Speed of chemical propellants
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Jim Watson
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We have partial combustion products because the system IS deficient in oxygen.
Now you are getting into P chem which I am over 50 years away from and never was very good at to start with.
Because not all cartridges are the same.
Because it is not operating in steady state and there are a lot of factors involved.
I'm getting tired of gedankenexperiments. I can visualize loading a round of ammo in a glove box inerted with helium, etc. to remove the effects of atmospheric oxygen but I don't know how to get a vacuum on a shooting range without a lot of unlikely cooperation from NASA.
Now you are getting into P chem which I am over 50 years away from and never was very good at to start with.
Because not all cartridges are the same.
Because it is not operating in steady state and there are a lot of factors involved.
I'm getting tired of gedankenexperiments. I can visualize loading a round of ammo in a glove box inerted with helium, etc. to remove the effects of atmospheric oxygen but I don't know how to get a vacuum on a shooting range without a lot of unlikely cooperation from NASA.
I got a glimpse.
Here's a link to a paper showing differences in evolved combustion products of nitrocellulose in different gas atmospheres for firefighting evaluation walking my fingers. I think it solidly backs up what I've been saying but I suppose someone will argue against it. If it didn't matter the evolved gas compositions would be identical. And if oxygen in the gas phase isn't required why the difference?
https://www.google.com/url?sa=t&sou...FjAKegQICRAB&usg=AOvVaw0cVbV-rYGl4eK7BFCPcycC
I'm 33 years from PChem and got all A's, same for thermodynamics, and lots more core classes for my degree plan. I graduated Summa Cum Laude 3.83 / 4.0 GPA.
I also found this experiment taking 3 minutes for a flame (combustion).front to travel in nitrocellulose. Far from instantaneous. Flame front propagation differences is a really good answer on cartridge case geometry influencing powder efficiency.
https://www.sciencedirect.com/topics/chemical-engineering/nitrocellulose
Of course under higher oxygen partial pressure, the reaction boundary layer has more oxygen readily available. And as it's consumed, that reaction rate decreases. First principles whether combusting nitrocellulose or converting natural gas and water (steam) to hydrogen and CO2 in a steam methane reformer (reactions are all gas phase) to dig sulfur atoms out of petroleum molecules converting them to H2S and providing CO2 to manufacture dry ice and make sodas, beer, and other carbonated beverages fizzy as examples. But that also uses multiple solid catalysts that in turn need to be managed.
I'm saying QED with supporting linked information.
Here's a link to a paper showing differences in evolved combustion products of nitrocellulose in different gas atmospheres for firefighting evaluation walking my fingers. I think it solidly backs up what I've been saying but I suppose someone will argue against it. If it didn't matter the evolved gas compositions would be identical. And if oxygen in the gas phase isn't required why the difference?
https://www.google.com/url?sa=t&sou...FjAKegQICRAB&usg=AOvVaw0cVbV-rYGl4eK7BFCPcycC
I'm 33 years from PChem and got all A's, same for thermodynamics, and lots more core classes for my degree plan. I graduated Summa Cum Laude 3.83 / 4.0 GPA.
I also found this experiment taking 3 minutes for a flame (combustion).front to travel in nitrocellulose. Far from instantaneous. Flame front propagation differences is a really good answer on cartridge case geometry influencing powder efficiency.
https://www.sciencedirect.com/topics/chemical-engineering/nitrocellulose
Of course under higher oxygen partial pressure, the reaction boundary layer has more oxygen readily available. And as it's consumed, that reaction rate decreases. First principles whether combusting nitrocellulose or converting natural gas and water (steam) to hydrogen and CO2 in a steam methane reformer (reactions are all gas phase) to dig sulfur atoms out of petroleum molecules converting them to H2S and providing CO2 to manufacture dry ice and make sodas, beer, and other carbonated beverages fizzy as examples. But that also uses multiple solid catalysts that in turn need to be managed.
I'm saying QED with supporting linked information.
