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my gun

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Unread postAuthor: DYI » Sun Feb 03, 2008 5:17 pm

To clarify, I was talking about the pneumatic, but I obviously f*cked up my post or accidentally deleted a line or something...

And making a pneumatic to 0.2:1 isn't silly at all. When you start with a ratio of 2:1, your chamber material is ~$15/foot, and your barrel material is ~$1/foot, it's a lot more practical to make the barrel longer than the chamber longer. Since pressure rated plastic pipe isn't available where I live, this is my reality, and efficiency for a given chamber size is imperative.
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Unread postAuthor: TwitchTheAussie » Sun Feb 03, 2008 6:43 pm

My only problem has been spoken. Not even my advanced combustion can hit 400m. 300 off MAPP gas is true. But with your size and as a spray n pray theres no way (hows that for alliteration :D )
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Unread postAuthor: jimmy101 » Mon Feb 04, 2008 12:19 pm

DYI wrote:And making a pneumatic to 0.2:1 isn't silly at all. When you start with a ratio of 2:1, your chamber material is ~$15/foot, and your barrel material is ~$1/foot, it's a lot more practical to make the barrel longer than the chamber longer. Since pressure rated plastic pipe isn't available where I live, this is my reality, and efficiency for a given chamber size is imperative.

Looking at typical GGDT output. If you start with an 0.2 CB gun then cut the barrel length in half you have minimal affect on the performance of the gun. That is why I say it is silly to make an 0.2 CB gun. You get essentially the same performance for a particular chamber volume with a barrel that is only half as long and a CB of 0.4. For mid-sized cannons that means shortening the total gun length by several feet. For a large gun that might be a barrel that is 10 feet shorter.

If you don't want the small velocity drop of the 0.4 CB then you can slightly (~5%) increase the volume of the chamber to make up for it.

Getting the optimal CB for a particular chamber requires a very long barrel and the relationship between velocity and barrel length is a very broad peak. To get that last ~10% performance out of the chamber requires a barrel that is roughly twice as long.
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Unread postAuthor: Ragnarok » Mon Feb 04, 2008 6:01 pm

TwitchTheAussie wrote:But with your size and as a spray n pray theres no way (hows that for alliteration)

Alliteration is matching sounds at the start of words, what you have there is internal masculine rhyming.
Or you could just ignore me - the thing is my mother has a degree in linguistics, so I've naturally grown up with an ingrained knowledge of language.

Talking about ratios, I like things around 1:1 for pneumatics. I might go down to 0.5:1 or up to 2:1.
HEAL, for example is a 1.5:1, for the specific reason that it was the longest chamber that would fit neatly under the 1 metre barrel, but I then put a slightly longer barrel on it, which lowered the ratio a touch. It also has the effect of keeping the chamber pressure above half of it's initial level during a shot, which doesn't exactly hurt performance.

It's not the most efficent ratio, which is slightly ironic when you bear in mind what the acronym actually stands for (High Efficency Air Launcher), but the truth is, the E in the name originally meant Energy, but then I did a George Lucas* on it, because I thought it would be a slightly better choice if I ever did have a run in with the rozzers.

*George Lucas: v. To retroactively change something that was well established, and probably annoy people in the process.

Although you could also consider it a backronym. Overall, the name is really a mis-apronym, in that the acronym is ironically counter to what it actually refers to.
And I'm on about linguistics again. Better wrap it up, or JSR will start making up his own unpronounceable words again. :D
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Unread postAuthor: McFear » Tue Feb 05, 2008 4:24 am

hey,

thanks for all your replies.
with the 10 acres block i think at the widest point it it only 100 - 150m (just guessing.) but the property is a big rectangle.
i can do the distances on google earth though and will get back to you.

i now have video of the combustion cannon and the recoil of it and i am yet to get a video of the combustion and its distance. i was stupid enough to video the distance but with the video camera not the digital so i cant download to the computer. i will get a video showing the cannons distance and will get some more photos and videos of the pnuematic one as well.

lol. this is my yr12 physics assignment so any help on improving the cannons can only get rid of errors when i have finished. before you ask i am comparing the distances of compression and combustion.

Ie. If 40PSI sends a ball mass ____ this far and cannon B using x amounts of this fuel sends a ball this far the approx PSI output would be this.

my teacher told me to do this topic. i gave him a list of things i was thinking about doing and he straight away pointed to the spud guns so i was happy. :)


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Unread postAuthor: jimmy101 » Tue Feb 05, 2008 1:12 pm

McFear wrote:lol. this is my yr12 physics assignment so any help on improving the cannons can only get rid of errors when i have finished. before you ask i am comparing the distances of compression and combustion.

Ie. If 40PSI sends a ball mass ____ this far and cannon B using x amounts of this fuel sends a ball this far the approx PSI output would be this.

my teacher told me to do this topic. i gave him a list of things i was thinking about doing and he straight away pointed to the spud guns so i was happy. :)

McFear

That is a great project!

Your approach is a very clever way to get an estimate of the pressure in a combustion gun.

Some thoughts;
1. The guns should have the same chamber volumes
2. The guns should have the same barrel dimensions (length and diameter)
3. The guns should be firing the same ammo
4. The guns should be fired with the same inclination (angle from horizontal) if you are using range as your measurment of performance.
5. You might consider building your own chronometer instead of relying on distance as your performance measurement.

You should probably fire the pneumatic at a couple different pressures, with a couple shots at each pressure. Make a graph (calibration curve) of pressure vs. distance. A couple shots at each pressure lets you do statistics (which your teacher will love).

