Been working on this cannon for about 2 weeks on and off, finally finished it off. The appearance is a little sloppy, but its 100% functional.

I haven't had a chance to actually fire a projectile yet, but I've done many blanks and the sound is deafening.






The secret to getting a good mix of fuel and air, a CPU fan inside the combustion chamber.


I removed the fan for this shot. I got a coil from a stun gun and re-wired it to jump across the screws in the chamber to ignite the mix. Voltage from a 9v battery is stepped up to 100,000 volts and can jump up to a 1 inch gap.


This barrel has seen all kinds of action in all the guns I have built in the last 3 years.


The same barrel as above attached to my newest gun. It is 5 feet long and has a diameter of 1.5 inches.


This is my 12 inch long, 2 inch diameter "noise maker" barrel. As far as projectiles it can only lob things a few feet but it is much more practical when you just want to make a big bang.


And lastly my monster 2.5 inch diameter "tennis ball" barrel. 2.5 inches is a perfect diameter for firing tennis balls. This barrel has never been used, but I cant wait to see the distance i get. I might have to shorten it for optimal performance. You can see one of my older guns in the background.




How it works (in case it isn't obvious to you)

The combustion chamber is 3300cc. The perfect ratio of air to propane is 4% by volume. This means I will need 132cc of propane at atmospheric pressure to get the best combustion. The propane meter pipe, which is the pipe between the 2 valves, is 34cc. A little physics says that 132cc of propane at atmospheric pressure is about the same amount as 34cc at approximately 58 PSI. That blue tank outputs about 90 PSI. That regulator on the tank is what I use to step down the PSI. It is variable, so if I want to do a lower power shot I can set it to 30 PSI, etc, however optimal is 58 and performance will decrease if I go above or below that.

Opening the first valve pressurizes the meter pipe to whatever the regulator is set to. Closing it seals off the meter pipe. Opening the second valve injects the perfect amount of propane directly into the chamber. The second valve is the closed to keep as much of the combustion out of the meter pipe as possible. The fan is turned on at this point with the first toggle switch (the one toward the rear). Only brushless CPU fans can be used, all other fans will have an arc within them that will ignite the mix prematurely. After a few seconds of the fan mixing the air and fuel, I arm the igniter with the second toggle switch (the one toward the front). When ready, I click the momentary switch at the front of the box and the spark will jump is gap, igniting the mix, and the rapidly expanding hot gas will push any projectile out with great force. If there is no projectile, most of the energy is converted to sound creating a very loud noise. I can fire off about 4 rounds per minute max, however it starts to get a little warm at that point. I usually give at least 1 or 2 minutes between shots with a longer break every 6 or so shots just as a precaution.


The weakest part of the gun is the 4 inch PVC which makes up most of the length of the combustion chamber, this pipe is rated to 220 PSI, and the combustion created makes no more than 50 PSI, so it is extremely safe when properly used. It is perfectly safe to hold in your hand and fire (takes 2 hands) however it can be a handful with all the switches and valves so I usually rest it on something.


The shop, mid-construction





Anybody else do this for a hobby?

Probably not like that, Christ. Pretty amazing, and I'm impressed and relieved by your apparent knowledge on what you are doing.

[edit- i guess i'm calling you christ, now]

Nice work, I suppose there are worse hobbies you could have

lets see it in action!

Seems like an awesome hobby! Beats building miniture boats.

Even better, he could use his cannon to SINK minature boats.

I demand videos.

pr0n?

Mount it on a golf cart and be a suburban/urban Pirate. y-Arrrrhh!

Right now I believe it legally qualifies as a "Recreational Launcher". Mounting it on any moving vehicle would not be legal

And yeah, countless hours/days were spent studying and researching every aspect of whats required to build one of these, and I dont recomend you try it unless you know exactly what you are doing.

Thinking up, designing, construction, and taking my time I put about 30 hours into this one. Not to mention all the past experiences with previous guns i've built, past failures, all the reading and studying i've done on the math and physics behind it. There is a lot involved, and there are some good places on the internet that will teach you all about them, however there are many BAD BAD designs out there too!

PM me if you are seriously interested in doing one for yourself and i'll show you the way.

