What is the highest velocity that can be achieved with a chemical propellant in a firearm? I know that there is a limit at which the gas expands which limits the bullets speed. So what is the highest velocity that a chemical propellant can push the bullet at?
Speed of chemical propellants
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mcb
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It is very very difficult to push a bullet faster than the speed of sound in the propellant gases pushing the bullet. Going faster than the speed of sound (of the propellant gases) forms a shock-wave in the barrel choking the gas flow and significantly lowering the pressure on the base of the projectile limiting further acceleration. In theory (IIRC), doing lots of funky things to optimize conditions like evacuating the air out of the barrel in front the projectile, things to reduce friction and similar difficult optimizations ~6500 fps should be theoretically possible with some modern propellants. In practice ~5700 fps is about the limit.
I believe the Rheinmetall Rh-120 (the main gun on the Abrams and the Leopard 2 tanks) holds the muzzle velocity max for a production weapon system using gun powder. The APFSDS rounds (tungsten or depleted uranium) can reach a bit over 5700 fps.
You can go much faster using two stage gun called a light gas guns. The first stage uses combustion like gun powder to compress the second stage of helium or hydrogen (speed of sound in these two gases is much higher) that accelerates the projectile. They have used light gas guns to launch projectiles at over 22,000 fps.
-rambling
I believe the Rheinmetall Rh-120 (the main gun on the Abrams and the Leopard 2 tanks) holds the muzzle velocity max for a production weapon system using gun powder. The APFSDS rounds (tungsten or depleted uranium) can reach a bit over 5700 fps.
You can go much faster using two stage gun called a light gas guns. The first stage uses combustion like gun powder to compress the second stage of helium or hydrogen (speed of sound in these two gases is much higher) that accelerates the projectile. They have used light gas guns to launch projectiles at over 22,000 fps.
-rambling
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LoneGoose
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Wow, that's a great question and a really great answer. It's gems like this that keep me surfing THR.
Thunderchicken
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I agree. Fascinating.
In re the two-stage gun thing, I wonder how long until someone makes a miraculous self-defense round with the powder packed in helium? Since we know the most powerful force accelerating ammo sales is ... marketing!
In re the two-stage gun thing, I wonder how long until someone makes a miraculous self-defense round with the powder packed in helium? Since we know the most powerful force accelerating ammo sales is ... marketing!
burrhead
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I'm missing something here. In dry, sea level air at 25 degrees Celsius, Mach 1 is equal to 340.29 meters per second, or 1,122.96 feet per second. That's handgun speeds. I'm not disagreeing with mcb but what does the speed of sound have to do with this? Not arguing, just confused.
The speed of sound is proportional to the temperature of the gas. So the higher the temperature, the higher speed of sound, the faster you can push a projectile. The pressure wave from the combustion can't travel faster than the speed of sound in the gas medium.
burrhead
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Got it. I'll have to roll that around in my head for a while but I see what you're saying.
mcb
Member
The speed of sound in a gas (assuming ideal gas principals hold) is predicted by this equation:
Speed of sound = (g R T / M)^0.5
g is the Adiabatic Constant, for air ~1.4, for most propellants ~1.23 For helium ~1.66
R is the Universal Gas Constant 8.314 J/mol K
T is the Absolute Temperature (Kelvin)
M the molecular mass of the gas in kg/mol. In the case of air or propellant gases we use the average since the gas is made up of a bunch if different gases. For air it is estimated at .02895 kg/mol. Helium is only .004 kg/mol. I could not find a good estimate for the byproducts of gun powder combustion but it has to be slightly higher than air.
Temperature comes from both the heat generated by the combustion and by the pressure created by that combustion.
As you can see due to helium's higher Adiabatic Constant and low molecular mass it has a very high speed of sound even at room temperature but remember in a light gas gun the first stage compresses the helium very fast and you get significant heating of the helium from adiabatic compression (adiabatic here simply means we are assuming no significant heat lost or gain to the environment) This is a safe assumption give how fast these event happens.
-rambling
ETA: I am a solid-mechanics/dynamics focused mechanical engineer and this is rapidly reaching the limit of my knowledge of the dark side (fluid/thermal) of mechanical engineering. It is dangerous to learn too much about the dark side...
Speed of sound = (g R T / M)^0.5
g is the Adiabatic Constant, for air ~1.4, for most propellants ~1.23 For helium ~1.66
R is the Universal Gas Constant 8.314 J/mol K
T is the Absolute Temperature (Kelvin)
M the molecular mass of the gas in kg/mol. In the case of air or propellant gases we use the average since the gas is made up of a bunch if different gases. For air it is estimated at .02895 kg/mol. Helium is only .004 kg/mol. I could not find a good estimate for the byproducts of gun powder combustion but it has to be slightly higher than air.
Temperature comes from both the heat generated by the combustion and by the pressure created by that combustion.
As you can see due to helium's higher Adiabatic Constant and low molecular mass it has a very high speed of sound even at room temperature but remember in a light gas gun the first stage compresses the helium very fast and you get significant heating of the helium from adiabatic compression (adiabatic here simply means we are assuming no significant heat lost or gain to the environment) This is a safe assumption give how fast these event happens.
