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 Post subject: MACH 1 !? Here's...
 Post Posted: Fri Jul 25, 2003 8:00 pm 
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Originaly posted by SwampFox,
my design plan to achive it. It's VERY similar to the gun on recent developments...3" sch 80 barrel at AROUND twenty feet long into a TEE which lets downward into the SUPAH VALVE PLUS then into a chamber 4" PVC...the ratio will be 1.5 to 1. The tee will alow to breach load pop cans. will this achive mach 1? what else sound i consider to achive more effeciancy to maybe even excede the sound barrier? thanks!


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 Post Posted: Fri Jul 25, 2003 9:00 pm 
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Originaly posted by thehalls,

...The limiting factor (read: theoretical maximum velocity) for ANY gun is the speed of sound in the working fluid. In the case of air guns, hey, that's Mach 1.0000000000000. But achieving theoretical maximums is just that - theory - you just can't do it in the real world.

Result: Anybody telling you that they've an air gun breaking Mach 1 is either (a) full of **** but they don't know it, (b) full of **** but they do know it, or (c) got a trick up their sleeve and they're not telling you the whole story. In most cases, it's gonna be 'a'.




For a bit more nerdy fun....

The speed of sound (typically designated as 'a') is dependent upon two things: the gas chemistry and it's temperature. More to the point:

a = sqrt(GAMMA * Rbar/MW * T)

Where:
GAMMA = ratio of specific heats for the gas (depends on gas chemistry but doesn't very MUCH)
Rbar = universal gas constant (a constant, period)
MW = molecular weight for the gas (another chemistry thing)
T = temperature on an absolute scale.

So how do you make a gun go fast? There are two ways....

1) Heat the gas up really hot. This is how firearms work.
2) Use a low MW gas like Helium or Hydrogen. This is how some insanely fast guns (called 2-stage light gas guns, do a google) work.



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 Post Posted: Fri Jul 25, 2003 9:00 pm 
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Originaly posted by thehalls,

...The limiting factor (read: theoretical maximum velocity) for ANY gun is the speed of sound in the working fluid. In the case of air guns, hey, that's Mach 1.0000000000000. But achieving theoretical maximums is just that - theory - you just can't do it in the real world.

Result: Anybody telling you that they've an air gun breaking Mach 1 is either (a) full of **** but they don't know it, (b) full of **** but they do know it, or (c) got a trick up their sleeve and they're not telling you the whole story. In most cases, it's gonna be 'a'.




For a bit more nerdy fun....

The speed of sound (typically designated as 'a') is dependent upon two things: the gas chemistry and it's temperature. More to the point:

a = sqrt(GAMMA * Rbar/MW * T)

Where:
GAMMA = ratio of specific heats for the gas (depends on gas chemistry but doesn't very MUCH)
Rbar = universal gas constant (a constant, period)
MW = molecular weight for the gas (another chemistry thing)
T = temperature on an absolute scale.

So how do you make a gun go fast? There are two ways....

1) Heat the gas up really hot. This is how firearms work.
2) Use a low MW gas like Helium or Hydrogen. This is how some insanely fast guns (called 2-stage light gas guns, do a google) work.



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 Post subject: he's not quite right
 Post Posted: Fri Jul 25, 2003 10:00 pm 
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Originaly posted by pacogoatboy,
that formula only applies to adiabatic conditions (changes that do not involve gains or losses of heat). a spudgun definitely is not an adiabatic system. also, this does not take into account the increased pressure of the gas inside of the chamber and barrel. the speed of sound is dependent upon a number of things, including pressure (and because it is a gas, increased pressure means increased density, in a big way). the increase in density in a pressurized system would allow the potato to exceed the speed of sound in sea level atmospheric air. while in the gun, the potato would never truly reach Mach 1, because the Mach number is the ratio of a body's speed to the speed of sound in that body. however, once the potato exits the gun, it is in regular atmospheric air (more or less) and may be in excess of the speed of sound in that air, roughly 1,100 ft/s at sea level.

if that formula held in all cases, it would be impossible for an object in vacuum to be propelled by a gas at all, as the sound will not propagate in a vacuum. i know that this is stupid, but i am just pointing out that the simplification that you used above does not apply to all situations.

the only way to truly resolve this is for someone with a chronograph to build a supersonic spudgun and measure the spud's speed. of course, you would need to calibrate the chronograph to ensure that it is measuring correctly, and i don't know what on earth you could use to calibrate it with. that's for someone else to figure out. i've thought too much tonight.


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 Post Posted: Fri Jul 25, 2003 10:00 pm 
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Originaly posted by Bartman007,
NT = Nice Transvestites


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 Post Posted: Sat Jul 26, 2003 6:00 am 
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Originaly posted by SwampFox,
I'm not doing that with a potato...i'm planning on fireing pop cans, so they SHOULD be better than potatos..also, I understand how combustion limits the speeds, but how could pneumatic guns be limited (other than their design)...so IF the gun was designed AND built very (not EXTREMELY) efficieantly well (and with a valve with a VERY high flow rate)...couldn't then reach high speeds like 1000 FPS? and what do you mean by "fluid"?

ALSO, the M 82 Barret gets OVER 2000 FPS (i may be wrong), but that's a TOTALLY different thing..


