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spudgun physics project

A place to ask general spud cannon related questions.
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spudgun physics project

Unread postAuthor: cocacola » Fri May 25, 2007 10:28 am

Hey..

Me and two guys from class are about to do a project in school about spudguns and the tech/physics around it! None of us has any experience with spudguns though. But it shouldn't be a problem because we have allot of time, the right tools and interest. We're not going to make this project half-assed or cheap. We're going to do our best in creating a cannon as reliable, powerful and "advanced" as possible. (No offense to hairspray, but it seems a bit too random to calculate.)

The project itself is going to represent parts of we have learned in our pre-engineer education, which means mathematics and physics is going to be involved. So basically we need to make a cannon and calculate (everything, i.e trajectory and stress.

All-in-all, we have about a year to do this. I guess we're going to need some of your assistance in this. Don't be intimidated by this long post, if you got any ideas on how we should do this, feel free to spread em'!

// Thanks alot.

PS. It doesn't seem to be illegal to create/fire spudguns in sweden. But if there's anything I should know of, please say.
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Last edited by cocacola on Sat May 26, 2007 11:27 am, edited 1 time in total.

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Unread postAuthor: rednecktatertosser » Fri May 25, 2007 10:34 am

first off id like to say welcome to the forums. secondly id say if you have around a year you should definately consider making a piston valve. lastly will you be shooting spuds or are you going for more exotic ammo(i.e. tennis balls, PVC rockets)?
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Unread postAuthor: paaiyan » Fri May 25, 2007 10:36 am

Well I'd say your best bet is a metered propane cannon. With metered propane you inject the stoiciometric amount of propane into the chamber so you can write up your calculations for the stoichiometric amount, you can calculate the volume of the gasses that it will produce, you can calculate the approximate energy that will be released by the reaction.

With metered propane you can calculate almost everything before you even take a shot, then take a shot and see how close you got.
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Unread postAuthor: mega_swordman » Fri May 25, 2007 10:55 am

It entirely depends on your interest. Whether you want to use a pneumatic cannon or a combustion cannon. I like paaiyan's idea without he measurements. With that, I would try to see 1. how far it will go just firing it in an open field. 2. how far will it travel after going through an obstacle (like plywood) 3. and how much energy is lost once it goes through an obstacle. How's that?
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Unread postAuthor: mark.f » Fri May 25, 2007 10:59 am

I'd say you should concern yourself with the actual physics of the spudgun, instead of the fueling. Calculating things like peak pressure, maybe judging the velocity my measuring the force exerted by recoil, calculating percent efficiency, reaction rates at increasing pressure, and things of that nature. Saying that 4.2% propane to air is optimal is simple gas stoichiometry, not physics. Ballistics is another field you can pursue. Also, you mentioned stress. A stress gradient at peak pressure running the length of the spudgun would be helpful to your grade, as well as us.

Anyways, good luck!
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Unread postAuthor: paaiyan » Fri May 25, 2007 11:03 am

Well I was jut saying, he's doing it for a physics project so the more calculations he makes and all, the more likely it is to get him a better grade. And as for the stoichiometry, that's just linear thinking. I meant he can calculate the stoichiometric amoutn of propane, then with that equation as a base, he can find approximate range, muzzle velocity, muzzle energy, energy of the projectile at 100 meters, physics stuff.

No, stoichiometry isn't physics, but it can make physics easier.
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Unread postAuthor: cocacola » Fri May 25, 2007 11:07 am

We will most likely be using tennisballs. They seem like the best alternative. With spuds I dont think I'll get the same consistency, and i guess it's going to be a little messy with potato peeling all over the place. Metered propane sounds pretty good, so does "stoichiometric" ( :? ). Although, none of us has experience with explosives so is it really a good idea? Sounds actually pretty interesting though!

Approximately, what's the pricetag of a homemade cannon?
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Unread postAuthor: Fnord » Fri May 25, 2007 11:12 am

(Edit: tennisballs are always a good choice too)
For your cannon I would use golf balls instead of the traditional potato, Just because they are extremely similar in weight, where a potato might be a couple oz different than the next.

If you are looking for power, go with a pnuematic. If you are looking for ease of use and tons of math/calculations, go with a combustion.
If you are really good at engineering, you might want to look at hybrids. I doubt anyone in school has ever seen a hybrid before, and they involve tons of physics related stuff. Just make sure its made out of metal for the safety of your audience.

Another edit: The pricetag varies greatly depending on where you live, what kind of cannon you want, and what materials/tools you have avalible.
Some people have spent hundreds of dollars on cannons(look up schmanman's SWAT gun somtime) and some are constucted for almost nothing. I have a combustion that will do 200 yards which I have spent $0.00 on, while my hybrid cost me about $40(cheap for a hybrid).
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Unread postAuthor: cocacola » Fri May 25, 2007 11:19 am

_Fnord wrote:(Edit: tennisballs are always a good choice too)
For your cannon I would use golf balls instead of the traditional potato, Just because they are extremely similar in weight, where a potato might be a couple oz different than the next.