-rambling
ETA: I am a solid-mechanics/dynamics focused mechanical engineer and this is rapidly reaching the limit of my knowledge of the dark side (fluid/thermal) of mechanical engineering. It is dangerous to learn too much about the dark side...
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illinoisburt
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So what you're saying is it's super-awesome-coolness'n'such! Yes, we all love math!ETA: I am a solid-mechanics/dynamics focused mechanical engineer and this is rapidly reaching the limit of my knowledge of the dark side (fluid/thermal) of mechanical engineering. It is dangerous to learn too much about the dark side...
Archie
Member
Seems to me the biggest problem with switching to other-than-conventional propellants and atmospheres is the ability to pack it all in a 'cartridge' and use it in a firearm without starting from scratch.
For that matter, I see little to be gained. F'finstance,a 230 grain, .45 caliber bullet at between 850 and 950 fps seems to work well for self-defense. The same bullet at 1500 fps would work just as well, (unless it penetrated too quickly) and would generate a lot more recoil. The same arguments apply to hunting arms.
Unless we change the projectiles to something lighter and shoot a 20 grain .223 caliber projectile at 5000 fps. But then the projectile will loose velocity rather quickly, I'd think.
Beam weapons. That's the ticket! Disintegrator rays and such!
For that matter, I see little to be gained. F'finstance,a 230 grain, .45 caliber bullet at between 850 and 950 fps seems to work well for self-defense. The same bullet at 1500 fps would work just as well, (unless it penetrated too quickly) and would generate a lot more recoil. The same arguments apply to hunting arms.
Unless we change the projectiles to something lighter and shoot a 20 grain .223 caliber projectile at 5000 fps. But then the projectile will loose velocity rather quickly, I'd think.
Beam weapons. That's the ticket! Disintegrator rays and such!
The speed of sound in a gas (assuming ideal gas principals hold) is predicted by this equation:
Speed of sound = (g R T / M)^0.5
g is the Adiabatic Constant, for air ~1.4, for most propellants ~1.23 For helium ~1.66
R is the Universal Gas Constant 8.314 J/mol K
T is the Absolute Temperature (Kelvin)
M the molecular mass of the gas in kg/mol. In the case of air or propellant gases we use the average since the gas is made up of a bunch if different gases. For air it is estimated at .02895 kg/mol. Helium is only .004 kg/mol. I could not find a good estimate for the byproducts of gun powder combustion but it has to be slightly higher than air.
Temperature comes from both the heat generated by the combustion and by the pressure created by that combustion.
As you can see due to helium's higher Adiabatic Constant and low molecular mass it has a very high speed of sound even at room temperature but remember in a light gas gun the first stage compresses the helium very fast and you get significant heating of the helium from adiabatic compression (adiabatic here simply means we are assuming no significant heat lost or gain to the environment) This is a safe assumption give how fast these event happens.
-rambling
ETA: I am a solid-mechanics/dynamics focused mechanical engineer and this is rapidly reaching the limit of my knowledge of the dark side (fluid/thermal) of mechanical engineering. It is dangerous to learn too much about the dark side...
I'm a Chemical Engineer who spent his career in the petroleum industry, primariy converting crude oil & natural gas into useful stuff and getting it to market.
From this viewer on the dark side, you did a good job. The other main point I would add is the Mach number varies not only with temperature, and molecular composition, but also pressure.
Those who are reloaders will pick up right away a cartridge's combustion gases are at a much higher pressure ratio to the ambient pressure at the muzzle, even than temperature. Both P & T considerations need to be done as absolute, not gauge pressures; Kelvin is one absolute temperature scale, Rankine is a handier one when dealing in temperature readings in °F
As far as filling cartridges with an inert gas like helium, neon, argon, what have you - you'll have a squib at best as the primer and powder still need oxygen to make combustion to produce considerably higher pressures and temperatures to propel a projectile.
The two stage gun cannot function with the cart in front of the horse.
Staying with slug throwers instead of jumping to beam weapons, rail guns are the next step.
whughett
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mcb
Member
I will take minor issue with one of your points. Gun powder does not need oxygen to burn. Black powder and modern smokeless powders both have oxidizers as part of their make-up. In black powder the charcoal and sulfur are the fuel and the potassium nitrate is the oxidizer producing all the oxigen the combustion will need. With modern smokeless powders the oxygen comes from the NO2 groups that are part of the nitrocellulose or nitroglycerin that is the primary ingredient of smokeless powders. Both black powder and smokeless powder will burn in the absence of gaseous oxygen in the atmosphere around them (assuming sufficient pressure is maintained to propagate the combustion). ie a gun will function in a vacuum or inert environment.
A light gas gun has to have two expansion chambers in tandem with each other but that do not mix.
This image is from the Wiki article on light gas gun. I think this will make the concept of the light gas gun more clear. They work great in a lab, NASA uses them for impact studies at orbital velocities. They do not make very practical small arms arms as you can imagine.