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Jim Watson
Member
I think a gunshot with powder enclosed away from bulk oxygen at 50000 psi and 2700 F for a couple of milliseconds may undergoing somewhat different reactions than an open burn.
One paper said the rate determining step of the decomposition was "reversible loss of NO2". Unfortunately it did not describe other steps in the process, which must amount to a good deal what with the lack of NO2 in the ejecta.
One paper said the rate determining step of the decomposition was "reversible loss of NO2". Unfortunately it did not describe other steps in the process, which must amount to a good deal what with the lack of NO2 in the ejecta.
So why did both reaction kinetics and reaction products change with atmosphere, and cease in a vacuum, based on first principles? No guessing. Honest critical thinking. Gas phase changes things, absolute lack of a gas phase stops the reactions. But using a vacuum to break the combustion of nitrocellulose is prima facia absolutely impractical for the group these tests were conducted for in 1992. No weasleing not being good at PChem voids ability to produce a rational reasoned answer supported at minimum by first principles.
The oxygen in the test flask mattered. The oxygen in a cartridge case also matters. The lack of oxygen in a vacuum matters.
What is the internal pressure in a cartridge case right before any combustion is initiated?.I'd say approximately one atmosphere absolute, same as the paper. I never said pressure does not increase when solids are converted to gas, in fact I've consistently said the opposite. Did you notice that yet?
I went back and put the most recent prior instance in bold font to help out.
The oxygen in the test flask mattered. The oxygen in a cartridge case also matters. The lack of oxygen in a vacuum matters.
What is the internal pressure in a cartridge case right before any combustion is initiated?.I'd say approximately one atmosphere absolute, same as the paper. I never said pressure does not increase when solids are converted to gas, in fact I've consistently said the opposite. Did you notice that yet?
I went back and put the most recent prior instance in bold font to help out.
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mcb
Member
Did you watch the video I posted earlier? In the video black powder in the open vacuum chamber would burn when directly hit with the laser but it could not sustain a chain reaction in the vacuum. Once the laser was removed the combustion stopped.
Then when the black powder was placed in a small glass vial. The vial has a pin hole in the lid so that when the vacuum was pulled it evacuated all the air from the small vial. Now with all the air (and thus oxygen) removed the laser was directed into the vial the black powder combusted and was able to sustain the reaction rapidly consuming all the powder and exploded the small vial. No gaseous oxygen was present but with the powder in a container small enough but not sealed to allow pressure to build the burning and vaporizing fuel and oxidizer and the thermal energy they had generated was kept from escaping the solid propellant still unburnt. This thermal energy is what sustain the reaction despite a lack of any outside sources of oxygen.
Another good video of black powder burning in a vacuum:
Scott Manly covers all the bases on on firing a gun in a vacuum.
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tark
Member
The Estes Co, who makes model rocket motors, say otherwise. They use compressed black powder as their propellant.
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tark
Member
mcb
Member
I did not recognize the bottle so I was not sure if it was actual black powder or a substitute. Either way it is still a self oxidizing propellant that burns in the absence of oxygen. In the same video he does burn rocket fuel from and Estes rocket which we know is a black powder.
tark
Member
tark
Member
I love showing threads like this one to one of my Son's friends. Malcolm lives in California (as does my son ) and he has a couple of Doctorial degrees.....Thermodynamics and Plasma Physics. He used to work at Lawrence Livermore National Laboratories. Guess what they do there......? Same thing they do at Scandia and Los Alamos.
Its always fun to get his assessment of the discussion.
Its always fun to get his assessment of the discussion.
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mcb
Member
One more video:
An actual 38/357 Mag revolver being fired in a vacuum chamber. Unfortunately at the end the explanation is for black powder though that gun was clearly a modern gun using smokeless powder. A valid explanation but not for the propellant used in their hardware example.
An actual 38/357 Mag revolver being fired in a vacuum chamber. Unfortunately at the end the explanation is for black powder though that gun was clearly a modern gun using smokeless powder. A valid explanation but not for the propellant used in their hardware example.
lysanderxiii
Member
While I agree that your upper limit is pretty close to the practical limit. The reasoning is a bit flawed. The flow does not choke, and the flow behind the bullet is supersonic.
