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Speed Limit of GunPowder.

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LAN_403

Bored Chemist

Tue Mar 17 2009, 07:06AM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
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Either of them use gunpowder, rather than cordite or some such?

Dave Marshall

Tue Mar 17 2009, 12:38PM

Registered Member #16 Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554

No, the only modern firearms that still use true 'black powder' (as its typically referred to by hobbyists to avoid confusion) are muzzle loading 'primitive' weapons and a small handful of specialty guns.

Gun cotton is also no longer used in any recognizable form. It had a nasty habit of killing people who mishandled it or manufactured it, though it does still play a part in propellants. Modern propellants are almost exclusively double-based or triple based creatures, typically a mix of nitrocellulose, nitroglycerin, and in triple-base propellants, nitrogaunidine as well.

I was pondering this whole argument and had an idea I'd like to know more about. Keep in mind I'm rather in the dark when it comes to pneumatics.

While gunpowder has a maximum expansion rate in free burn, this does not apply in a rifle barrel, which is for all purposes a Venturi. Wouldn't this have a dramatic effect on the actual muzzle velocity of the expanding gas within the rifle?

Dave

Bored Chemist

Tue Mar 17 2009, 06:39PM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
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Posts: 1022

Let's try to simplify the idea. Instead of an explosive charge generating high pressure gas lets pretend that, like an air canon, there's a reservoir of compressed gas and a fast acting valve connecting it to the barrel. For this debate lets also assume that the valve opens "infinitely" quickly and has no resistance once it's open. Also we can ignore the viscosity of the gas.

The molecules of gas are bouncing about in the reservoir- they bounce off the walls and off one another. In doing so they distribute their energy randomly among themselves. Some of that energy might be stored as vibrational energy as the molecules of the gas vibrate but, at least at low temperatures that's not a big part of the question. Also, just to make it easy, we can use helium as the gas- it doesn't have any bonds to vibrate so all the energy it has is present as the kinetic energy of the atom.
if we heat the gas up we supply energy to those atoms. The only option atoms have to take on that extra energy is by moving faster. The average energy of the atoms is, for an ideal gas, proportional to the temperature and so the average velocity is proportional to the square root of the temperatrue.
Of course, not all the atoms are moving at the same speed. The distribution of energies is given by the Boltzmann distribution. From this you can calculate a distribution of velocities and there's a peak in it. This is the average velocity of the atoms (strictly a mode rather than a mean). There are atoms traveling faster than this peak but, as you look at increasing velocity, there are fewer and fewer atoms traveling that fast.

Imagine you suddenly open the valve and let this gas flood into the barrel. Initially, the gas atoms bump into the walls, each other, and the projectile. When they hit the projectile it starts to move forwards. In doing so it extracts energy from the expanding gas. (After the atoms bounce off the receding projectile they have a lower velocity than when they hit it). This leaves the gas atoms with a lower average velocity so it is cooler than it was.
As the projectile picks up speed two things happen. The gas behind it gets cooler and also the fraction of atoms moving fast enough to catch it up and push on it falls.
After a while it is moving so fast that the friction forces acting on it and the work it has to do pushing the air out of its way take as much energy as the relatively few atoms hitting it from behind are able to give. At this point it has reached a maximum velocity.

What we are trying to do is maximise the velocity when this happens; how can we do that?

Well, for a start we can use a gas whose atoms are moving quickly. There are essentially 2 ways to do this- we can use a gas with light atoms so that, to carry their share of the energy they have to move quickly or we cam make sure that each of them is carrying a lot of energy because it's hot; to do that it would have to move quickly.
So, if you want to build a really impressive "air canon" you don't use air, you use hydrogen and you heat it.
You get one of these

which can bring a projectile to 7000m/s- rather faster than the APFSDS can manage.

With an ordinary gun driving the projectile you use hot gas, but the molecules of the gases produced by burning the propellant are relatively heavy so they don't go very fast.

The other thing you can do is use a lot of gas.
If you use a lot of gas then there are more atoms or molecules available and so more of them will be fast enough to push the projectile, even when it already traveiling very fast. Of course, there are practical limits to the extent to which you can raise the pressure before the barrel bursts. On the other hand, since the military think that nuclear weapons are perfectly practical, it's no surprise that they can get very high muzzle velocities by simply using great big charges and rather small shells. It wastes a lot of energy but that doesn't trouble them- explosives aren't that expensive.

Using a high pressure (or roughly equivalently, a big charge) doesn't make the molecules of gas move faster- it just means that there are so many that the relatively few "maverick" ones that are fast enough to catch up with the projectile are able to push it even faster.

hotcrazyfruit

Tue Mar 17 2009, 09:01PM

banned on 5/26/2009
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Bored Chemist wrote ...

After a while it is moving so fast that the friction forces acting on it and the work it has to do pushing the air out of its way take as much energy as the relatively few atoms hitting it from behind are able to give. At this point it has reached a maximum velocity.

that IS true but if you increase the pressure (meaning more density) that point will be pushed further on, because there will be more molecules to continue increasing. i am very bad at explaining things by the way.