And do you know there's sufficient oxidizers for complete combustion before the bullet leaves the barrel? I think reloaders know that answer can be "no" with air in the case with the powder. There's software called Quickload that displays % burn of a load for reloaders, using air as the fluid in the cartridge.
It's like saying gasoline doesn't need oxygen to burn because it's blended with ethanol, or even better saying ethanol doesn't need oxygen to burn.
Stoichiometry. Activation energy. Reaction pathways of all intermediate reactions. Important considerations.
Try filling a cartridge with an inert gas displacing the air, along with the primer & powder, sealed with a bullet, and see if you get a squib.
It's like saying gasoline doesn't need oxygen to burn because it's blended with ethanol, or even better saying ethanol doesn't need oxygen to burn.
Stoichiometry. Activation energy. Reaction pathways of all intermediate reactions. Important considerations.
Try filling a cartridge with an inert gas displacing the air, along with the primer & powder, sealed with a bullet, and see if you get a squib.
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Now think about why muzzle flash exists. I have yet to see or hear of a military or civilian, smokeless or black powder, that causes zero muzzle flash. Reduced, yes, but why not zero?
If your postulation is correct, why isn't the military especially not specifying their ammunition cases be filled with helium? Or the propellant charges of the 120mm Main Battle Tank cannon you mentioned, if it's all so simple? Those are smoothbores, the sabot provides a ballistic guidance seal along the walls and the gas seal at the rear of the projectile, right? No rifling to engrave. Energy on target is important enough to manufacture depleted uranium penetrators as you noticed, but with collateral side effects as noted in The Gulf War when that stuff vaporizes. But it sure looks good on paper.
If your postulation is correct, why isn't the military especially not specifying their ammunition cases be filled with helium? Or the propellant charges of the 120mm Main Battle Tank cannon you mentioned, if it's all so simple? Those are smoothbores, the sabot provides a ballistic guidance seal along the walls and the gas seal at the rear of the projectile, right? No rifling to engrave. Energy on target is important enough to manufacture depleted uranium penetrators as you noticed, but with collateral side effects as noted in The Gulf War when that stuff vaporizes. But it sure looks good on paper.
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mcb
Member
Yes there is sufficient oxidizers present to burn completely without external oxygen. With black powder the ratio of 75% potassium nitrate, 15% charcole, 10% sulfur results in a roughly stoichiometric balance of fuel (charcoal and sulfer) and oxidizer (potassium nitrate). In the case of single base smokeless propellants (nitrocellulose being that single base) there is again a near stoichiometric balance of fuel and oxidizer (assuming all the cellulose is fully nitrated) present in the nitrocellulose. If we are talking about a double base powder that also includes nitroglycerin then there will be an surplus of oxygen as for every two molecules of nitroglycerin that decomposes they produces an extra O2 molecule not consumed in the decomposition.
Do a google search there are lots of articles and videos on the fact that gun powder will burn in the absence of oxygen. The problem with burning gun powder in a vacuum is not the lack of oxygen but the lack of pressure. If you do not have pressure then the gun powder will frequently stop burning due to the combustion front not being able to propagate. You see this even in standard atmosphere conditions. Go shoot a bunch of pistol ammo in short barreled handgun and you will find a bunch of partially burnt powder grain on the floor down range (380 ACP in pocket pistols due to its lower pressure is very good at this.) The powder does not completely burn before the bullet exits the short barrel and at atmospheric pressure (and the grains moving very fast through it) they can be extinguished and don't fully burn despite having sufficient oxygen.
One nice example of black powder burning and not burning in a vacuum due to the ability to produce pressure or not:
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But black powder and smokeless powder are NOT solid rocket fuels.
So explain why the military doesn't specify oxygen free cartridge casings displacing the oxygen with helium. The US has great access to helium. Plus those 120mm tank cannon propellant charges. And every artillery piece if it's so simple.
And record a test of you loading and firing such a cartridge.
mcb
Member
Exactly and the same principle. They mixed that solid rocket motor compound to have a stoichiometric balanced mix of fuel to oxidizer. This works from the simplest hobby sugar powered rockets (sugar and potassium nitrate) to the solid boosters on the Space Shuttle or Atlas
They can be use for solid rockets and have a similar balanced mix of fuel and oxidizer. Most of the smaller hobby rocket motor kids use for model rockets use black powder as the propellant.
mcb
Member
What? The helium in a conventional cartridge would not do anything. I think your confusing conventional guns with the light gas gun I went off on a tangent about. Go back up and look at the diagram i posted of how a light gas gun works. Putting helium in a conventional round would have no real effect.
Go back and look what I wrote that you quoted first.
Would the helium not change the mach number behind the projectile based on your post?
Is that nothing?
So what creates the pressure?
Would loading a cartridge with no fluid between the powder interstitials and other void space similarly BUILD no appreciable pressure?
How does the pressure rise but not in space? Or helium?
Would the helium not change the mach number behind the projectile based on your post?
Is that nothing?
So what creates the pressure?
Would loading a cartridge with no fluid between the powder interstitials and other void space similarly BUILD no appreciable pressure?
How does the pressure rise but not in space? Or helium?
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