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 Post Posted: Sat Jul 26, 2003 3:00 pm 
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Originaly posted by IsaacKuo,
If you ever got past high school chemistry you would know that an amorphous solid is an amorphous SOLID. It's not a liquid, period. Click on any of the links in the following Google search:

http://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=glass+flow+myth+liquid


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 Post Posted: Sat Jul 26, 2003 4:00 pm 
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Originaly posted by IsaacKuo,
For example, look at these muzzle velocity tests for Beeman's RX-2

Beeman RX-2

There is NO absolute speed limit on a gas gun. However, the efficiency of trying to push a projectile beyond the speed of sound of the propelling gas goes down extremely dramatically. It's trivially easy to prove the possibility with this thought experiment:

Consider a gas gun designed to launch a projectile at 60% of the speed of sound (of its gas). This is a long tube closed off on one end and open in the other. The high pressure gas is just behind the bullet and it's ready to go.

Now place this gas gun inside a bigger gas gun, also designed to launch its projectile at 60% of the speed of sound. In this case, the projectile is the entire gun described above! Obviously, this larger gun is MUCH larger and more powerful than the first one.

Imagine firing both of these gas guns at almost the same time--first the big one and then the little one. The first gas gun accelerates the little gas gun to 60% of the speed of sound. Then the second gas gun accelerates the bullet to 120% of the speed of sound. The bullet is fired off at a supersonic speed, no big deal.

Now here comes the interesting part of the thought experiment--you can modify this scenario in steps to eliminate the small gas gun entirely. First, remove the little gas gun's barrel--it uses the big gas gun's barrel to hold pressure in (the big gas gun's barrel needs its bore reduced and may need its length increased). This just leaves a cylindrical base "plug" for the little gas gun. Second, remove that "plug" entirely. It does nothing to increase the bullet's acceleration, and in fact all it really does is leach energy away from the system.

Now, the removal of the unnecessary little gas gun from the system increases overall efficiency a lot. However, it doesn't change the fact that a huge amount of increased gas is required to get a small amount of increased muzzle velocity.

Notice that if you conceptually add a third gas gun stage, or a fourth stage, and so on, you can theoretically acheive ANY muzzle velocity. However, it doesn't take many stages before you're looking at a huge gun the size of Texas...


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 Post subject: Show me the money....
 Post Posted: Sat Jul 26, 2003 4:00 pm 
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Originaly posted by thehalls,

"For example, look at these muzzle velocity tests for Beeman's RX-2"

Yup. I did. The fastest one there is 1092 fps. That's Mach 0.93. Note that 0.93 is not greater than or equal to 1.0.

As for your hypothetical "gun shooting a gun"... Sorry, it doesn't work that way. While a gun shooting a gun could work as you first discuss, when you start removing those plugs your assumptions break down. Go look at how REAL 2-stage gas guns work.


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 Post Posted: Sat Jul 26, 2003 4:00 pm 
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Originaly posted by thehalls,

OK, if you want to discuss density effects on the speed of sound, you can do that. Certainly it's within the realm of real-world fluid mech. HOWEVER, when dealing with gases, those effects are negligable!


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 Post subject: sure.
 Post Posted: Sat Jul 26, 2003 4:00 pm 
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Originaly posted by pacogoatboy,
first off, i am talking about the speed of the potato whereever it happens to be at the moment, inside the gun, outside of the gun, whereever. it really doesn't matter, as long as it is exceeding the speed of sound in that medium. that's why i wrote that you weren't QUITE right. i agree entirely that it is impossible for an object to be propelled by a fluid faster than the speed of sound in that fluid.

a spud gun is not adiabatic. in an adiabatic system, all energy that is lost must be converted into work. in this system, there IS friction, and thus heat loss. it may be over quickly, but that doesn't matter. you can't just say that something didn't happen because it happened fast. the entire system moves quickly, that's why we like them.

all of these formulae assume air to be ideal as well, which it is not. at pressures that deviate significantly from atmospheric, air behaves badly.

http://gsd.ime.usp.br/~yili/SpeedOfSound/Speed.html
I can't remember where I originally saw the equation for speed of sound / pressure dependence, but I did find this in about ten seconds. This is a link to a page with a java applet for calculating the speed of sound as a function of T and humidity. it lists a complete formula from an article in the Journal of Acoustical Society of America, admittedly not the engineering text you asked for, but still... there are about a hundred factors in the formula, including T, P, and water and C02 concentration. if you do the math, there is a small pressure dependence in the speed of sound. in this case, the speed of sound increases by about 1 ft/s at 100psi. it's not much, but it's enough. (read carefully, not QUITE right, no need to be so stinking defensive.)

i am pretty sure that we would actually agree with one another on this if we were talking, but instead, we are writing in a forum, so misunderstandings are easy.

finally, i am happy that you have calibrated chronographs. please shoot a potato with a very well designed spudgun (huge chamber/barrel ratio, long barrel, great valve) and measure its velocity for me. i honestly would like to know. have a nice day.