The speed of sound is governed by:
v = sqrt [(gamma x R x T)/M]
with (metric units):
v = the speed of sound in m/s
gamma = the adiabatic constant, characteristic of the specific gas, for propellant gas, this is 1.24,
R = universal gas constant (8.314 J/mol K),
T = absolute temperature (typical propellant gas temperatures for small arms is in the 2500 K range), and
M = the average molar mass, for propellant gas, which is somewhere in the .035 to .040 kg/mol range.
Throw all that in a calculator the speed of sound for that gas would be around 800 m/s, well short of the 1750 m/s of a tank gun. It's actually a little short of your high velocity small caliber bullets like 5.56mm which run in the 1000 m/s range. Really hot tank gun propellants might reach 3500 K, but that only raises the speed of sound to around 950 m/s.
For those that don't speak metric 800 m/s = 2625 fps; 1000m/s = 3300fps; and 1750 m/s = 5750 fps,
lysanderxiii
Member
Muzzle flash is caused by the fact that the combustion products of propellant burning are combustible, but there is a lack of oxygen in the barrel behind the bullet. After the bullet exits the barrel the combustion products mix with the ambient air and are ignited by a pressure wave formed by the expanding gas ejected from the barrel.
mcb
Member
Still trying to figure out the details, clearly you have found a flaw in my presentation. That equation above, that we have both referenced, assumes that our particular gas behaves close enough to an ideal gas that the ideal gas assumptions holds. Given your analysis I am either wrong (it happens on rare occasions
) or our gas comprised of combustion byproducts does not behave as an idea gas or we are using the wrong temperature.I am fairly certain that the speed of sound in the working fluid plays a critical role in what limits its maximum velocity since that is not my assertion but what I have read in several credible sources. It is also given as the reason light gas guns exists since the speed of sound in these light gases used to push the projectile is significantly higher than combustion gases from gun powder, again not my assertion but one I have read and am repeating here. It is also use to explain why spring/gas piston airguns can achieve higher velocities than pump up or pre-charged airguns (adiabatic heating raising the speed of sound in the air pushing the pellet) Bullets very well may be exceeding the speed of sound in thr combustion gas as I said it is very difficult to do this from my understanding but also not impossible.
Going to a more fundamental equation for the speed of sound:
v = sqrt(K/r)
K = modulus of bulk elasticity for a gas
r = density
but I can't find good values for these for products of combustion either...
Looking in my one internal ballistic text books (Ballistics Theory & Design of Guns and Ammunition By Carlucci & Jacobson) I am thinking we are not using a proper temperature. As you state typical propellant flame temperatures are ~2500 K but we have to remember that that temperature is the flame temperature of the combustion but as the propellant burns producing heat we are also producing pressure and the act of compressing a gas raises its temperature even in the absence of the heat from combustion (ie adiabatic heating). I suspect actual bore temperature are going above flame temperature. In one problem in the text book they reference temperatures reaching nearly 4000 K in closed bomb testing setups. That higher temperature as you point out does not solve the problem but it does indicate that flame temperature may be too low.
I will keep looking and see if I can make a better more accurate explanation of it. If anyone else finds something useful throw it in.
-rambling
mcb
Member
Can't sleep...
I am really starting to think that the ideal gas laws do not work for the pressures typically seen in firearms. 60,000 psi (450 Mpa) and 2500 K that is well above the critical pressure/temperature for many of the products of combustion; CO2, N2, H2O. At these pressures and temperatures there is no distinction between a gas and fluid. Even at only 10,000 psi and 1500 K we are still above the critical pressure for all three of those byproducts. I am starting to believe that the formula v = sqrt [(gamma x R x T)/M] based on the ideal gas law is not valid for the combustion gases at in-bore pressures and temperatures. I believe what might work is the speed of sound derived from van der Waals equation instead of the ideal gas law but its making my head hurt trying to find the additional needed parameters.
