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Supersonic pneumatic

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Supersonic pneumatic

Unread postAuthor: DYI » Sun Sep 09, 2007 9:31 am

I am currently in the process of designing a small caliber (0.75") pneumatic to push a light projectile slightly over the speed of sound. GGDT doesn't work properly with supersonic velocities in any gas, so I was hoping that someone would have an idea on how to achieve this. GGDT says that the design I have should reach ~1400 fps with a paintball, which would be nice, but doesn't seem possible.

The problem is, I will be using nitrogen, which has a speed of sound almost identical to that of air. I am open to suggestions as to how to achieve this without torching the entire chamber for 15 minutes to raise the temperature, because that isn't any fun.

I had an idea about dead space, but I don't know if it would work, due to the fact that even though the small volume of gas in the dead space is compressed rapidly to 400+ psi, the entire volume of gas decreases in pressure, and therefore temperature, which could negate the effect of the compression of the smaller volume. Does anyone know if this would work?

The whole reason for not using helium is in the price of regulators, $370 for a 0-200 psi regulator where I live. Eventually, I will buy a helium tank and a low pressure output reg. for it, so that I can use N2/He mixtures which would have a higher speed of sound, but this won't happen for at least another year, due to the finite amount of money in my possesion.
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Unread postAuthor: veginator » Sun Sep 09, 2007 9:53 am

You do know that you can get helium regulators from walmart for party baloons. I dont know wat the maxium psi is though.
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Unread postAuthor: jimmy101 » Sun Sep 09, 2007 10:11 am

Well, the speed of sound is a funtion of temperature. If you can heat the gases in the chamber you can reduce the problem of getting them moving at ambient supersonic speeds. The speed of sound scales as the square root of the absolute temperature. So, if you could heat the gases from 300K (80F) to 600K (620F) the speed of sound would increase to about 1500 FPS.

GasEq will calculate the speed of sound as a funciton of temperature.

Of course, you still have the problem of getting the air in front of the round moving supersonic as well as the problem of the very high drag there is going to be between the moving gas and the barrel.
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Unread postAuthor: jackssmirkingrevenge » Sun Sep 09, 2007 10:14 am

A burst disc pneumatic with say 400 psi of nitrogen, a lightweight projectile in a long barrel with a big enough chamber should break the sound barrier easily.
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Unread postAuthor: DYI » Sun Sep 09, 2007 10:40 am

We seem to have a conflict of opinion here. I had thought of the problem of the air in front of the projectile, and I don't really know how to circumvent it, short of evacuating the barrel.

@veginator: those balloon regs can rarely exceed 60 psi output, and don't usually have threads on the output.

@Jack: How would it break the sound barrier?
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Unread postAuthor: jackssmirkingrevenge » Sun Sep 09, 2007 10:51 am

You can break the sound barrier using compressed air through a valve, that's a fact. Nitrogen is slightly less dense than air so that gives you a bit more of an edge.
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Unread postAuthor: dongfang » Sun Sep 09, 2007 1:23 pm

Hi,

It could actually be interesting if someone here with a high-performance pneumatic just tried it, with a light projectile and / or sabot...

Jack, there´s talk about a phenomenon in barrels: Something about irregularities in the barrel wall creating a back pressure that will rise sharply as Mach 1 is approached (yes there should be no such thing as back pressure above, but the problem is that it gets too high before then). Whether it is this that makes the loud "sonic noise" when releasing high pressure air through a big enough valve, I don´t know.

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Unread postAuthor: LikimysCrotchus5 » Sun Sep 09, 2007 1:50 pm

It would be easier to with a rifled pvc barrel because it will work on the same principles as a regular gun that has a rifled barrel, more accuracy and distance and speed.
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Unread postAuthor: jackssmirkingrevenge » Sun Sep 09, 2007 1:53 pm

There certainly is a bit of a sonic crack from air breaking the sound barrier with some launchers, even if the projectile itself is subsonic.

It would be easier to with a rifled pvc barrel because it will work on the same principles as a regular gun that has a rifled barrel


Actually rifiling causes drag which slows down the projectile, indeed one of the advantages of smoothbore barrels in modern tank guns is that higher velocities are available than from comparable rifled guns.
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Unread postAuthor: DYI » Sun Sep 09, 2007 2:06 pm

A rifled barrel would make the whole thing even more difficult. The barrel should be as smooth as possible.

I should have the launcher to do this done in about 1 week, and tested by the end of September.

The setup will be as follows: .75" burst disk valve
50 ci chamber
.74"x12' barrel
500 psi nitrogen
5 gram ammo
GGDT predicts 1904 fps with this setup (which I highly doubt, considering that it is above the particle speed of nitrogen). The strange thing is, when I lower the projectile mass to 3 grams, the efficiency actually INCREASES!
Any ideas as to what causes this effect, or is it just the program acting up because of the irregular parameters?

If anyone has any suggestions to further increase the probability of this thing achieving supersonic velocities, please tell me.
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Unread postAuthor: jackssmirkingrevenge » Sun Sep 09, 2007 2:09 pm

At high velocities GGDT is unreliable, I wouldn't pay too much attention to the predicted figures. With the specifications you posted, I'd say the speed of sound is well within reach.
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Unread postAuthor: jimmy101 » Sun Sep 09, 2007 2:24 pm

I don't think you'll come very close to the speed of sound. Perhaps 900~950 FPS.

The drag on the near sonic gas as it goes through the barrel is going to be huge. I wouldn't be surprised if the drag was more than the force needed to accelerate the projectile.
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Unread postAuthor: SpudBlaster15 » Sun Sep 09, 2007 2:28 pm

GasEq predicts that the speed of sound in room temperature nitrogen is ~20fps higher than the speed of sound in room temperature air. This means that it is possible to break the (air's) speed of sound using room temperature nitrogen, albeit much more difficult than I want to think about.

However, as many fail to realize, the compressed gas in the chamber cools substantially when it expands into the barrel, lowering the speed of sound of the gas. If you use GGDT to graph your hypothetical launcher's barrel temperature VS projectile position, you will see that the temperature of the gases in the barrel falls below freezing before the projectile has moved 1ft. The speed of sound in the gas propelling the projectile has now fallen 40fps below the speed of sound in air at room temperature.

You will not break the speed of sound without heating the gas, or using a gas that has a much higher speed of sound at room temperature. Fortunately for you, pressurizing a chamber to 500psi generates quite a lot of heat.
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Unread postAuthor: dongfang » Sun Sep 09, 2007 3:40 pm

Hi,

One other idea: Get a few large electronics heat sinks, ideally some high quality copper ones (the CPU overclocking market??) Some with long, unbroken fins, or some other relatively aerodynamic shape. Or or may have to braze something together yourself.

Bolt some high power resistors on those, maybe 500 Watts total at mains voltage.

Now you can pre-heat the thing with electricity, and it should dump heat to the air when firing. How much, I don´t know. It probably would pay off to try make some estimates or scaled experiments before trying this full scale.

Of course the easiest way to heat air is it burn fuel in it, but I understand that you don´t want a hybrid?

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Unread postAuthor: LikimysCrotchus5 » Sun Sep 09, 2007 3:49 pm

well i can see now why rifling wouldnt work thanks for correcting me but if he is going to use a burst disk what kind of disks could withstand 400 to 600 psi?
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