FRIHOSTFORUMSSEARCHFAQTOSBLOGSCOMPETITIONS
You are invited to Log in or Register a free Frihost Account!


Fuel Injected Spud Gun





polly-gone
Okay, so you, a regular reader of the Frihost forums, may or may not have noticed that I am an unstable, slightly insane specimen of a human being and a purveyor of made-up words. Well, during a flash of insane geniusosity, I came up with arguably the best idea ever. EVER.

See, here's the thing: there are two kinds of potato cannons (spun guns, potato guns, whatever you want to call them), compressed air propelled potato cannons and combustion propelled potato cannons, each with its own pros and cons.

The main advantage to a compressed air potato cannon is that it can be quickly launched and it provides consistent results every time. The main disadvantages are that one can sometimes be very bulky and very hard to assemble if you don't know what you are doing.

Combustion potato cannons are advantageous because they can be quickly loaded and reloaded, their fuel is cheap, and they can provide the same amount of power while being much more compact. Their main problem is that they do not fire consistently, and if you put more fuel in than the gun was designed for (fuel is usually a combustible aerosol like hairspray), there won't be enough oxygen and the gun won't fire at all.

So here's the idea: What is you had a combination combustion/compressed air potato cannon?

The design would be simple. You would have the central potato cannon like you would find on any potato cannon design -- a large PVC air chamber on the back of a more narrow PVC barrel into which you shove a potato. Out the back of the air chamber would come two tubes, basically some sort of line that can handle compressed air, each with a valve attached to seal off the chamber.

On the left side, attached to one line, would be a relatively narrow PVC pipe that is opened on one end. On the right side would be a relatively large PVC pipe about 3/4 the size of the air chamber. Each side pipe would have some sort of piston in it that is either activated by hand or activated by some sort of pneumatic piston.

You would then completely fill the narrow pipe with an excess of combustible fuel like hairspray and push the piston in. The valve to that pipe would be shut, trapping the hairspray in the combustion chamber. The piston would be pulled out and that procedure would be repeated one or two more times. (I haven't worked out the chemistry of it yet.)

You would then push the large piston on the other side in all the way, seal the valve, pull it out, and repeat until the combustion chamber is thoroughly compressed with regular air.

Now, when you activate the gun to fire, there is a massive amount of fuel and oxygen that can undergo combustion and your potato even gets a little push from all the other non-combustible gases in the chamber (like the nitrogen that makes up 79% of our air.) World distance record? I think so.

What do you think?

-Nick Smile Smile Smile
deanhills
You should patent it! Very Happy
Magicman
It seems like that should work. You are trying to get the right concentration of fuel to air which takes care of the cons of using combustion propellant. You also get some of the benefits of an air pressure propelled canon. With a consistent mixture like this you should probably be able to get a more consistent shot. Try building it and see if it works!
ocalhoun
1- Be sure the tube where the compression/explosion happens is EXTREMELY strong. You're basically building a cannon, and there's a very good reason that cannons have thick, heavy sides: If the explosion pressure is too high, the tube will break, releasing the explosive power (and plenty of shrapnel) back towards the operator.
2- Try using pure oxygen as your compressed gas; that should greatly magnify the power of the fuel you're using.
3- Your apparatus is actually very similar -- in both form and function -- to a cylinder in a car's engine. You may be able to use car parts (like fuel injectors, spark plugs, or even valves) as cheap, off-the-shelf components.
polly-gone
Well, if I build it, I actually plan to use high pressure industrial valves which I could probably pick up for about 10 bucks a piece.

To make sure the cylinder doesn't explode, it will be covered with a nylon rope that has spaces of about 3-4in. Then the rope will be secured down with Kevlar tape. Then wrapped with another rope grid. Then completely covered in Kevlar tape. It will be a little heavy, but I just dare that thing to explode. Smile Plus, I can use a remote detonator. A sparker from a model rocket launcher that is radio activated should do the trick.

And chemistry is kinda my thing, so I can look up the optimal concentration of air to gas, then magnify it by a whole crapload.

