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New to spud guns - a few questions

Discuss miniature cannons built from things such as pill bottles and pen caps.
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New to spud guns - a few questions

Unread postAuthor: KnooBill » Fri Nov 23, 2007 8:34 pm

First, hello everyone, this is my 1st post here :)
Me and a friend had the idea to build a potato gun, so I did some research on it but still there are a few questions I got...
Since size matters, having a huge potato gun isn't really of our liking, we'd prefer to make a mini gun.
From what I've seen so far, chamber:barrel ratios vary quite a bit, (here, spudtech archive, burnt latke), so what would the ideal ratio for a mini would be?
I'm thinking to use propane/ possibly deodorant and potato as ammo, the chamber diameter would be from 1-2.4 cm (I've calculated various builds keeping in mind ratio as 1:1 [chamber:barrel always] but the chamber gets a weird shape).
It gets a werd shape because I read somewhere (can't remember now) that if you use a fan it increases performance, so I'm thnking to use a 5cm fan or more unlikely a 3cm one. Will they make a notable difference (since they are quite weak).
I hope it's not too much, thanks for your help in advance.
8)
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Unread postAuthor: The Kid » Fri Nov 23, 2007 8:40 pm

welcome to spudfiles knoobill

if your making a mini try a pnematic instead of combustion more reliable and powerful and minis can be pumped up with a bike pump easily. A 50mm chamber would be pritty ideal and ratios arn't really important in a pnematic and for a valve try modding a sprinkler valve feel free to ask questions
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Unread postAuthor: Novacastrian » Fri Nov 23, 2007 8:43 pm

Hey Knoobill, welcome to spudfiles.
I use a ratio of around 3:1 on my minis, that said it's not ideal. You will notice an increase in power to the order of 30% if you use a fan- don't build your gun without one :wink:
Tell us what you have in the way of materials to work with and we can steer you in the right direction. I'm thinking maybe a 6" long section of 2" pvc. Minis are cool! :)
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Unread postAuthor: mark.f » Fri Nov 23, 2007 8:55 pm

If you can find a fan small enough, that would increase performance slightly. My friend had the good idea a while ago to take the propeller off an RC boat and mount it on a small 3v DC motor, (the type found in most small electronics and VERY cheap RC cars). He fit that in a 3/2" chamber, so you might be able to find a propeller and motor combination to fit in a mini.

As for C:B ratios, in theory, it would still be the optimal 80% chamber:barrel using propane in air. In practice, though, the small size of minis makes fueling a PITA, which means you will get less power out of a given chamber size because your fuel mixture will be slightly off. You can go for a "bigger is better" approach, and simply step up the chamber size while holding your barrel constant, (increasing C:B ratio), or you can test. If you've read the Burnt Latke site, you've read the general procedure for testing C:B ratio. Since mini materials are cheap... why not? Set up your chamber first and then buy a cheap, long barrel. Test, then get your optimal length.
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Unread postAuthor: TwitchTheAussie » Sat Nov 24, 2007 4:43 am

The Kid wrote:welcome to spudfiles knoobill

if your making a mini try a pnematic instead of combustion more reliable and powerful and minis can be pumped up with a bike pump easily. A 50mm chamber would be pritty ideal and ratios arn't really important in a pnematic and for a valve try modding a sprinkler valve feel free to ask questions


First off size matters here with pneumatics especially c:b ratios. a 1.8:1 ratio is optimal for a pneumatic but ask around. Im not too experienced with pneumatics. Also just try a simple blow gun valve or piston. Dont go sprinkler.
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Unread postAuthor: The Kid » Sat Nov 24, 2007 5:04 am

why not go with a sprinkler there fast opening may have reduced flow but it will be a small gun (i imagine about 1500mL chamber volume). i've heard the ideal pneumatic barrel to chamber ratio is 2:1 but as long as the chambers at least half the volume of your chamber you should be fine i might be wrong but thats what i've heard.
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Unread postAuthor: BigGrib » Sat Nov 24, 2007 6:05 am

If you are making a mini pnuematic would be the way to go because you can build them very simply, my first took all of a hour to make even though it isn't a mini. I would reccommend using 1" pvc to make the chamber however long you want. put a scrader valve through one of your double layers. mod a 1" sprinkler valve (search the wiki) which you can pick up at home depot for about 17 bucks. you can go down in barrel diameter to 3/4 just pick up a pvc adapter and i think that would be a fine mini.
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Unread postAuthor: TwitchTheAussie » Sat Nov 24, 2007 7:36 am

Aw better yet depending on what ya got make it out of metal. And what kind of size were you planning on making it?
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Unread postAuthor: The Kid » Sat Nov 24, 2007 7:53 am

TwitchTheAussie wrote:Aw better yet depending on what ya got make it out of metal. And what kind of size were you planning on making it?


he said 10-40mm chamber size

a full metal is probably the best way to go when making a pneumatic for a valve a piston would be best since sprinkler valve is only rated to 150 and in an all metal you might go over 100psi or more? but if your not concerned all that much about power you might go a ball valve (they open slow but oh well) do some research before building it.

