D
dtknowles
Guest
I'm a broken record
The challange is going to be the model for pressure rise time, all the real important (hard to model) suff happens before the bullet is inches into the rifling. This includes (in my unsubstatiated opinion) the temperature sensitivity. It is not a barrel chilling the expanding gasses it is the slower ignition of the powder if its cold. Burn rate is so sensitive to pressure that if you get the pressure rise rate wrong you are going to miss the peak pressure. Some powders have a serious sensitivity to certain pressure levels (very peaky) being off on the pressure rise will put you way off on peak pressure depending whether you predict getting to the sensitive pressure range while there is still powder to burn. The mechanics are going to be more important than you might think (driving the thermodynamics) Neck Tension, Freebore, Bullet Hardness, Bearing Area, even the shape and angle of the Leed (Lead). It can even be necessary to model the powder position and shoulder angle. Powder position has be scapegoated for detonation. I have seen personally where powder position has caused significant velocity variation to the degree where it could be detected by audible differences in the report. Case shape has been reported to influnce cartridge efficiency, for the same volume, shorter, fatter with sharp shoulder produces higher velocity with equal powder.
Just throwing this stuff out there becasue I think so are raising interesting points that are at most secondary to accurate pressure modeling.
Tim
The challange is going to be the model for pressure rise time, all the real important (hard to model) suff happens before the bullet is inches into the rifling. This includes (in my unsubstatiated opinion) the temperature sensitivity. It is not a barrel chilling the expanding gasses it is the slower ignition of the powder if its cold. Burn rate is so sensitive to pressure that if you get the pressure rise rate wrong you are going to miss the peak pressure. Some powders have a serious sensitivity to certain pressure levels (very peaky) being off on the pressure rise will put you way off on peak pressure depending whether you predict getting to the sensitive pressure range while there is still powder to burn. The mechanics are going to be more important than you might think (driving the thermodynamics) Neck Tension, Freebore, Bullet Hardness, Bearing Area, even the shape and angle of the Leed (Lead). It can even be necessary to model the powder position and shoulder angle. Powder position has be scapegoated for detonation. I have seen personally where powder position has caused significant velocity variation to the degree where it could be detected by audible differences in the report. Case shape has been reported to influnce cartridge efficiency, for the same volume, shorter, fatter with sharp shoulder produces higher velocity with equal powder.
Just throwing this stuff out there becasue I think so are raising interesting points that are at most secondary to accurate pressure modeling.
Tim
243winxb
Member
"suff happens before the bullet is inches into the rifling."
Army Research Laboratory Experimental studies of the NO. 41 Primer and Ignition of 5.56mm Ammunitions http://www.arl.army.mil/arlreports/2006/ARL-TR-3922.pdf
Army Research Laboratory Experimental studies of the NO. 41 Primer and Ignition of 5.56mm Ammunitions http://www.arl.army.mil/arlreports/2006/ARL-TR-3922.pdf
M
mks
Guest
Tim,
If that were the case, then powder temperature, but not barrel temperature, should affect maximum chamber pressure, right? There are some well-documented experiments here (www.shootingsoftware.com/ftp/Pressure Factors.pdf) that show that both ammo and barrel temperature affect maximum chamber pressure.
A simple calculation (from the current tuner thread) shows that the thermal capacity of an 80 oz stainless steel barrel is 1135 joules per degree Kelvin, while the energy that we get out of the gunpowder (for a 30BR) is about 2200 ft lb or 2983 joules. So the barrel, by soaking up enough heat to raise its temperature one degree Kelvin quenches over 1/3 as much energy as we get out of the charge.
How about this for a hypothesis: powder temperature, neck tension, seating depth, etc., determine peak pressure during the first couple of inches of bullet travel, but barrel temperature (and maybe several other things) determines energy (proportional to the area under the pressure vs. time curve)?
Cheers,
Keith
Bob Kingsbury
New member
Mks
I think your theory is well accepted
I think your theory is well accepted
D
dtknowles
Guest
Very little difference
Heat transfer from powder gasses to the barrel is not very dependant on the barrel temperature. That heat transfer is taking place whether the barrel is cold or hot because the barrel is always much colder than the combustion temperature. It is slightly greater when the barrel is cold because of the slightly greater delta T but I don't believe that the difference is significant (meaning make a meaning full difference and even then it is not large perfromance driver but is detectable) until you are getting barrel temperture in the sub freezing or sub zero range. I also think you have an error in you heat transfer analysis, I think you used the bullet KE instead of total energy. Either need to use the energy of liberation of the chemical reaction (burning powder) or the sum of all energy created during the shot, bullet KE plus bullet temp rise, gas KE plus gas temperature rise, and barrel temperature rise due to bullet friction. This leads to the additional error in your the barrel rise temperature analysis how much of the barrel temperature rise is due to the friction of the bullet passing thru the bore at high velocity and how much is due to heat transfer from the gas. You are underestimating the energy available for heat transfer to the barrel and overestimating the heat transfer contribution of the combustion gasses.