And if works, I am DEFINITELY patenting it. There are online stores dedicated to potato cannons and one that can launch a few thousand feet would definitely sell. I would just need somewhere to test it... hmm... haha

Nick Smile Smile Smile
ocalhoun
polly-gone wrote:

To make sure the cylinder doesn't explode, it will be covered with a nylon rope that has spaces of about 3-4in. Then the rope will be secured down with Kevlar tape. Then wrapped with another rope grid. Then completely covered in Kevlar tape. It will be a little heavy, but I just dare that thing to explode. Smile

As the spud travels down the barrel, the pressure behind it decreases rapidly (due to the extra room now available to expand into).
Because of this, the lowest portion, closest to the fuel is subject to the highest pressures.

If your rope & kevlar doesn't hold up, you can use a heavier alternative just for that section, while using lighter materials for the rest. That way, you can add a lot of protection where its needed, without increasing the weight more than needed.
(Which is why many cannons are much thicker at the base of the barrel than at the end of the barrel.)
Since you're basically building a cannon*, might I suggest researching cannon design?
You might also benefit from the use of 'wadding', especially since a spud isn't perfectly round.

*Just replace the spud with a cannon ball, and replace the fuel/air mix with gunpowder...


And, by the way, setting if off from a (long) distance is definitely the way to go, especially the first time!
Bikerman
The maximum pressure is determined by the resistance of the object in the barrel and the speed of burn of the fuel/propellant. Providing the object does not offer very large resistance then the pressure will not be too great. You want a propellant with a moderate burn rate, not a very fast one.


* Example - I have made a tennis ball cannon a few times to amuse my nephew. It consists of 4 beer tins/cans - which are very thin-walled. Select tins which a tennis ball can just slide down. Cut the top and bottom off two tins - that is your barrel. Tape them together with gaffa tape. Cut the top and 1/2 the bottom off the third tin - this is where the tennis ball will sit. Gaffa that to the barrel. Cut the top off the fourth tin and perforate the bottom with 20 or so holes. That is your expansion chamber. Gaffa that to the other 3.
Put a thimble-full of white spirit in at the top whilst holding your hand over the perforated bottom. Give it a good shake. Slide the tennis ball in from the top. Aim upwards (the tennis ball will go pretty far and would really hurt at short range - typically this thing will shoot a ball a couple of hundred metres) and hold a naked flame under the perforated bottom. Boom.

* This is pretty safe for someone with common sense, but please don't attempt it without someone responsible with you, and do not use other fuels - stick with white spirit.
ocalhoun
Bikerman wrote:
Providing the object does not offer very large resistance then the pressure will not be too great.


Got to remember that the inertia of the object will contribute to its resistance to motion though...
It would only matter for a short time, but that would be the most critical time for high pressure problems.
(A potato is much heavier than a tennis ball, and from the sound of it, it should be accelerating very fast.)
polly-gone
Well, in a typical design of a spud gun, wadding isn't necessary because the opening of the barrel slices the potato to be form fitting when you load it. It forms an air tight seal with the wall of the cannon and no gases can escape. That's also why it is usually a good idea to lubricate the walls of the barrel in some way to make sure that the potato leaves the barrel properly.

I was thinking maybe reinforce the lower barrel and barrel/chamber joint with fiberglass or even carbon fiber so that it doesn't separate. That would suck.

-Nick Smile Smile Smile
ocalhoun
polly-gone wrote:

I was thinking maybe reinforce the lower barrel and barrel/chamber joint with fiberglass or even carbon fiber so that it doesn't separate. That would suck.


Go ahead and try it... But if I was building such a thing, I would use a steel pipe as the barrel, with a thread-on end cap for the bottom of the chamber... And would still consider reinforcing it, depending on the thickness of the pipe.

But as long as you fire it from a safe distance the first few times, trial and error would be a fine way to find out just how strong it needs to be.
HoytJolly
My friend came over to my house with a spud gun once. He used the thickest PVC pipe available in home depot to build it. It had a flint lighter embedded in the main chamber and a screw-in cap in the back. We filled it with mapp gas and jammed a spud down the barrel. Even though the PVC was thick, I could still see the tube glow every time my friend fired it. I made a point to stand back a few feet.
dwxco
Bikerman wrote:
The maximum pressure is determined by the resistance of the object in the barrel and the speed of burn of the fuel/propellant. Providing the object does not offer very large resistance then the pressure will not be too great. You want a propellant with a moderate burn rate, not a very fast one.


