containing the burnt gasses behind the projectile after the initial "Blast" effect .
Unless free bore is so great to allow instantaneous release of pressure around the projectile.
I'm sure you will find a range/distance advantage to the longer tube . How long a tube  to diminishing returns is another question .

[Double D]

I don't think you will get an effective burn of the powder without confining it in a chamber or something resembling one.   

That is exactly the point I am trying to make. 

Blast is unconfined. 

A chamber is confined, the gases  expelled are directional and  equal bore pressurization.

[1Southpaw]

More thinking ....
I'm wondering if powder chamber design such as wide shallow , narrow deep , would have more effect on range .
 
DD maybe we need to make a multichambered  BB motar to test different configurations  of powder chambers , using the samer powder charge .
A series of interchanageable powder chambers in the same base block ?
We could use your new mile shooter as a test base . Could make inter -fitting  smaller chambers in your 1 lb powder chamber .

[Double D]

More thinking ....
I'm wondering if powder chamber design such as wide shallow , narrow deep , would have more effect on range .
 
DD maybe we need to make a multichambered  BB motar to test different configurations  of powder chambers , using the samer powder charge .
A series of interchanageable powder chambers in the same base block ?

Ernie,

Gibbons did some work on this and I believe Muller discussed it also.   

Hey you got a full tank on that Motor home?  We could make quick run back to PA and have Dom run us over to that steel yard and we could pick up some more of those rounds. Wonder what Ron would say if we dropped 5 or 6  with the tube and tooling.

Tracy and I discussed the chamber shape issue and it really wasn't what they want to look at.

I do not think what they want to look at  is the effect of the unconfined blast. 

I think they want to look at the effect of controlled directional blast (actually pressure) coming out of a chamber without an expansion chamber to contain the expanding gas behind a projectile, a bore with zero pressure.    Then they will add lengths of expansion chamber to see if range increases or decreases as the gases are contained and focused on the projectile for a longer periods of time.

 

 

[seacoastartillery]

More thinking ....I'm wondering if powder chamber design such as wide shallow , narrow deep , would have more effect on range .  DD maybe we need to make a multichambered  BB motar to test different configurations  of powder chambers , using the same powder charge . A series of interchanageable powder chambers in the same base block ?

Ernie,I think they want to look at the effect of controlled directional blast (actually pressure) coming out of a chamber without an expansion chamber to contain the expanding gas behind a projectile, a bore with zero pressure.    Then they will add lengths of expansion chamber to see if range increases or decreases as the gases are contained and focused on the projectile for a longer periods of time. [/color]

     Ernie,    Mike and I have been thinking about doing just what your comments in  Red  above describe for about two years now.  We have gathered a few historical sketches and drawings of chamber shapes used in the past.  We think a series of experiments using removable chamber pieces that screw into a common breech plug would be educational and lots of fun too.  We think a series including the cylindrical, the gomer, the spherical, the ellipsoidal and the hemispherical chamber shapes would be a good start.  This project will have to wait until July of this year, however.  Too much to do before that time.

     Double D.,    What you said above is another way of describing what our intent is in pursuing the ZERO to TWO Caliber tube length "power of projection" experiment.

    If someone with the means to accomplish it, wants to do a series of "unconfined blast effect" experiments in outer space with the charge and the projectile separated and hanging from a solar sail, please Go For It!  We would eagerly read about your findings, especially if you decorated the projectile with "dingle balls" which are almost garranteed to augment the flight characteristics, if any.  FYI, "Dingle balls" are those little balls composed of cotton thread hung by a short piece of thin cotton cord around the periphery of windshields in the 50s and 60s in the USA.

     We hope to have some ancient, "drafting pencil" drawings done by Sunday which will show a unique method of chamber piece attachment to allow easy removal and replacement.

Mike and Tracy



   

[seacoastartillery]

     We are only about 2 1/2 weeks late on this drawing, but we have so much to do to finally get those Brooke Seacoast guns delivered in June that we hardly have time to spit these days.  Although we hope that you guys think the design is OK, we won't have the time to cut steel and weld steel until we get back from the east coast in early July.  So, this is the time to discuss the Zero Cal. Mortar design.

     What do you think?    Remember that this thing will be shooting Coffee Cans full of concrete, so that is why the bore walls go .125" ABOVE the chamber piece which supports the projectile's weight.  This feature should keep the projo from slipping off.

Your opinions are important to us!  Please don't hold back any comments either positive or negative.  Chamber should hold between 200 and 300 grains of 1Fg Black Powder.

Mike and Tracy


ZERO CAL MORTAR Drawing shown in 3X Scale.

[KABAR2]

  M&T,
 With this item it would be interesting to see if there is any difference from what you have drawn up and just setting a ball on top of the powder chamber by itself.... I have a feeling that any lift generated will cause the ball to fall to one side or the other and lift will be negligible

[exlimey]

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     Easy one for me; its Ft William Henry at the south end of Lake George NY.A real dog and pony show and the same view that a Fort gunner had a couple of years ago when he fired off a Ram rod that hit a tourist OOPS!