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Discussion Starter #1 (Edited)
Knee Mortar Physics?

Topic:
Topic author: Ronin48
Subject: Knee Mortar Physics?
Posted on: 06/21/2005 1:37:13 PM
Message:
The BATF 'says' to destroy (their term) a knee mortar a hole the diameter of the barrel must be drilled in the high pressure area of the barrel(near the receiver ring) and an obstruction welded in the barrel.

NASA has shown that a rocket motor can be fired in a vacumn (space) and propel the rocket. The exhaust gases do not need a solid object to thrust against.

The range adjustment on the knee mortar moves the firing pin up or down. If the pin is up on a mortar with the hole drilled, but no obstruction in the barrel and a round in the barrel and the trigger is pulled, would not the round function? Even if he pin was down with a live round in the tube the thrust of the gasses from the bottom of the round would propel it upward. Most gas would escape out the hole, but how would that effect the thrust/velocity of the round?

I had "C"s in Physics (two semesters of general and one of Geophysics) and that was a few years before the Tokagawa shogun became head-honcho. Some of you young, educated whipper-snappers reeducate me. What I'm asking is what/where is the 'high pressure' area in a knee mortar barrel? Is it related to the location of the firing pin? And what is the relation to the rounds thrust in a drilled 89 and an undrilled 89? And who, besides me, gives a fat rat's a$$?

Replies:

Reply author: strangms
Replied on: 06/21/2005 5:47:28 PM
Message:
I happen to think that the round leaves the tube so fast that gases which do not escape,
would still push the round out of the barrel. I think that any lost of the gases would affect the
distance of the round.
The amount of gases that builds up would have to do with the position of the firing pin.

strangms

Reply author: gtbehary
Replied on: 06/21/2005 8:30:26 PM
Message:
I wouldn't want to be holding the barrel when that thing went off!


Reply author: fredh
Replied on: 06/21/2005 8:51:39 PM
Message:
Doss, a hole near the base the size of the bore prevents the pressure X area = force acting on the round. The blocked hole at the exit prevents insertion of the round, so BATF has covered the variables. If there were no exit block, you're right in saying that there would be some small net force acting on the base of the round as the round is ignited.


Reply author: arisakadogs
Replied on: 06/21/2005 10:44:43 PM
Message:
Interesting how this one was "destroyed". They took the piece removed with the hole saw and welded it back in place with a rod that crosses the barrel. So you have your required hole and the barrel obstruction in one fairly neat operation.


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Reply author: rambob
Replied on: 06/22/2005 09:38:14 AM
Message:
I have always had a bee in my bonnet about the fact that the BATF still considers these knee mortar relics as dangerous weapons, and requires them to be mutilated. Has anyone ever figured out a way to fire ANY projectile out of a knee mortar?? I believe that if the ammunition is unobtainable for a weapon, then it is only dangerous if you hit someone over the head with it. Interestingly, on ebay I can buy an M7 grenade launcher for my M1 Garand and the rifle grenade projectiles to fire from it. It wouldn't be hard to take a practice rifle grenade and rig it up with powder and some kind of impact fuze. A ball bearing, epoxyed to a shotgun primer would do the trick. So why is it that the M7 launchers are a dime a dozen, while an antiquated relic like a knee mortar is still regulated. I feel that the BATF needs to get with the 21st century and update their idiotic lists on what constitutes a USEABLE, dangerous weapon!!!


Reply author: Ronin48
Replied on: 06/22/2005 3:23:14 PM
Message:

what I should have asked - Take an operational mortar, unscrew and remove the barrel, balance a live round on the firing pin 'stalk,' pull the trigger. How would velocity compare with one shot from mortar?




Reply author: fredh
Replied on: 06/22/2005 5:33:02 PM
Message:
Without a tube to develop the pressure in and guide the round, don't expect it to go anywhere. Would probably make someone close by pretty miserable. I envision uneven breakage of the copper disk in the base and the round spinning around. Think I've got that right if memory is correct on how the round is configured.


Reply author: Ronin48
Replied on: 06/23/2005 2:27:43 PM
Message:
Fred, Thanks for the reply. Here's what is confusing me. The various parts of the moon vehicle, Command/Service/Lunar Modules worked in a vacumn with nothing to 'push' against (unless you believe the images of the moon missions were all done in Hollywood).


How does the T-89 or 91 round differ from a solid fuel rocket when fired? Guess I could hit the nerest used bookstore and try to locate an elementry physics text.

Reply author: fredh
Replied on: 06/23/2005 5:57:10 PM
Message:
Mannnnn, you're picking on me.