Tim
Heat transfer from powder gasses to the barrel is not very dependant on the barrel temperature. That heat transfer is taking place whether the barrel is cold or hot because the barrel is always much colder than the combustion temperature. It is slightly greater when the barrel is cold because of the slightly greater delta T but I don't believe that the difference is significant (meaning make a meaning full difference and even then it is not large perfromance driver but is detectable) until you are getting barrel temperture in the sub freezing or sub zero range. I also think you have an error in you heat transfer analysis, I think you used the bullet KE instead of total energy. Either need to use the energy of liberation of the chemical reaction (burning powder) or the sum of all energy created during the shot, bullet KE plus bullet temp rise, gas KE plus gas temperature rise, and barrel temperature rise due to bullet friction. This leads to the additional error in your the barrel rise temperature analysis how much of the barrel temperature rise is due to the friction of the bullet passing thru the bore at high velocity and how much is due to heat transfer from the gas. You are underestimating the energy available for heat transfer to the barrel and overestimating the heat transfer contribution of the combustion gasses.
Tim
D
dtknowles
Guest
I think the author explained it very well
If your read all the way to the end the author got to where I was going while I was reading the article. The barrel temperature effect is in its effect on primer temperature not on the cooling of the combustion gases. Heat transfer from the barrel to the cartridge case and primer is much faster that to the powder or the thermocoulple in the middle of the case.
Tim
If your read all the way to the end the author got to where I was going while I was reading the article. The barrel temperature effect is in its effect on primer temperature not on the cooling of the combustion gases. Heat transfer from the barrel to the cartridge case and primer is much faster that to the powder or the thermocoulple in the middle of the case.
Tim
M
mks
Guest
Tim,
In a second round of experiments, Bramwell found that primer temperature had no effect on chamber pressure or muzzle velocity (www.snipershide.com/UserFiles/.../powder_primer_temp_experiment_2.pdf). However, cartridge and barrel temperature were both important, with barrel temperature being more important than cartridge temperature. Here is what he says:
"The fundamental reason that ammunition temperature has an effect on pressure and velocity is that heat energy absorbed by the cartridge components is energy lost to propulsion. Warm cartridges absorb less heat than cold cartridges.
Similarly, cold receivers and barrels absorb more propulsion energy than warm ones. Since the thermal mass of a barrel and receiver is much larger than that of a cartridge, this effect is much stronger than the effect of cartridge temperature."
Keith
M
mks
Guest
Tim,
My calculation of 1135 joules per degree K is just the result of multiplying the mass of the barrel times its specific heat. It is just a property of the steel, and doesn't depend on kinetic energy or friction.
A surprisingly small change in heat transfer could possibly explain significant changes in muzzle velocity. A 1% change in muzzle velocity is large (30 fps change from 3000 fps nominal). A 1% change in heat transfer could result from a 22 K barrel temperature change (2500 K flame temperature, 300 K nominal barrel temperature). That's about a 40 F change. I'm not ready to rule out the effect of barrel temperature just yet.
Cheers,
Keith
PS. For those who are keeping track, I underestimated flame temperature over in the tuner thread. Sorry.
D
dtknowles
Guest
Keith
The link for exp. #2 did not work for me but I would be glad if primer temperature did not effect velocity. I did not intend to inticate that barrel temperature or "Gun Temperature" did not effect velocity, my point was that the effects were small and if I was to build a model for internal ballistics for small arms that the other factors I mentioned would be where I would put most of my effort.
Tim
D
dtknowles
Guest
I was not questioning your calculation of the heat capacity of the barrel, I was saying that you were massivly underestimating the energy available to heat the barrel. While the colder barrel robs the gas of more heat there is a lot of energy available to heat the barrel and not all the rise in barrel temperature is do to energy robbed from the gas much comes from friction between the bullet and the barrel. It has been noted that Moly coated bullets cause much less barrel heating than uncoated bullets.
You imply that a 1% increase in heat transfer from the gas to the barrel would cause a 1% decrease in velocity, would you care to substantiate that implication. The greatest part of the heat transfer occurs during the part of a bullets travel that produces the least increase in velocity, the bullets travel down the last half of the barrel.
I also think that in most cases a 1% variation in muzzle velocity is statisically insegnificant not large as you stated. Most data I have seen on cartridges with velocities in the 3000 fps range 30 fps is less than the standard deviation.
Tim
D
dtknowles
Guest
Keith
I did not read the tuner thread but its mention leads me to make this clarification relating to Bench Rest shooting. The temperature effects certainly are great enough to effect tuning and if that was your point in defending the story about how gun temperature effects are real than your point is granted.
My point is just in building a ballistics model there are many factors that have to be modelled that have a much greater effect on velocity and pressure than gun temperature.
If someone could build a model that could predict the load need to maintain tune based on conditions his skills are need some place else beside benchrest shooting because he is a genuis.
Tim
M
mks
Guest
Oops, looks like the link got abbreviated. Here is the full address: http://www.snipershide.com /UserFiles/Image/articles/powder_primer_temp_experiment_2.pdf
Just copy the address into your browser and delete the space after ".com"