* Example - I have made a tennis ball cannon a few times to amuse my nephew. It consists of 4 beer tins/cans - which are very thin-walled. Select tins which a tennis ball can just slide down. Cut the top and bottom off two tins - that is your barrel. Tape them together with gaffa tape. Cut the top and 1/2 the bottom off the third tin - this is where the tennis ball will sit. Gaffa that to the barrel. Cut the top off the fourth tin and perforate the bottom with 20 or so holes. That is your expansion chamber. Gaffa that to the other 3.
Put a thimble-full of white spirit in at the top whilst holding your hand over the perforated bottom. Give it a good shake. Slide the tennis ball in from the top. Aim upwards (the tennis ball will go pretty far and would really hurt at short range - typically this thing will shoot a ball a couple of hundred metres) and hold a naked flame under the perforated bottom. Boom.

* This is pretty safe for someone with common sense, but please don't attempt it without someone responsible with you, and do not use other fuels - stick with white spirit.


This sounds like a good, easy project. Thanks for posting such clear directions.
polly-gone
ocalhoun wrote:
polly-gone wrote:

I was thinking maybe reinforce the lower barrel and barrel/chamber joint with fiberglass or even carbon fiber so that it doesn't separate. That would suck.


Go ahead and try it... But if I was building such a thing, I would use a steel pipe as the barrel, with a thread-on end cap for the bottom of the chamber... And would still consider reinforcing it, depending on the thickness of the pipe.

But as long as you fire it from a safe distance the first few times, trial and error would be a fine way to find out just how strong it needs to be.


I think steel pipe would be way too cumbersome. I ran some numbers (and by "ran some numbers" I mean that I looked up the strengths of the various materials and guesstimated because I don't have time to run numbers these days), and I think that just applying plenty of reinforcement to it would work just fine.

But yeah, I think there would need to be a minimum of a hundred foot radius around it for the first few test runs to give everyone time to hit the ground if the whole things just decided to up and explode. I figure that if after a few test runs, if a detailed inspection shows that the entire thing is okay, it would be safe to reduce the radius around it.

But there is another thing that I didn't think of...

The entire premise of this thing is that compressed air and fuel together will allow there to be more fuel to fire the gun with. An added advantage would be that any air that isn't used in the combustion would also help propel the spud out of the barrel. But what if because of the pressurization, the very beginning of the combustion process was enough to launch the spud out of the gun and the pressurized fuel and oxygen expanded and combusted to shoot flames out of the barrel? Can anyone say "baked potato"? And/or flaming potato?

-Nick Smile Smile Smile
ocalhoun
polly-gone wrote:

But there is another thing that I didn't think of...

The entire premise of this thing is that compressed air and fuel together will allow there to be more fuel to fire the gun with. An added advantage would be that any air that isn't used in the combustion would also help propel the spud out of the barrel. But what if because of the pressurization, the very beginning of the combustion process was enough to launch the spud out of the gun and the pressurized fuel and oxygen expanded and combusted to shoot flames out of the barrel? Can anyone say "baked potato"? And/or flaming potato?


It would probably be best to keep it at relatively low pressure, at least for the first prototype. Even a normal-atmospheric-pressure fuel-air mixture has quite a bit of power.

Otherwise, you'd have to do one of these complicated things:
1- Pressurize the barrel of the tube as well. -Difficult, and could decrease performance.
2- Devise a method to keep the spud in place until pressure built to a certain level (i.e. More than mixture pressure, but less than ignition pressure) -Would be a relatively complex mechanism, prone to failure... Possibly catastrophic failure.
3- Have exact timing so that ignition and pressurization happen almost instantly, giving the spud no time to move much before ignition. Have the pressurization be nearly instant, and also simultaneously cause the fuel to be mixed in, and have the ignition triggered by a pressure switch? -Could be complicated, but would probably be the best bet for a super-powerful gun.

