SpudFiles

Okay. This might be a very, very, very amateurish question. But I have to know..



I've now made three air 'rifles'; one made from PVC, and two from galvanised steel.

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The first of the two steel rifles has an air reservoir about two and a half feet long. The other has an air reservoir about one foot long.

Both go up to about 300 PSI (the limit of my pushing-strength using a bike pump).

So- my question is, does the PSI alone determine the force of the projectile (I'm trying to go through half an inch of pine- so far, no luck) or does volume of air contained in the rifle play a part?

i.e, is it worth it trying to build a super-long air reservoir attached to the barrel, or is that just pointless?

I know this might seem a very stupid question, but I am new to this, and very keen to learn. So if any of you have the answer, I would be incredibly grateful.

Thanks. Eli.

They both do.

If you think of the projectile moving down the barrel, a higher pressure will result in a larger force initially, resulting in higher accelerations and a greater muzzle velocity. Making the chamber bigger maintains this pressure further down the barrel before it has dropped off significantly.

So to answer your question, higher pressures help velocity, and larger chambers help velocity up to a point where making the chamber bigger starts making less of a difference.

As always, have a look at GGDT to see these relations.

The 2 factors work hand in hand with the valve you're using, so really 3 factors. This is the simple answer. Link to your cannon??? Did you post this cannon and specs?

Would suggest barrel and chamber volumes being equal (1:1) at low pressure (<120psi) and chamber volume being half of barrel volume (.5:1) at higher pressure (>300psi). Chambers get smaller yet as pressure increases to 1000 of psi.

Playing with variables in GGDT is a great way to decide how much pumping effort to trade off for how much power.

Aside...1/2" pine should not be presenting much of an obstacle...likely an issue with the launcher.

Need some pics and a better description of it's workings.

Welcome to Spudfiles, Eli. You probably already know this but since you said you were a newbie, I thought I should include this in the discussion:

The weight, shape and position of your projectile can also be a factor. Make sure your projectile is of sufficient weight and large enough to fit snuggly in the barrel (with little to no air escaping around the projectile). Also make sure it is postitioned all the way to the back of the breech.

As mark and Gippeto said, you can use GGDT to see how these different factors interact.

Hi all! Firstly, thank you all so much for your replies. I cannot tell you how much I appreciate you sharing your expertise with a newbie.
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So. I was pretty hesitant about sharing an image of my little project, simply because it's so crude looking compared to all of the amazing designs I've seen on here. But remember, I'm still very new. Be gentle! So here goes (apologies for the poor quality photo):



↑ ↑ ↑ ↑ ↑ This is the larger of the two steel ones I've made.

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As you can see- it's as simple as it gets. Just a couple of pieces of galvanised steel pipe, a brass ball-valve and a shrader valve drilled through the pipe cap stuck on the end. The threads of the pipe are sealed with two-part epoxy, and, as I said in the first post, hold air with no leakage to AT LEAST 300 PSI (the limit of my bike-pumping strength).

The barrel has a 10mm-diameter, and is a 12-inch long piece of copper pipe (air-tight glued into a couple of larger pipe-pieces to fit into the ball-valve thread). The barrel has a screw in it near the ball-valve so the projectile won't get lodged in the valve.

The air reservoir is 1 foot long (I guessed in my original post that it was 'two and a half feet long' - that goes to show how stupid I am [though, to be fair, we don't use 'feet' as a unit of measurement in my country]) and has a diameter of 20mm.

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As for projectile - well, this is an area I've been concerned about too.

So far I've been using sawn-off pieces of bolts- about 7mm in length / 9mm in diameter. However, I noticed that they have the tendency to spin around almost as soon as they leave the barrel - so I can only get a clean entry to a piece of board from about two feet away. Which is not ideal!

I've decided to try ball-shaped projectiles. Do you think this would improve things? I've ordered some appropriately-sized ball bearings on the internet (you literally can't buy them here because even sling-shots are banned where I live) and will see how that works.
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So, I think that's about all the info I have. If any of you have additional advice- please feel free to give it to me! I really admire your experience and how hard you guys have worked to build far superior cannons / rifles, and appreciate any help I can get.

If making the air reservoir larger would help, I am willing to try that with my next design! I mean, at the end of the day I guess it doesn't really matter, but it would be nice to build something with my own hands that will impress my friends.

Thanks. Eli.

Your main limiting factor there will be the ball valve.

Here's some simulated results I did a few years ago to demonstrate:



The X-axis is the relative volume of the chamber compared to the barrel, or the "C:B ratio" as it's called. A value of 1 is an equally sized valve and chamber, a value of 2 is a chamber double the volume of the barrel and so on and so forth.

The Y-axis shows the muzzle energy relative to the theoretical maximum (with an infinitely sized chamber and no flow losses anywhere in the system.)

The blue line is performance with no flow losses, but a finitely sized chamber. As you can see, increasing chamber size does increase muzzle energy, but at heavily diminishing returns.
A C:B ratio of 1:1 has ~60% of the theoretical performance, but a C:B ratio of 2:1 only improves that to ~75% - in other words, twice as much pumping for only a quarter more energy.