The resulting equation for the speed of sound is much more complicated...
I should have taken the blue pill...
I am really starting to think that the ideal gas laws do not work for the pressures typically seen in firearms. 60,000 psi (450 Mpa) and 2500 K that is well above the critical pressure/temperature for many of the products of combustion; CO2, N2, H2O. At these pressures and temperatures there is no distinction between a gas and fluid. Even at only 10,000 psi and 1500 K we are still above the critical pressure for all three of those byproducts. I am starting to believe that the formula v = sqrt [(gamma x R x T)/M] based on the ideal gas law is not valid for the combustion gases at in-bore pressures and temperatures. I believe what might work is the speed of sound derived from van der Waals equation instead of the ideal gas law but its making my head hurt trying to find the additional needed parameters.
The resulting equation for the speed of sound is much more complicated...
I should have taken the blue pill...
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Yes, this is what I keep seeing from those obstinately not comprehending smokeless propellant stoichiometry instead deliberately choosing to make demonstrably false statements in that regard much less intermediate chemical reactions to produce oxygen ions which are vapor state. Absent a container to bounce back from the reaction ends as the mean free path becomes too great to reach the solid boundary layer while the energy state is above the minimum activation energy. Jim Watson, who admits he once studied Physical Chemistry, an advanced university chemistry course with prerequisites, wouldn't even acknowledge oxygen ions during decomposition to answer how the various atoms get rearranged, and ions and oxidation and reduction reactions are high school chemistry education fundamentals. Nor reaction kinetics, "everything all goes at once".
And beyond that not only obstinately refuses the scientific definition of vacuum requesting data from a pressure transducer in a vacuum. Zero absolute pressure is zero absolute pressure no matter whether pounds per square inch absolute, bar absolute, millimeters mercury absolute, inches of mercury absolute, ad infinitum. This here even describes imperfect vacuum but has clearly been deliberately avoided by the same individual deliberately choosing to post demonstrated false information regarding smokeless propellant stoichiometry including the nitrocellulose component which is at minimum also has stabilizers added
https://en.m.wikipedia.org/wiki/Vacuum
As already pointed out, Dalton's Law, also studiously ignored, shows total pressure is the sum of the partial pressures, so when total absolute pressure approaches and is effectively zero, all partial pressures do the same. When different gases including helium, are introduced instead of a vacuum, the partial pressure of oxygen is proportionately decreased demonstrating Dalton's Law and changes the boundary layer composition at the solid surface in a non-vacuum test chamber.
The oxygen in the cartridge case matters, both atmospheric air and evolved during decomposition.
It's obvious where the dark side exists and that's the deliberate dishonesty the electrons demonstrate in the face of deliberate attempt to deny that was performed when the dishonesty was shown to be without a doubt.
And beyond that not only obstinately refuses the scientific definition of vacuum requesting data from a pressure transducer in a vacuum. Zero absolute pressure is zero absolute pressure no matter whether pounds per square inch absolute, bar absolute, millimeters mercury absolute, inches of mercury absolute, ad infinitum. This here even describes imperfect vacuum but has clearly been deliberately avoided by the same individual deliberately choosing to post demonstrated false information regarding smokeless propellant stoichiometry including the nitrocellulose component which is at minimum also has stabilizers added
https://en.m.wikipedia.org/wiki/Vacuum
As already pointed out, Dalton's Law, also studiously ignored, shows total pressure is the sum of the partial pressures, so when total absolute pressure approaches and is effectively zero, all partial pressures do the same. When different gases including helium, are introduced instead of a vacuum, the partial pressure of oxygen is proportionately decreased demonstrating Dalton's Law and changes the boundary layer composition at the solid surface in a non-vacuum test chamber.
The oxygen in the cartridge case matters, both atmospheric air and evolved during decomposition.
It's obvious where the dark side exists and that's the deliberate dishonesty the electrons demonstrate in the face of deliberate attempt to deny that was performed when the dishonesty was shown to be without a doubt.