A low-pressure mixture should be fine for recreational firing though.
A high-pressure mix might end up powerful enough to cause real damage to the target (or disintegrate the spud)... It would be entertaining to see a spud gun with enough power to take out a car though. ^.^
A well-designed gun like that might be able to achieve bullet-like speeds... And that's a big, heavy projectile at that speed... And it would spread on impact, making the full force of the shot be absorbed into the target.
At that point, the only thing that would prevent it from being tactically effective is that it would certainly have very poor accuracy (due to the irregularity of the spud projectile). Also, the range might be more limited than it would otherwise be, because the spud may not fly in an aerodynamic fashion at all.


[begin side-track]
Twisted Evil
Golf balls, on the other hand, are very regularly shaped and very aerodynamic... And they can even be hollowed out and filled with your choice of payload.
A golf ball cannon built along the same lines as your spud gun could be terrifyingly effective by home-made artillery standards.
If one got the help of a machinist, one could even envision a revolver-like semi-auto version, though it would have to be double-action. Have a revolving wheel of barrel sections, with the stop just before connecting with the rest of the barrel being loaded from a hopper full of golf balls.
(1) Turn wheel, (2) prime ignition switch, (3) fire (pressurize), repeat. ^.^
A clever design could integrate 1 and 2 into one lever... Or even all three... That would be scary. Use the recoil to push that lever, and voila; full-auto. Shocked
[end side track]


-On an unrelated note, you probably want to put in some kind of provision to absorb recoil, especially when it is fired remotely... This will help it last longer without tearing itself apart. Mount it on springs, or on a wheeled base, or anything else that will absorb the (possibly intense) recoil.
Parkour_Jarrod
My dad an I actually built one of these a few years ago...

Initially we used a 6 inch diameter PVC pipe 2 foot long connected to a 3 inch diameter PVC pipe 1 foot long...

We made sure the 6 inch pipe wouldn't explode by using denim wrapped around it multiple times to catch the explosive force and release the air if something did go wrong.

Using a pure oxygen tank on the bottom we used 1/3 oxygen and 2/3 butane for the reaction, we shot the cannon into an old Blue Gum tree (aussie hardwood) from 200m and it went in about.... 4 inches into the trunk... by calculations, that should dent a tank...
ocalhoun
Parkour_Jarrod wrote:
we used 1/3 oxygen and 2/3 butane for the reaction,

Using a gaseous fuel would make the fuel-mixing process far simpler...

You could even completely mix it before hand, then pressurize it.
Though that pressurized fuel/oxygen tank would then become an extremely dangerous object. If it should get too hot (like in a fire) or somehow get a spark inside, it would just be one big bomb...
polly-gone
ocalhoun wrote:
It would probably be best to keep it at relatively low pressure, at least for the first prototype. Even a normal-atmospheric-pressure fuel-air mixture has quite a bit of power.

Otherwise, you'd have to do one of these complicated things:
1- Pressurize the barrel of the tube as well. -Difficult, and could decrease performance.
2- Devise a method to keep the spud in place until pressure built to a certain level (i.e. More than mixture pressure, but less than ignition pressure) -Would be a relatively complex mechanism, prone to failure... Possibly catastrophic failure.
3- Have exact timing so that ignition and pressurization happen almost instantly, giving the spud no time to move much before ignition. Have the pressurization be nearly instant, and also simultaneously cause the fuel to be mixed in, and have the ignition triggered by a pressure switch? -Could be complicated, but would probably be the best bet for a super-powerful gun.


Ha HA! You doubt me ocalhoun! The way to keep the gas in the chamber would be to have a valve separating the chamber and the barrel; the same kind as a safety release valve. It would be pressurized up to the point that a tiny amount of gas is released, then when ignition occurs, there would be so much pressure behind the valve that it would fully open to release all of the gas. It would probably need to be specially designed, but I'm capable of that. It might need to be one that needs replaced after every shot, but a replacement could be inserted through the barrel.

Now... golf ball cannon... I am currently designing a paintball gattling gun, and I think with a few minor modifications it could shoot golf balls... mwahahahaha...

I think I've covered all your points now...

-Nick Smile Smile Smile
ocalhoun
polly-gone wrote:

Now... golf ball cannon... I am currently designing a paintball gattling gun, and I think with a few minor modifications it could shoot golf balls... mwahahahaha...

Way ahead of me there, I see...
Well, if you finish it, I'd like to see it in action!
Parkour_Jarrod
polly-gone wrote:

Now... golf ball cannon... I am currently designing a paintball gattling gun, and I think with a few minor modifications it could shoot golf balls... mwahahahaha...