The red line is with a full-bore piston valve (a good valve, but nonetheless limited by the laws of physics) and a moderate weight projectile. Energy does still improve as the chamber size increases, but it's much more limited. By a ratio of about 6:1, the curve is almost flat, and there's very few gains to be hand.
Also note that it "peels away" from the perfect blue line - It's pretty close to the same performance below about 1:1, but the relative performance gap is proportionally much bigger at higher ratios.

This effect is even more pronounced with the green line, which is essentially the same valve as above, but half the diameter (and thus a quarter of the flow). While it's similar to the other systems below about 0.25:1, the line is nearly flat by about 3:1.
In this system, increasing the C:B ratio from 5:1 to 10:1 would have absolutely no discernible effect, because the limiting factor is the valve.

These are the results that most resemble your system, because while ball valves have excellent flow when they're fully open, they're very slow to open. If you're opening it fairly fast, it might take a tenth of a second, but even pushed by the little air that gets through the valve, a small projectile like that will be gone in a fraction of that.

If the valve isn't letting most of the air through anyway, then adding extra chamber volume is nearly pointless - that extra air just won't get into the barrel in time, and all the added effort is just turning into sound and fury, signifying nothing. (To quote Shakespeare).

Personally, I'd say even a really good valve would be hitting the sane limits of diminishing returns at around about 2:1 C:B ratio, and you're already at around 4:1.
You'd get massively more out of lengthening your barrel (which gives the projectile longer to accelerate, and also gives a slow valve more time to open) and/or improving the valve.

My recommendation for the valve would be a spring-loaded ball valve (as a suitably beefy spring can open the valve a lot faster and smoother than is possible by hand - just make sure you don't pinch your fingers in it)- while this won't be as fast as a pneumatically actuated valve, it will be a lot simpler to build; things like piston valves can be a bit fiddly at early stages, and you might find it difficult to find a commercial quick exhaust valve.

Hi Ragnarok,

Wow. Thank you so, so much for your amazingly detailed and extremely thorough response. I'm blown away- I cannot thank you enough for taking the time to give me all of this information.


As for your valve idea- do you know of any online source which provides instructions on how to build a spring-loaded ball valve (or do you know if there are already-made commercial ones available which are used for another purpose)?

Thank you again,

Eli.

I can't recall anything that provides a step by step plan, but Mathias Wandel's airgun is one of the better examples I can recall*, and he's documented it all with some good photos and useful explanations that you can hopefully get the basic ideas from.

*Although I would absolutely advocate a sturdier initial construction rather than his "this bit is here to catch this other part if it comes loose" method.
If you can't stop it failing in the first place, what you now have is a bullet shaped like a launcher part, and trying to catch it will go about as well as any other attempt to catch a bullet will.

Just an opinion but... Generally it's easier to just mod a sprinkler valve or similar rather than attempt to build a spring-actuated ball valve.

And even easier than modding a sprinkler valve? Just buying a QEV.

I did think about QEVs, but they're a fairly specialist part which I don't know the German name for, making it hard to help him find one locally, and rather less transparent in how they work.

Ragnarok wrote:I did think about QEVs, but they're a fairly specialist part which I don't know the German name for, making it hard to help him find one locally, and rather less transparent in how they work.

Not sure what they'd be known as in German either, but to the OP I would list another couples names in English for the same valves: Pulse Valves and Dust Valves.

I'm from Finland but I think they (QEVs) are called "Schnellentlüftungsventile" in Germany

Example (site in German language): (http://store.norgren.com/de/de/search.aspx?q=T70C2800)

And QEVs are very normal thing in pneumatics, nearly every pneumatic valve manufacturer has some models, they are used with pneumatic cylinders to get higher speeds.
If need for over 1" size valve, then maybe dust collector valves etc. would be easier to find

Thank you so much for your help, guys. However- just to clear up something- I'm not actually Austrian! I just clicked the wrong country in the drop down menu (as an Australian, you would be surprised how often this happens).


As for the QEV valve solution; I've had a look at youtube videos of cannon's utilising these but... simply put, I can't understand them at all. I'm really not as technically-minded as the rest of you (a euphemistic way of saying I'm perhaps not as smart). The ball-valve works so well for me because it comes ready made, and is simple enough for a Chimpanzee to use.

I am really keen to try this QEV method, so I will keep trying to learn. However, when I look at pictures of a QEV valve (using Google Images), all I see is a rectangle with three holes in it. I can't translate it to my own physical design. But I will keep trying.

As for my own cannon's- shortly after my first post (as soon as I was given the information about the importance of volume / barrel length), I built a hasty 7-foot-long steel 'rifle' and shot it. It goes through a solid inch of pine! I know this is a childish question, but that kind of force is technically 'lethal' right? I'm certainly not planning on killing anyone with a 7 foot home made spud cannon, but it would just be kind of cool to know that something I made was capable of such things.

Thank you all again for your amazing help and apologies for the nationality-confusion!

It's powerful enough to hurt someone.
Seriously.
You or someone close to you perhaps.
So handle with care..it's not a toy.
Treat it like a firearm.

We have a thing called the Spudwiki ( found in the buttons on the top of the screen)
In there you will find lots of information ...such as: http://www.spudfiles.com/spud_wiki/index.php?title=QEV