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For providing thrust in vacuum, as previously asked?
Untrue from stoichiometry to begin with. Plus containment of oxygen ions is required to sustain the reaction. The boundary layer is where the reactions occur including the external surface of individual solid propellant particles plus the pores. Without renewal of fresh oxygen whether atmospheric or evolved along that boundary layer while in a state above reaction energy threshold reaction ceases. Reaction kinetics are a teal world plank of science including these solid propellants, it must propagate. It is not quantum behavior with nothing intermediate existing between "ladder rungs" of reactants to products. CUP test pieces do not spring from fresh untested to elongated as "ladder rungs", but elastic deformation over time with intermediate states. Pressure curves are real outside true vacuum by definition of vacuum, which is absolute zero pressure; containment of any and all evolved gases is not vacuum. Dalton's Law applies.
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mcb
Member
I did a little more research (even touched my fluids and thermal text books {ewww...}) and I am fairly convinced we cannot treat these combustion gases as an ideal gas. Nearly all the products of combustion are supercritical fluids while in the gun bore and thus the ideal gas law no longer works. The ideal gas law assumes there is more space than particles and that the particle do not chemical react or are not attracted to each other, they simple act like perfectly elastic particles bumping into each other. CO2 at 20,000 psi and 800K is nearly the same density as water at room temperature. And there is no doubt that the particles in the gun barrel are interacting as more than simple elastic particles since a fair portion of them are combusting with each other.
I have found research paper measuring CO2 (a product of combustion in our discussion) above the critical point (7.39 Mpa/1071 psi and 31C/88F) and they have experimental measure the speed of sound in super critical CO2 at over 500 m/s at ~10MPA (1500 psi) only slightly elevated temperatures (~400K). Take that temperature up to 2500+ K and I think we would have a very high speed of sound.
In many ways supercritical fluids behave like liquids rather than gasses. The speed of sound in air at standard conditions is, as we all know, ~340 m/s in a liquid like water at room temperature is nearly 1500 m/s. As I stated in the first paragraph CO2 at only 20,000 psi has nearly the density of water. In that light I think a speed of sound in combustion gases approaching 2000 m/s seem plausible. That said I really wish I could find something more definitive.
-rambling
I have found research paper measuring CO2 (a product of combustion in our discussion) above the critical point (7.39 Mpa/1071 psi and 31C/88F) and they have experimental measure the speed of sound in super critical CO2 at over 500 m/s at ~10MPA (1500 psi) only slightly elevated temperatures (~400K). Take that temperature up to 2500+ K and I think we would have a very high speed of sound.
In many ways supercritical fluids behave like liquids rather than gasses. The speed of sound in air at standard conditions is, as we all know, ~340 m/s in a liquid like water at room temperature is nearly 1500 m/s. As I stated in the first paragraph CO2 at only 20,000 psi has nearly the density of water. In that light I think a speed of sound in combustion gases approaching 2000 m/s seem plausible. That said I really wish I could find something more definitive.
-rambling
Supercritical fluids do not exist in vacuum. Zero Fluids. So including evolved supercritical fluids.
Are oxygen ions supercritical fluids?
https://en.m.wikipedia.org/wiki/Vacuum
I worked with industrial scale supercritical fluid extraction units during my career, I possess both education and hands-on experience in the field. This is one such licensed process technology for such I worked with. This is a mass transfer application.
https://www.google.com/url?sa=t&sou...FjAAegQIARAB&usg=AOvVaw0PWNYIhJ-2_w0PGoXUX3CF
The boundary layer must be renewed with fresh reactants to ionize in order to react. Period. In the absence of mechanical agitation, and exothermic (producing heat) conditions, fluid convection would be a pretty good way to accomplish this. Hotter fluid products move out, (relatively) cooler fluid reactants move in, along the boundary layer.
In a helium atmosphere, the boundary layer is crowded with helium molecules, retarding overall reaction kinetics and shifting intermediate reaction pathways to different proportions of reaction products.