Golf-ball cannons don't work, the cannon requires the nozzle to be airtight to the projectile, whereas golfballs have the small dents (~400) to make them aerodynamic in the wind, these dents stop the airtight and cause it to fail...
Flakky
Good luck with it, you should create it xD
polly-gone
Parkour_Jarrod wrote:
polly-gone wrote:

Now... golf ball cannon... I am currently designing a paintball gattling gun, and I think with a few minor modifications it could shoot golf balls... mwahahahaha...


Golf-ball cannons don't work, the cannon requires the nozzle to be airtight to the projectile, whereas golfballs have the small dents (~400) to make them aerodynamic in the wind, these dents stop the airtight and cause it to fail...


Projectile firing -- for lack of a better word -- weapons definitely do not need to be airtight. If the force of the escaping gas behind the projectile is strong enough, it will easily shoot the projectile out of the cannon. Look at paintball guns: the sizes of paintballs are very inconsistent, ranging from .67 caliber to .71 caliber. This variable sizing is just due to inconsistencies in the manufacturing process. The barrel of a .69 caliber paintball gun is actually about .73 caliber to accommodate paintballs of every size.

Anyway, the divots in the golf ball would actually help to propel it out of the cannon because they would catch the air.

-Nick Cool Cool Cool
Parkour_Jarrod
polly-gone wrote:
Parkour_Jarrod wrote:
polly-gone wrote:

Now... golf ball cannon... I am currently designing a paintball gattling gun, and I think with a few minor modifications it could shoot golf balls... mwahahahaha...


Golf-ball cannons don't work, the cannon requires the nozzle to be airtight to the projectile, whereas golfballs have the small dents (~400) to make them aerodynamic in the wind, these dents stop the airtight and cause it to fail...


Projectile firing -- for lack of a better word -- weapons definitely do not need to be airtight. If the force of the escaping gas behind the projectile is strong enough, it will easily shoot the projectile out of the cannon. Look at paintball guns: the sizes of paintballs are very inconsistent, ranging from .67 caliber to .71 caliber. This variable sizing is just due to inconsistencies in the manufacturing process. The barrel of a .69 caliber paintball gun is actually about .73 caliber to accommodate paintballs of every size.

Anyway, the divots in the golf ball would actually help to propel it out of the cannon because they would catch the air.

-Nick Cool Cool Cool


Understandable but in the theory of a fuel injected spud-gun the way it is fired is by creating mass amounts of pressure behind the spud (projectile) Now in a paintball or a firearm where the projectile is is airtight but the barrel is not... For Paintball guns the paintball it stuck behind a rubber seal and when the gun initiates the gas propellant the ball flys past the rubber seal which is the weakest (therefore easiest) place to equalize pressure.
polly-gone
Sorry, I forgot my proof. Smile

http://m.youtube.com/index?desktop_uri=%2F&gl=US#/watch?xl=xl_blazer&v=ylbWorbZeOM

-Nick
Parkour_Jarrod
polly-gone wrote:
Sorry, I forgot my proof. Smile

http://m.youtube.com/index?desktop_uri=%2F&gl=US#/watch?xl=xl_blazer&v=ylbWorbZeOM

-Nick

WheezyWaiter is your proof?
polly-gone
Did my phone butcher the URL?

http://www.youtube.com/watch?v=ylbWorbZeOM

Golf ball cannon in action.
Related topics
Firm claims fuel cell breakthrough
Your favourite movie quote
Urban Legends About the Iraq War
Need help in ps2 game "GUN"
Potato Guns
Spud Guns
[-Jeu-] La photo mystère.
Making ECU for old 4 stroke machine
info requires (hobbies / interests / field / profession ...)
Has you car a carburator?
HHO: Hydrogen Electrolysis
A new car company.
HHO Gas Generators for Cars
2006 Pontiac Pursuit/Cheverolet Cobalt
Reply to topic    Frihost Forum Index -> Lifestyle and News -> Amazing Projects

FRIHOST HOME | FAQ | TOS | ABOUT US | CONTACT US | SITE MAP
© 2005-2011 Frihost, forums powered by phpBB.