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 Post subject: So.......
 Post Posted: Sat Jul 26, 2003 5:00 pm 
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Originaly posted by SwampFox,
it's so far UNDETERMINED whether a projectile (potatos, pop cans, soda bottles, ETC) can travel at or excede the speed of sound with pnuematics, but what I don't get is what limits the fluids (the compressed air in the chambers) to travel at mach one? I see now, however, that in my opinion it is either imposible to achieve mach one OR very hard. As for the different types of gases thing, I DO have acess to helium and hyrogen but I'm not ready to go there...

But anyway, if I wanted to achieve a VERY high FPS, what should I consider OTHER than the valve of the cannon (barrel length, C:B ratio, projectile demensions, etc...)?


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 Post subject: Glass Does Not Flow
 Post Posted: Sat Jul 26, 2003 5:00 pm 
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Originaly posted by mta,
The erroneous belief that glass flows is widespread. I even had a High School Chemistry (or maybe it was Physics) teacher explain to me that glass is not a true solid and that it flows, as evidenced by the thicker glass at the bottom of windows in ancient cathedrals. The teacher was otherwise very good, but in this instance he was very wrong. I believed it at the time, and college chemistry and physics courses did not disabuse me of the idea. I now know better.


Do your own google search and you will see. In the old days, glass could not be made of uniform thickness. When installing a pane of glass, it was natural to put the thicker part of the glass at the bottom. But, this was not always done. There are windows that have been in place since medevial times that are thicker at the top, on the right and on the left.. due to the particular installation.

Glass does not flow. Google it and see.



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 Post Posted: Sat Jul 26, 2003 9:00 pm 
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Originaly posted by corncob909,


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 Post Posted: Sat Jul 26, 2003 11:00 pm 
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Originaly posted by IsaacKuo,
I know how exactly how 2 stage gas guns work--do you know how normal 1 stage gas guns work?

Seriously, it's well known that airguns can exceed the speed of sound, but it's terribly inefficient. The thought experiment I describe gives a theoretical backing for why it's possible. You say that "assumptions break down", but in fact you can't find any flaw in the reasoning.

The simple fact is that air travelling down the barrel of a gun has no idea how fast it's going relative to the outside world and due to basic physics it doesn't care. The true practical speed limits have more to do with specific energy densities and wasting energy into accelerating air vs the projectile than with the speed of sound. Make a barrel long enough and frictionless enough, and the original kinetic energy is going into the air/bullet, regardless of whether that results in a final velocity greater than any particular speed of sound.

What you fail to understand is that while the speed of sound in a medium does limit the speed at which a force can propogate, the medium itself may be moving! That is what happens when air gets accelerated down a barrel--the air itself has a particular velocity. A sound which propogates within this moving air may indeed be limited to the speed of sound in the frame of reference of the moving air, but relative to the rest of the world this is faster than the speed of sound.


Also, it's also well known that the speed of sound is NOT constant--it depends on altitude and other atmospheric conditions. You're simply presuming a high speed of sound whereas those people actually doing the chronograph testing surely knew their speed of sound in the conditions being tested.


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 Post subject: Alright, Dang It!
 Post Posted: Sun Jul 27, 2003 8:00 pm 
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Originaly posted by man_o_brass,
I don't know one way or another whether or not a pneumatic can break mach 1. Nobody's been able to convince me one way or another. SOMEBODY needs to give an actual text reference for some backup. Personally, I'd like somebody to explain to me the different properties of sound waves and mass waves(probably not correct terminology). For instance, you can hear a breeze blowing through the trees seconds before you feel the actual air mass blow past your face. Both are caused by pressure, but exhibit very different qualities. In a rifle, the "mass wave" hits the outside air traveling at high mach numbers, but dissipates rapidly. The report of the shot, however can be heard for sometimes a mile or more.

It seems to me that the only difference between performance in rifles and in airguns is energy density of the propellant. Initial chamber pressures in rifles often exceed 50,000psi. I can't help but think that if you compressed regular air to similar pressures, you would get similar results. I prefer to think of combustion as nothing more than a means of generating high pressures. Am I wrong? If so, then give me something verifiable and not just opinions. Let's put this debate to an actual end, instead of an argument that just pisses people off.


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