A pure oxygen atmosphere would likewise shift proportions of reaction products in the absence of atmospheric nitrogen (NOx species),
https://en.m.wikipedia.org/wiki/NOx
but that, sensibly, was not part of the Army's experiment framework regarding potential fire retardants. It *should* go without saying, not advisable in the test apparatus used, in addition.
Are oxygen ions supercritical fluids?
https://en.m.wikipedia.org/wiki/Vacuum
I worked with industrial scale supercritical fluid extraction units during my career, I possess both education and hands-on experience in the field. This is one such licensed process technology for such I worked with. This is a mass transfer application.
https://www.google.com/url?sa=t&sou...FjAAegQIARAB&usg=AOvVaw0PWNYIhJ-2_w0PGoXUX3CF
The boundary layer must be renewed with fresh reactants to ionize in order to react. Period. In the absence of mechanical agitation, and exothermic (producing heat) conditions, fluid convection would be a pretty good way to accomplish this. Hotter fluid products move out, (relatively) cooler fluid reactants move in, along the boundary layer.
In a helium atmosphere, the boundary layer is crowded with helium molecules, retarding overall reaction kinetics and shifting intermediate reaction pathways to different proportions of reaction products.
A pure oxygen atmosphere would likewise shift proportions of reaction products in the absence of atmospheric nitrogen (NOx species),
https://en.m.wikipedia.org/wiki/NOx
but that, sensibly, was not part of the Army's experiment framework regarding potential fire retardants. It *should* go without saying, not advisable in the test apparatus used, in addition.
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Here's a link regarding thermobaric weapons I mentioned. @Jim Watson may be interested as well. The reactions here demonstrably propagate, but could "seem" to be all at once.
The Celanese chemical plant a few miles from the university I graduated from used, and stored, ethylene oxide as an intermediate, it was shipped there by rail cars
The storage tanks were covered by multiple layers of earth and concrete. They stopped doing that shortly before I graduated. On a AICHE student chapter tour, I struck up a conversation with my guide. He told us newly required risk modeling had shown if they had ever exploded / detonated / uncontrollably oxidized (take your pick) the small town beside the plant would be leveled and every window in the city where that university was would be stattered, at minimum The empty tanks were still there in under their blanket at that time.
Legacy of the "It's never happened yet" mindset to risk awareness and risk management was no longer propagated!
The Celanese chemical plant a few miles from the university I graduated from used, and stored, ethylene oxide as an intermediate, it was shipped there by rail cars
The storage tanks were covered by multiple layers of earth and concrete. They stopped doing that shortly before I graduated. On a AICHE student chapter tour, I struck up a conversation with my guide. He told us newly required risk modeling had shown if they had ever exploded / detonated / uncontrollably oxidized (take your pick) the small town beside the plant would be leveled and every window in the city where that university was would be stattered, at minimum The empty tanks were still there in under their blanket at that time.Legacy of the "It's never happened yet" mindset to risk awareness and risk management was no longer propagated!
Before someone uses Youtube videos to define what ions are, I'll put this here although whether it'll honestly be used is unlikely.
https://en.m.wikipedia.org/wiki/Ion
An important piece of info:
"Ions in their gas-like state are highly reactive and will rapidly interact with ions of opposite charge to give neutral molecules or ionic salts."
https://en.m.wikipedia.org/wiki/Ion
An important piece of info:
"Ions in their gas-like state are highly reactive and will rapidly interact with ions of opposite charge to give neutral molecules or ionic salts."
wally
Member
If you are constrained to not destroying the barrel you are limited to basically the speed of sound in what ever pressure/temperature your barrel can withstand. If single shot is OK, the RPG etc get 20,000 FPS+ compressing a copper cone into a projectile slug with a detonation.
The "light gas gun" idea with a vacuum has been used to great effect to launch ping-pong balls super-sonic and blow a hole through the paddle!
The "light gas gun" idea with a vacuum has been used to great effect to launch ping-pong balls super-sonic and blow a hole through the paddle!
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