Bored Chemist wrote ...

Using a high pressure (or roughly equivalently, a big charge) doesn't make the molecules of gas move faster- it just means that there are so many that the relatively few "maverick" ones that are fast enough to catch up with the projectile are able to push it even faster.

so 1 pound of c4 in a heavy metal box with a hole a one end, will generate as much pressure and speed that 4 pounds of c4 in the same box?

[Edit: Double posting is cruel]

Bored Chemist

Wed Mar 18 2009, 06:58AM

Registered Member #193 Joined: Fri Feb 17 2006, 07:04AM
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Posts: 1022

"so 1 pound of c4 in a heavy metal box with a hole a one end, will generate as much pressure and speed that 4 pounds of c4 in the same box?"
You keep linking two things together.
It will create a lot more pressure, but not a lot more speed (of course, it also depends on the design of the experiment too).
The fastest things leaving the box will be gas molecules and their speed depends on temperature, not pressure.

Steve Conner

Wed Mar 18 2009, 10:44AM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
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I agree with Bored Chemist: You really won't get the projectile to go much faster than the speed of sound under the barrel conditions. Adding more charge will just make a bigger muzzle flash after the bullet has left, and eventually burst the barrel.

That is the thinking behind light gas guns: the light gas has a higher speed of sound than the gas mixture inside a regular gun barrel.

Now with a railgun, the "propellant" is electromagnetic fields, and the relevant "speed limit" for those is the speed of light. Hence why the military seem interested in them nowadays, they can shoot things faster than any gas-powered weapon.

Dr. Slack

Wed Mar 18 2009, 11:25AM

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Let's just remember that pressure is a derived concept. Pressure is a useful average that can help us in certain circumstances. A gas consists of (at the energy/temperature relevant to a gun) hard little bits of stuff pinging around in all directions, and bouncing elastically off each other and the walls. At any wall, over some period of time, a whole bunch of them will hit and recoil, transferring their momentum to the wall. It is useful to call that change of momentum per time per area "pressure", when dealing with static or near-static situations, like the force needed to keep the breech block attached to the back of the barrel. When things are static, the pressure is uniform throughout the gas. The mean speed is a function of the temperature. When we increase the pressure of the gas by adding more gas to the box, keeping the temperature constant, the speeds stay the same, but the number of collisions and hence momentum transferred per second has increased.

However, with a very dynamic situtation, where the wall (back of the round) is racing away from the gas at a speed similar to the pinging bits of stuff, it makes no sense to say "as the pressure in the bulk of the gas is X, therefore I would expect to see a force on the back of this fast moving wall because force = area * pressure". This makes the assumption that pressure is uniform thoroughout the container, which it isn't. It is the force on each wall that defines the pressure there, not the other way around. Even if the pressure in the breach is X, the "pressure" on the back of the round is much less, because the pinging bits are hitting it with a lower closing speed, many are not reaching it at all.

4lbs of C4 in a box may generate 4x as much pressure as 1lb when measured on the static walls of the box, but if measured on the back of a wall racing away (for some reason) at higher than the mean speed of the gas molecules, then both situations would measure zero pressure (though I grant that the 4lb zero would probably be 4x the size of the 1lb zero

speakerguy79

Wed Mar 18 2009, 06:11PM

Registered Member #1460 Joined: Thu May 01 2008, 12:45AM
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Posts: 19

Smokeless powders are essentially desensitized high-order explosive mixtures. The deflagrate (burn) instead of detonate. In the extremely unfortunate situation where it might detonate, your gun turns into an expensive hand grenade and you will be lucky to live.

I don't have much to add, other than I shoot rifles recreationally. The fastest any commercially available loading will do is about 4500fps out of a .204 Ruger as previously stated. It is a super high velocity varmint round firing a very light weight projectile (32 grains) that practically explodes upon hitting a prairie dog/ground squirrel/whatever. Barrels don't last very long with these fast projectiles, maybe a few thousand rounds at most before accuracy degrades.

More typical cartridges (.30-06, .308, 5.56mm, etc) are about 3000fps muzzle velocity. I personally shoot a .270 Winchester that fires a 130gr projectile at 3060fps muzzle velocity.

Shotguns are lower pressure devices shooting a heavier mass of projectiles at lower velocities. 1200-1800fps depending on the loading (2.75" bird shot vs. 3.5" magnum turkey loads in 12 gauge). Handguns run from about 800fps (.38 special and other older, low pressure cartridges) to maybe 1500fps or more (more modern high pressure rounds like .357 magnum)

And in closing, theoretical limits matter little when practical considerations like barrel life come into the picture.

hotcrazyfruit

Wed Mar 18 2009, 09:14PM

banned on 5/26/2009
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meh i suppose you are right. i have been focusing on my coil gun mortar latley. so i havent been reading up on exposives as much as i used to. xD

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