If you are looking for power, go with a pnuematic. If you are looking for ease of use and tons of math/calculations, go with a combustion.
If you are really good at engineering, you might want to look at hybrids. I doubt anyone in school has ever seen a hybrid before, and they involve tons of physics related stuff. Just make sure its made out of metal for the safety of your audience.

Another edit: The pricetag varies greatly depending on where you live, what kind of cannon you want, and what materials/tools you have avalible.
Some people have spent hundreds of dollars on cannons(look up schmanman's SWAT gun somtime) and some are constucted for almost nothing. I have a combustion that will do 200 yards which I have spent $0.00 on, while my hybrid cost me about $40(cheap for a hybrid).
Golfballs was taken into consideration, however It seems a little too dangerous. Tennisballs seems to be just fine.

When you drop a tennisball into the barrel, how do you want it to fit? Should it stick, roll or a little bit of both?

Edit: shanman's gun seems a little too big. We're thinking of a portable shoulder held cannon, bazooka-style. Something like this: http://i145.photobucket.com/albums/r205 ... CT0086.jpg

Were going to make some kind of "holding-device" though, which allows us to measure angles.
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Last edited by cocacola on Fri May 25, 2007 11:31 am, edited 1 time in total.

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Unread postAuthor: mark.f » Fri May 25, 2007 11:25 am

Tennis balls squish into a barrel, and you have to use a ram-rod to push them down. Indeed, you can make a breech loading barrel to make this simpler. The fuzz burns off due to friction when fired, so expect a smelly, (and colorful!), barrel interior after several shots. :wink:
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Unread postAuthor: Fnord » Fri May 25, 2007 11:28 am

Cannons are dangerous by nature, but as long as you use your head you wont get hurt. Golf balls tend to ricochet if you shoot them at something too hard, but you should never be shooting point blank anyway.

Although I've never made a tennisball gun before, 2.5" sch40 pvc pipe is the ideal barrel to use for one. You probably won't get the distance you would with a golfball, but for a science project they are still a good choice.
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Unread postAuthor: jimmy101 » Fri May 25, 2007 12:40 pm

If you actually want to be able to predict the performance of the gun with any kind of accuracy you should build a compressed air gun.

I don't think paaiyan is right with "With metered propane you can calculate almost everything before you even take a shot, then take a shot and see how close you got." You can do that, but the predictions won't be anywhere near the actual performance of the gun. (I'm talking about predictions using basic physics here. If you simply scale the measured performance of someone else's gun then you get pretty good predictions, that is how Evbec works.)

So you can easily calculate a lot of stuff, but none of it is particularly useful in predicting the performance of a combustion gun. To actually model the behavior of a combustion gun is very difficult. For example, using basic physics I don't think there is anyway to calculate that the optimum C:B is ~0.8:1. Basic physics says it is in the vicinity of ~0.25:1. The combustion process is just too complex for "basic physics" to work.

A compressed air gun can be predicted with at least reasonable accuracy. GGDT works pretty well. With a little effort you can recreate most of the mathematics/physics that GGDT uses for your project.

If you want to model the internal ballistics then it is probably a lot easier to go with a compressed air gun. If you are just modeling the external ballistics then either type of gun would be fine.
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Unread postAuthor: jackssmirkingrevenge » Fri May 25, 2007 1:04 pm

How about this lascannon, proven to function well in northern europe :)

...though as jimmy says, in this case I think a pneumatic is a better idea for your purposes.
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Unread postAuthor: cocacola » Fri May 25, 2007 3:13 pm

I'm sorry jimmy101, my english is limited just as it is. C:B, Evbec and GGDT, I have never heard the terms before.. However I get your point. Even though, going with combustion is going to be more of a challange, I guess. But then again, if I get unreliable results it's pretty much worthless.

If anyone of you reading this has already done what im trying to do. Please share your experience, i.e how accurate the launch was comparing to the estimation from the calculation.
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Unread postAuthor: paaiyan » Fri May 25, 2007 3:25 pm

cocacola wrote:I'm sorry jimmy101, my english is limited just as it is. C:B, Evbec and GGDT, I have never heard the terms before.. However I get your point. Even though, going with combustion is going to be more of a challange, I guess. But then again, if I get unreliable results it's pretty much worthless.

If anyone of you reading this has already done what im trying to do. Please share your experience, i.e how accurate the launch was comparing to the estimation from the calculation.


C:B is the term for the ratio of chamber volume to barrel volume

Evbec and GGDT are tools that help you calculate certain things for your cannon. GGDT is the Gas Gun Design Tool and can tell you the approximate range of a pneumatic cannon if you input the chamber and barrel volumes, the psi you charge it to, and the mass of the projectile. Evbec i don't have any experience with, but I believe it is a similar tool.

EDIT: <a href="http://www.spudfiles.com/spud_wiki/index.php?title=EVBEC">This link</a> will tell you about Evbec.
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