You need the graph since the distance vs. pressure won't be a linear relationship.

Lets see, IIRC, folks usually figure a combustion gun peaks out at about 60~70PSIG. So, you might want to shoot the pneu' at say 40 to 100 PSIG in steps of 10 PSI.

The peak pressure in a combustion gun is very dependent on the mass and friction of the ammo.

You might want to think about the difference between the pressure versus time curve in a pneumatic versus combustion, they aren't the same. The pneumatic starts the ammo at the max pressure in the chamber and the pressure is always dropping as the ammo moves through the barrel. The combustion won't reach max pressure until the ammo has already started to move.

This really is a cool experiment. Basically you'll be able to say "a propane + air combustion has the same energy as a pneumatic (with such-and-such a valve) operating at XXX PSIG when firing YYY ammo".

(BTW, when you plot pressure versus distance for the pneumatic you should probably use absolute pressure and not gauge pressure.)
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Unread postAuthor: jook13 » Tue Feb 05, 2008 2:37 pm

he said he shot across his 10 acre property. my dads 10 acre property (its square) is 660 feet across, so about 220 yards, or right around 200 meters. so if the property he is shooting across is square, he is shooting around 200 meters.
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Unread postAuthor: Ragnarok » Tue Feb 05, 2008 2:44 pm

jimmy101 wrote:Your approach is a very clever way to get an estimate of the pressure in a combustion gun.

My greatest concern is that the ball valve will greatly limit the performance of the pneumatic, so he's going to need a higher pressure than the combustion is actually developing to equal it.

The other slight concern is the higher speed of sound in a combustion means also that it's more "pressure efficent" if you can see what I mean - for the same average pressure, you'll get slightly more performance. Much like a helium pneumatic will beat an air one, which will beat a CO2 one, even if they all use the same pressure, at just about any velocity.

However, at lower velocities, the effect will be lost in experimental error. Changes in ball valve opening time will add at least that much change.

I normally reckon on the average pressure in a combustion of a sensible ratio being around 2 bar, or 30 psi, if I'm doing a rough mental calculation.
However, flow restrictions at the ball valve will mean the pneumatic will need more to reach that same performance.

The peak pressure is another matter entirely. I did once reverse engineer the latke data to get something around 5 or 6 bar IIRC - the problem was I never knew how heavy the gasket slugs were, which creates an uncertainty in the energy value, and therefore the force, and ultimately the pressure. The 3/4" data is the only one that covers that region, which makes it tougher.

The 2 bar average was mainly based on the 2.5" data, because I know what tennis balls weigh, and a rough friction value for tennis balls in 2.5" SCH 40, which reduces guessing error.

By the way, McFear, feel free to nick bits of my logic for your project write up if you want.

One last question - I've got this impression from some things you've said that you're probably not from the US like most members (around 2/3rds as it seems). Don't get me wrong, there's no problem with that - I'm not either.
It's mostly that you're using metric units, which is unusual in America, but also some of the words and terms you're using stand out a little.
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Unread postAuthor: Fnord » Tue Feb 05, 2008 3:35 pm

One last question - I've got this impression from some things you've said that you're probably not from the US like most members (around 2/3rds as it seems). Don't get me wrong, there's no problem with that - I'm not either.
It's mostly that you're using metric units, which is unusual in America, but also some of the words and terms you're using stand out a little.


In that bit of rambling, you managed to not actually ask a question.
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Unread postAuthor: MrCrowley » Tue Feb 05, 2008 3:41 pm

@Rag

He's Australian.
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Unread postAuthor: Ragnarok » Tue Feb 05, 2008 4:42 pm

MrCrowley wrote:@Rag: He's Australian.

Ah, is this use of mod power?
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Unread postAuthor: POLAND_SPUD » Tue Feb 05, 2008 5:00 pm

rag what is wrong with using metric units...? i would rather say that using Imperial units is wrong, there are only few third-world countries that still use them: Liberia, Myanmar (Burma),the United States of America and UK

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Unread postAuthor: MrCrowley » Tue Feb 05, 2008 6:32 pm

Ragnarok wrote:
MrCrowley wrote:@Rag: He's Australian.

Ah, is this use of mod power?


Sure is, I could tell you what he had for breakfast if you like :D
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Unread postAuthor: DYI » Tue Feb 05, 2008 6:41 pm

rag what is wrong with using metric units...? i would rather say that using Imperial units is wrong, there are only few third-world countries that still use them: Liberia, Myanmar (Burma),the United States of America and UK


Don't forget Canada! While we claim to use metric, almost everything here is done in imperial. Metric makes a hell of a lot more sense, but being good at both is a very useful skill.
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Unread postAuthor: frogy » Tue Feb 05, 2008 6:50 pm

jook13 wrote:he said he shot across his 10 acre property. my dads 10 acre property (its square) is 660 feet across, so about 220 yards, or right around 200 meters. so if the property he is shooting across is square, he is shooting around 200 meters.


McFear wrote:with the 10 acres block i think at the widest point it it only 100 - 150m (just guessing.) but the property is a big rectangle.


10 acres is about 40,468m IIRC... The square root of that being about 201. With a guesstimated (an actual word btw) property width of 100-150m, that gives you a 100m x 400m or a 150m x 270m rectangle...

Assuming your property is 100m wide, the father you could shoot while still landing in your property would be 400m... With you combustion setup and tennis balls, your looking at 200m or so. So I propose recalculating :)

I like your Physics project very much... Hopefully you can impress us with some data :wink:
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