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 Post Posted: Sat Jul 26, 2003 11:00 am 
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Originaly posted by pacogoatboy,
this isn't all of the picture, but it is probably enough more than enough for anyone who is confused. i hope it helps clear things up a bit.

for anyone who doesn't want to read all of this, in essence, the reason you can't break the sound barrier of the propelling fluid is because the molecules in the air can't keep up with the potato. for more (probably too much), read on.

first off, fluids are basically anything that flows. this could be water or oil, like we normally think of fluids being, but it also includes things like gases, silly putty, and even glass. (glass just flows VERY slowly, look at really old windows and you can see that they are thinner at the top than at the bottom.)

ok, for air, water, or any other fluid to transmit mechanical energy, it must be able to move at the speed of the thing it is transferring energy to. when the potato comes out of one of our guns, it is doing so because a ridiculously large number of molecules of oxygen and nitrogen, along with some others (i'm going to call them air molecules from here on) are hitting the backside of the potato and transferring some of their energy to it. when you pressurize a gas (or a liquid or a solid), you are storing mechanical energy in it. the nature of things is for that energy to want to escape, creating an equilibrium with the outside environment. the way it escapes in our guns is by transferring its energy to a potato, which makes the potato move down the barrel and into the air, bringing us a strange joy.

the limits for the speed at which this energy can be transferred are the same things that determine the speed of sound. sound is essentially just a transfer of mechanical energy through a medium. think of the last time you were standing beside a speaker with the volume waaay up and you could actually feel the air moving away from it, that's the energy i'm talking about. the main things that limit that speed are the density of the substance and the temperature of the substance.

density:
in air, the speed of sound is about 1,100 feet per second. in water, which is denser, it is about 3 times faster (if i remember correctly). in steel, it is faster still. when you pressurize air, it becomes more dense and is able to transmit energy more efficiently, which increases the speed of sound in the air. (this isn't EXACTLY accurate, but it is close enough for government work)

temperature:
temperature is just a measure of the average kinetic energy of the substance being measured. kinetic energy is the energy that is stored in the motion of an object. as the temperature rises, the molecules of air gain kinetic energy, which means that they are bouncing around faster and faster. now, imagine a potato that is hurtling along the barrel at the speed of sound. the molecules of air behind it have to hit it on the backside to make it move faster. because the potato is already moving as fast as the speed of sound, the air molecules aren't able to catch up to it to give it more of their energy, so the potato can't speed up any more. all the while, the air molecules in front of the potato are smacking it (more weakly) trying to get it to slow down. also, as the air expands, it loses energy (because it has done work on the potato to make it move forward), lowering the temperature inside of the gun. this is why pneumatics "smoke" when they fire, the air exiting the gun has dropped to a temperature at which water condenses out of it. the lower temperature means that the air molecules are moving slower, and are able to push the potato less strongly, and at a slower maximum speed.

if the gun was designed and built PERFECTLY, the projectile still could not exceed the speed of sound in the gas that is propelling it. also, a pop can might make a better projectile than a potato, just because it ought to be easier to transfer energy to than a spud, but it still won't make a difference.

finally, concerning real guns, it isn't an entirely different thing at all. when a cartridge is fired, the powder in the shell burns, creating a small volume of very hot gas under very high pressure. this combination of high temperature and pressure shoots the bullet out of the gun at an enormous rate because the speed of sound in the gas is much higher than it is in regular air. in essence, the gun powder is just an exquisitely high pressure package of gases, and burning the powder releases them.


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 Post subject: Glass Does Not Flow!
 Post Posted: Sat Jul 26, 2003 12:00 pm 
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Originaly posted by mta,
Glass Does Not Flow!

Please do not perpetuate the myth that glass flows. It does not.



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 Post subject: Care to back that up?
 Post Posted: Sat Jul 26, 2003 12:00 pm 
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Originaly posted by thehalls,
First, I would question your assertion that a spud gun is not adiabatic. The timelines involved (factions of a second) do not allow for significant heat transfer outside the system.

Second, please cite me a thermodynamics or fluid mech text that lists the speed of sound as being non-negligably effected by density. Oh, I know, you'll see equations that list a pressure over density term, but going back to the basics, that's nothing more than a substitution for temperature. While you're at it, explain the isothermal region in the upper atmosphere (from memory, about 35kft to 50kft). The speed of sound is *constant* while the pressure sure as hell drops. Rectify this with your assertions.

As for a vacuum. Well duh, a gun filled with a vacuum can't push ANYTHING out as there's no gas to push the projectile out of the gun. DUH! Me thinks you're getting confused on where I'm talking about Mach 1. I'm talking about the properties of the gas INSIDE the gun.

Oh, and I've already got access calibrated chronographs ;).

(Hint: I've worked in gun design for the military.)


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 Post subject: lol you are wrong mta...
 Post Posted: Sat Jul 26, 2003 3:00 pm 
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Originaly posted by taimaishu1103,
the myth about glass flowing is not a myth, it's true.

if you ever got past high school chemistry you would know about a substance called an amorphous solid. an amorphous solid is a substance without a definite crystalline structure, which means that with time it will conform to a force on it ( in most cases gravity)

Glass does flow, and the common example is old glass, like in cathedrals. That two hundred year old glass is in fact enarly twice as thick at the bottom as it is at the top.

now your wondering why glass doesnt just bend when you hit it. Glass shatters when you hit it because the impulse energy that you impart on it overcomes the electromagnetic charges holding the glass molecules in shape, and thus glass shatters.

if you were however to place a piece of glass in a black hole, it would theoretically flow like water.


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