Loosedhorse
member
Psssst, denton...
Don't look now, but check out post #12.
Don't look now, but check out post #12.
Firearms thermodynamic efficiency
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denton
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It's one of the joys of being forgetful. Everything old is new again.
PercyShelley
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OK, I see it now. Yes, if you measure the thermodynamic efficiency of the rocket in terms of increase in velocity of the rocket from a stationary point of reference, yes you do get wonky efficiency numbers like that.
Which is why it makes more sense to measure efficiency as percent of thermal energy converted to kinetic energy of the propellant.
You'd get similar weirdness (although to a lesser degree because of, as you point out, the more skewed mass ratio) if you measured the efficiency of a firearm in space as a function of propellant energy converted to projectile motion relative to a stationary point of reference. As the gun floated back with each shot, it would get less and less efficient.
The BAR tests are interesting. I would not have guessed that you lose almost as much energy to heating the case up as you do to barrel friction!
At SHOT there was a new plastic-cased ammo company that mentioned that they were getting a slight velocity boost with the same loads as regular ammo just from switching the case material. I wonder if it's because less energy was going into heating up the case, or because the case is insulative enough that it's preventing some of the energy from leaking into the firing chamber. It could also be because plastic is bulkier and the case has less volume, which would lead to higher efficiency as well. Perhaps it's all three.
Cahrle's law,like therdieal gas law does indded requoire assumptions about reactivity.
2H + 1O yields 1 H2O.
The volume law just failed.
The ideal gas law also fails since tt encompasses Charle's law.
If the slightest bit of condensation occurs, the ideal gas law fails horribly.
It cannot work in a state change, or if liquid gas is present.
you can keep taking gas out with liquid preset above the boiling point with zero change in pressure until all the liquid vaporizes.
That is why pressure gauges cannot tell the contents of a propane tank or Dewar with any liquid nitrogen oxygen, or refrigerant. The liquid is above boiling, and any removal f gas immediately cases vaporization of the liquid.
The laws can be used carefully for short times compared to any further reaction rates, but temperature changes and composition changes can easily make the gas so far from ideal things come apart.
Even the ideal gas constant is slightly different among ideal gases.
The pressure will not decrease til the liquid is evaporated.
The military is interested in thermodynamic performance, especially in machine guns and other weapons that can fire rapidly compered to their ability to shed heat.
It is not the efficiency they care so much about, but the heating produced from deflagration.
Hot steel wears faster than cooler steel from friction.
SimplyChad
Member
Everytime I think no we wouldnt burn money like that. Well im proved otherwise.
denton
Member
But they work for polyatomic gasses, which are not ideal.
I Thought It Might be Helpful
if physics terms could mean the same thing each time they're used, and no matter who uses them. Too often such terms are used indiscriminately, or even interchangeably, when in fact they have specific definitions and produce nonproductive conclusions when not used correctly.
So -- here:
ENERGY: the ability to perform work
WORK: a force acting through a distance; the product of force times distance
KINETIC ENERGY: the energy which an object possesses due to its motion; the work needed to accelerate a body of a given mass from rest to a specific velocity.
FORCE: an influence that causes an object to undergo a change in speed, a change in direction, or a change in shape
POWER: A measurement of the rate at which energy is dissipated; i.e. transferred, used, or transformed
MOMENTUM: This is a difficult concept to narrow down to a short definition. One of the best descriptions that I have seen applied to the term “momentum” is “the amount of force required to stop a moving object” It’s not perfect, but for our purposes here that description will suffice.
And never forget, the formula for kinetic energy is one-half the mass times the square of the velocity.
For momentum, just plain mass times velocity.
Momentum and energy are not the same thing.
Have fun!
if physics terms could mean the same thing each time they're used, and no matter who uses them. Too often such terms are used indiscriminately, or even interchangeably, when in fact they have specific definitions and produce nonproductive conclusions when not used correctly.
So -- here:
ENERGY: the ability to perform work
WORK: a force acting through a distance; the product of force times distance
KINETIC ENERGY: the energy which an object possesses due to its motion; the work needed to accelerate a body of a given mass from rest to a specific velocity.
FORCE: an influence that causes an object to undergo a change in speed, a change in direction, or a change in shape
POWER: A measurement of the rate at which energy is dissipated; i.e. transferred, used, or transformed
MOMENTUM: This is a difficult concept to narrow down to a short definition. One of the best descriptions that I have seen applied to the term “momentum” is “the amount of force required to stop a moving object” It’s not perfect, but for our purposes here that description will suffice.
And never forget, the formula for kinetic energy is one-half the mass times the square of the velocity.
For momentum, just plain mass times velocity.
Momentum and energy are not the same thing.
Have fun!
Certaindeaf
member
I wonder what Clark would say. lol
Loosedhorse
member
Airborne Falcon
Member
Loosedhorse ... I lol'd.
Yoda
Member
So, a guy throwing a rock, or flinging one in a sling, is more efficient that a rifle, because there is no barrel to heat up?
Airborne Falcon
Member
That's over simplifying it just a bit, is it not?
Loosedhorse
member
No one said otherwise.
O2, N2 are diatomic and behave reasonably well.
The are relatively non-reactive in a pure mixture.
Even better if you use the gas ideal gas constants for THE CORRECT GAS.
Even the 'ideal' constant varies with gases and non-reacting ,mixtures of gases.
If you mix H2 and O2 the mixture behaves 'nicely' right up until the reaction starts.
And the resulting hot water vapor behaves reasonably well until any of it condenses
Not all gases behave under all conditions as ideal gases, and state changes and reactions are two if the big poisoners.
Try using it with any liquified gas present in the container and see how far you get.
Propane, carbon dioxide, even a Dewar with liquid oxygen or nitrogen.
The ideal gas law fails with state changes or chemical reactions in the gases.
PercyShelley
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My vague recollection is that railguns have pretty good efficiency, it's coilguns that have absolutely terrible, single digit efficiency.
Hmmm...
Is it a mistake to assume that the pressure detected by a strain gauge mounted on the barrel somewhere is representative of the pressure behind the projectile? Things are happening fast enough in a gun barrel that I'm not sure Pascal's Law applies. I don't think this would work if the changes in pressure were being applied instantaneously across the entire bore area.
I also recall reading a small arms design manual which mentioned that propellant gasses have net velocity, and that this has practical consequences. Erosion tends to be more aggressive on the borewards side of gas ports, for example, because the gas is moving in the same direction as the bullet and tends to smack into that side of the port and wear it down faster.
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