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Temprature drop

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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Temprature drop

Unread postAuthor: vovka351 » Wed Jul 02, 2008 2:01 pm

Due to the nature of a pneumatic, when you shoot, the temprature of the air goes down, A LOT, how bad of an issue is this? Because i was testing an idea in GGDT and the temprature went from 80*F to -140 in about 15 ms.
Is this really going to be a problem?
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Unread postAuthor: MrCrowley » Wed Jul 02, 2008 2:12 pm

No, you'll just get a cloud forming after each shot at the barrel.
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Unread postAuthor: starman » Wed Jul 02, 2008 2:15 pm

It happens in a combustion as well, of course after a temperature spike measured in the 1000s of degrees. The phenom is caused by the very fast pressure drop in the chamber and barrel.
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Unread postAuthor: FishBoy » Wed Jul 02, 2008 2:39 pm

soooo... the longer you leave a pvc chamber pressurized, the more likely it will fail? :shock:
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Unread postAuthor: starman » Wed Jul 02, 2008 2:44 pm

FishBoy wrote:soooo... the longer you leave a pvc chamber pressurized, the more likely it will fail? :shock:


No, the cool down happens only at the moment of pressure release. Otherwise things stay at room temperature, or whatever the air temperature coming out of the compressor is/was.
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Unread postAuthor: psycix » Wed Jul 02, 2008 5:27 pm

The gas acts out force on the projectile, and the projectile gains kinetic energy. Now when a gas does that, it loses that energy in a thermal way, and thus gets colder.
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Unread postAuthor: SEAKING9006 » Wed Jul 02, 2008 5:29 pm

Hmmm.... That being said, if there was a way to keep the temp of the gas up, either by heating the barrel (not good for PVC) or by adding more air when it's fired, would that increase performance any?
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Unread postAuthor: DYI » Wed Jul 02, 2008 5:54 pm

There will always be a temperature drop in a pneumatic due to the nature of its operation. The barrel heating necessary to counteract that would have to be able to output as much or more energy than the projectile receives in the time that the projectile is moving in the barrel, and that's only to keep the gases at their original temperature. It's more complicated than that, but that is the basic idea.

So it probably wouldn't be worth it. If you want a higher SOS, it's better to heat the gases before firing, like an LGG does.
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Unread postAuthor: SEAKING9006 » Wed Jul 02, 2008 5:57 pm

Oh yeah. Don't those things get near HYPERVELOCITY shots during use? Some of them are actuated by 12ga shotgun shells, if I recall.
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Unread postAuthor: DYI » Wed Jul 02, 2008 6:29 pm

Oh yeah. Don't those things get near HYPERVELOCITY shots during use? Some of them are actuated by 12ga shotgun shells, if I recall.


The best LGGs can reach over 7km/s muzzle velocity. Hypervelocity is generally defined as over Mach 5, which is less than 2km/s. I've never heard of an LGG so small it could be actuated by a 12 gauge shell. The piston is usually accelerated by several pounds of smokeless powder or similar. But I don't think we're supposed to talk about standard LGGs here, so we'd better leave it at that.

Electrothermal guns are another way of getting the propellant gas to insane temperatures and pressures, and are probably a lot less problematic than LGGs, although all current high velocity propulsion methods have some quirk that makes them impractical for all but intermittent use.
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Unread postAuthor: SEAKING9006 » Wed Jul 02, 2008 6:30 pm

Ah, such is the plight of the rail/electrothermal/Light Gas gun maker. All for the first shot, none for the next.
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Unread postAuthor: psycix » Thu Jul 03, 2008 6:12 am

Back on the subject of the temperature drop: this does not only lower the pressure of the gases, but isn't it true that the speed of sound lowers and thus the gases flow slower?
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Unread postAuthor: D_Hall » Thu Jul 03, 2008 8:35 am

DYI wrote:I've never heard of an LGG so small it could be actuated by a 12 gauge shell.

Not a 12 gauge shell, but I've personally used one that used a .50 BMG casing.
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Unread postAuthor: jimmy101 » Thu Jul 03, 2008 2:35 pm

psycix wrote:Back on the subject of the temperature drop: this does not only lower the pressure of the gases, but isn't it true that the speed of sound lowers and thus the gases flow slower?

Yep. That's why it is so difficult to get a supersonic shot out of a basic pneumatic. You need to either start with hot gases or switch to a lower density gas like helium (which gives you a higher starting speed of sound).

Combustion guns don't have this problem, the high temperatures make the speed of sound inside the chamber much greater than ~1100 FPS. In a closed propane+air combustion chamber the speed of sound peaks at about 3300 FPS (roughly 3X the speed in air at RT). Combustion guns have other characteristics that make reaching the speed of sound a challenge.
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Unread postAuthor: psycix » Fri Jul 04, 2008 6:19 am

jimmy101 wrote:Combustion guns have other characteristics that make reaching the speed of sound a challenge.

Like?
The only one I can imagine is the lack of high pressures, which can be overcome by hybrids.
The short time the pressure lasts doesn't seem much of an issue to me, unless you want to use a really long barrel.
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