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Unread postAuthor: TwitchTheAussie » Sat Nov 24, 2007 8:10 am

Ball valves are good for high pressures. Trust me :wink: . But if you dont have the know how for metal working and welding dont try it.
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Unread postAuthor: The Kid » Sat Nov 24, 2007 8:46 am

TwitchTheAussie wrote:Ball valves are good for high pressures. Trust me :wink: . But if you dont have the know how for metal working and welding dont try it.


i can get welding or soldering done for free or at least very cheap at my local plumbing store

moral of the story: theres alway someone near you who can weld or solder
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Unread postAuthor: mark.f » Sat Nov 24, 2007 9:18 am

Guys, if he wants to build a combustion mini, let him build a combustion mini. I've always said to myself, give me an unlimited amount of time and materials and I can make an atmospheric combustion that outperforms a pneumatic...

Twitch, the optimal C:B ratio for a pneumatic is not 180%. Pneumatics have no "real" optimal ratio. As you increase chamber volume in respect to barrel volume, you add performance... up to a point. The law of diminishing returns comes into place as you keep adding more chamber volume and increasing the C:B ratio, and your performance increases less and less. This is because the instantaneous acceleration over the entire length of the barrel is higher because of the lower pressure drop in the chamber.

Because of the behaviour of performance vs. C:B ratio in function form, we can say that there is a limit at infinity, greater than zero, (a horizontal asymphote to the graph), to which performance can approach but never reach. Specifically, this would be the horizontal asymphote to the definite integral from 0 to the barrel length of the acceleration function of the projectile, which is a rational function, that represents the maximum velocity obtainable from a barrel of n" radius and n" length for a chamber at a constant pressure and varying chamber size, disregarding flow restrictions, friction, and so forth.

So, for a pneumatic optimal C:B ratio... there is none. "Bigger is better, but only up to a point." A larger C:B ratio will outperform a relatively small one, but a larger one still will improve performance slightly more, a larger still will outperform that by a teeny bit more, and a larger one still is just wasting time and materials, etc., etc...

Now, there may be some C:B ratio at which the increase in performance obtained by increasing the C:B ratio any further will be less than a certain "significant" value. Depending on what this value would be, (personal preference, really, do you appreciate the loss of 3 FPS or do you not care? It's all up to the user...), you could find a C:B ratio after which the performance will not change "significantly", in your definition of "significant".
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Unread postAuthor: The Kid » Sat Nov 24, 2007 9:20 am

markfh11q wrote:Guys, if he wants to build a combustion mini, let him build a combustion mini. I've always said to myself, give me an unlimited amount of time and materials and I can make an atmospheric combustion that outperforms a pneumatic...

Twitch, the optimal C:B ratio for a pneumatic is not 180%. Pneumatics have no "real" optimal ratio. As you increase chamber volume in respect to barrel volume, you add performance... up to a point. The law of diminishing returns comes into place as you keep adding more chamber volume and increasing the C:B ratio, and your performance increases less and less. This is because the instantaneous acceleration over the entire length of the barrel is higher because of the lower pressure drop in the chamber.

Because of the behaviour of performance vs. C:B ratio in function form, we can say that there is a limit at infinity, greater than zero, (a horizontal asymphote to the graph), to which performance can approach but never reach. Specifically, this would be the horizontal asymphote to the definite integral from 0 to the barrel length of the acceleration function of the projectile, which is a rational function, that represents the maximum velocity obtainable from a barrel of n" radius and n" length for a chamber at a constant pressure and varying chamber size, disregarding flow restrictions, friction, and so forth.

So, for a pneumatic optimal C:B ratio... there is none. "Bigger is better, but only up to a point." A larger C:B ratio will outperform a relatively small one, but a larger one still will improve performance slightly more, a larger still will outperform that by a teeny bit more, and a larger one still is just wasting time and materials, etc., etc...

Now, there may be some C:B ratio at which the increase in performance obtained by increasing the C:B ratio any further will be less than a certain "significant" value. Depending on what this value would be, (personal preference, really, do you appreciate the loss of 3 FPS or do you not care? It's all up to the user...), you could find a C:B ratio after which the performance will not change "significantly", in your definition of "significant".


good point
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Unread postAuthor: KnooBill » Sat Nov 24, 2007 11:43 pm

Thanks for the replies, I appreciate them. I thought of making a pneumatic gun instead, but we have no clue where do get the equipment needed (talking about a minor town in the english countryside). The plan is coming to life slowly now that I see a fan should (or must) be included. Still things are shady about ratios though, seing 3:1 and 0.8:1...

UPDATE (couldn't post the first time, but as I saw little lag I saved the message to clipboard :P )
I worked out an approximate 3:1 C:B ratio: (given diameters)
Chamber: 4.7cm x 5.5cm
Volume: 95.4cm³
Barrel: 1.6cm x 16cm
Volume: 32.1cm³

In the chamber a 3cm fan can be fitted too.
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Unread postAuthor: The Kid » Sun Nov 25, 2007 12:29 am

knoobill said: Thanks for the replies, I appreciate them. I thought of making a pneumatic gun instead, but we have no clue where do get the equipment needed (talking about a minor town in the english countryside). The plan is coming to life slowly now that I see a fan should (or must) be included. Still things are shady about ratios though, seing 3:1 and 0.8:1...

it shouldn't be that hard to find to stuff you need some pvc a ball valve tire valve solvent weld epoxy pretty common stuff but if you want to make a combustion i have no problem with that iits your gun make it however you want, good luck with it

peace
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