I'm going to try making this simple for non-rocket guys reading this thread. There are 2 components to thrust (or force=F) from a rocket nozzle: 1) static pressure effect and 2) velocity of gasses. The two components are additive at the nozzle exit. In space the static pressure effect is usually negligible. The other effect is gas flow out the nozzle exit. That's what you see in launch photos. It is termed the mass flow effect and is basically (rho)(V)(V)(A)= F, where rho = gas density, V = velocity of gas, and A = nozzle exit cross-sectional area. Think of travelling down the road and sticking your hand out the open window of your car. The effect of wind velocity against an open palm is significant and is the force (F) I'm referring to. The mortar round does not experience that effect. The rocket engine does.

After ignition of the round in the mortar tube (barrel), gas exits the small holes in the base of the mortar round. While in the barrel, the primary means of round launch is static pressure build-up (number 1 in the previous paragraph), or (pressure)(tube cross-sectional area), or F = (P)(A). After the round leaves the barrel, there may be a small component of force (or thrust) from gas flow effect (number 2 in the previous paragraph), but I'll bet the gasses have already burned completely by time of barrel exit. By barrel exit, the round continues on based on exit velocity. You might compare the idea of static pressure to the effect you would feel standing 100 feet under water, the effect of all that 100 feet of water above you acting on your body.

I've designed combustion chambers and nozzles for production engines, for example the RL10B-2 on the Boeing Delta rocket, and live with this stuff every day. I don't know whether I made the answer simple or not.

Reply author: Ronin48
Replied on: 06/23/2005 7:52:40 PM
Message:
"It's as clear as mud, but it covers the ground..." You need the damn tube if you plan to fire the mortar round!


Reply author: fredh
Replied on: 06/23/2005 9:05:00 PM
Message:
Doss, I knew I was gonna get into trouble trying to explain the difference, but you did ask how the mortar differed from the rocket. And I squirmed and wrote, then squirmed and wrote some more. It's actually much more complex than my 2 paragraphs.


Reply author: arisakadogs
Replied on: 06/23/2005 10:50:29 PM
Message:
No rocket scientist here. The copper band expands at the same time. Not only does it grip the rifling, providing rotation, but it acts as a seal - preventing blow by. It actually works much like the Civil War Minie Bullet in the rifled musket. Both are muzzle fed, rifled, weapons.


Reply author: rcb
Replied on: 06/23/2005 10:57:04 PM
Message:
I think you have explained how the truck load of ferterlizer did not blow up the Murray Building - There was no confinement to push against. rcb


Reply author: Ronin48
Replied on: 06/24/2005 08:03:20 AM
Message:
Fred, Mearly 'joshing.' I did follow your explanation and appreciate your input. Any chance a 6th Ed. out any time soon. (I'm 68, prostate problem, cardiact anamoly (skipping beats like crazy) arthritis,, etc, Only thing keeping me in this existance is (1) the hope of a 6th Ed. of "Military Rifles of Japan" and (2) to observe the look on Adogs face when he sees the 'Nagoya-made' T-2 carbine sniper!)


Reply author: arisakadogs
Replied on: 06/24/2005 09:00:04 AM
Message:




quote: Originally posted by Eloldehombre1
(2) to observe the look on Adogs face when he sees the 'Nagoya-made' T-2 carbine sniper!)
Will it be in the 6th edition??


Reply author: Ronin48
Replied on: 06/24/2005 3:08:24 PM
Message:
Much too rare for a book on 'everyday' Japanese rifles. Look for the upcoming book, "The very-extremely rare Japanese T-99 Sniper Carbines Issued to the very-extremely small Japanese men of Kohido Island, Japan." Coming to a bookstore near you, very-extremely soon.

Reply author: fredh
Replied on: 06/24/2005 4:19:06 PM
Message:
Doss, what may have thrown you off is that the mortar round appears to have a nozzle. Actually, the holes in the base let the products of combustion exit the enclosed chamber in the round and fill up the tube on the mortar. This is the same action as the explosion within a rifle cartridge case, for example, except the cartridge case has a single exit hole. The other factor that makes the mortar round appear as a nozzle is that the combustion chamber is moving out the barrel as part of the round and spewing hot gas behind it, but it serves only the same purpose as a stationary case in a rifle chamber, and that is to pressurize the barrel (tube).


I wouldn't take the atrial fib, arthritus, prostate, stuff too seriously. That's the territory we're in. My father had an enlarged heart and lived with the skipping heart for over 50 years. I would prescribe more bourbon for that condition.

Doss, Dawg, my plate is too full to even think about the books. Maybe in a few years.

Reply author: arisakadogs
Replied on: 06/24/2005 10:37:15 PM
Message:
There are also ports to allow the gases to push out on the copper band to provide sealing and rotation. Like I said, it works just like a Minie bullet.


Reply author: fredh
Replied on: 06/25/2005 08:58:22 AM
Message:
Yep, the copper band expands in much the same way the rifle case expands in a chamber to seal the products of combustion in place. And, dawg, I've even started using your quote about beer and chuckle everytime I read it.


Reply author: arisakadogs
Replied on: 06/25/2005 10:21:03 AM
Message:
Thanks Fred. It is a good quote. Ben Franklin must have been quite a character.

 
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