Terrorist!

Wait, so, mounting a weapon on a moving vehicle is illegal?

Damn, there go my plans for a laser turret in the back of my truck.

Are you good at making land mines? My folks just put up a new fence by the highway and we don't want anyone to put graffetti on it so I was just thinking...

I made one of those a few summers ago. My range was awesome, and after a while I started experimenting with my projecticles.

I may or may not have fired a living creature as ammunition. <_< >_>

PVC? That **** shatters when it explodes, you know that right? Use ABS instead which will simply burst in one area, whereas PVC will send HOT SHARDS OF PLASTIC FLYING INTO YOUR FLESH!

It's not so much the PSI rating, as it is the WAY that it fails when that limit is exceeded. ABS and PVC with the same rating are not the same level of safeness for this purpose. ABS is hundreds of times safer.

Ladies and gentlemen CLASSIC example of somebody who does NOT know what the hell they are talking about. PVC is PRESSURE RATED, which is why it is typically used in HIGH PRESSURE situations. When was the last time you saw black ABS pipe used for ANYTHING besides a drain pipe?

What do you mean "PVC and ABS with the same rating?" ABS has NO rating whatsoever.

Look on the side of any ABS pipe and you will see the words CELLULAR CORE. This means that there are actually 3 layers within the pipe wall, two very thin plastic layers on the outside (the hard part that you see) however IN BETWEEN these plastic layers is the cellular core, a spongy plastic substance that has multiple air bubbles WHIPPED into it before the pipe is formed to make the pipe WEIGH LESS. Sure, if ABS pipe fails it will only split in two, but the chances of it failing are MUCH HIGHER. ABS has no pressure rating at all (which is way it is rated for use as drain pipe only). Sure, it may work fine for you and never fail on a combustion cannon, and I will admit that it has been proven to be an acceptable building material, but there is NO guarantee and I do not think it is wise to take the chance, especially if you like experimenting. If you proporly clean, prime, and glue PVC pipe that is structurely sound and give it propor time to cure it will never fail and is much safer. The pipe wall of PVC is solid all the way through.

Once you move on to things like compressed air (which is WAY more powerful than combustion) using ABS pipe would be suicide.

If you build a hybrid combustion cannon which involves injecting 2x, 3x, 4x etc the ammount of required propane for the given chamber volume and than compressing the chamber with 2x, 3x, 4x atmospheric air pressure to make up for the extra propane (making a combustion that is 2x, 3x, 4x etc more powerful) nothing less than reinforced SCH-80 or SCH-120 PVC pipe is acceptable, and if you want to go up to things like 6x, 7x, 8x etc you want to build your cannon out of steel.

My gun is made out of pressure rated SCH-40 PVC. The 4" pipe is rated at 220 PSI. I'm not sure what the 1.5" barrel is rated at because I painted over it, but that is moot because the smaller the pipe the higher the rating. I believe it is rated at about 420 PSI. The 2.5" is around 300 PSI. I could only dream of creating combustions with this cannon that even come CLOSE to generating 100 PSI of pressure, let alone the 220 PSI that the WEAKEST part of my cannon is rated for. Build and use your gun right and you wont even have to worry about where or how it will fail.

Get your **** together before you get somebody hurt.



Oh and another thing...while i'm at it...dont even think about using acetylene (welding gas) as fuel, you will turn your gun into a grenade no matter what material you use to construct it.

Do you watch mythbusters? hahah

Actually i've never seen the show, but a friend of mine told me something about an episode where they used a big cannon kinda like mine to fire whole chickens at an airplane window or something to see if it would break :p lol.

I'm not sure what either of them are talking about, but Dash said more words, so I think he won.

Eh, maybe I'll double check what you are saying. I trust people who make plastic things for a living more though. Especially when they shoot chickens at cockpit windows. Really, they make the canopies here, and they shoot chickens.




Since acetylene self ignites at 14 (might be 12, actually) PSI in a free state, yeah, that would be a bad idea.

I didnt mean to come off so harsh, I just see way too many idiots around who dont know what they are doing and come way to close to getting somebody hurt or killed by mis-using home-made guns, and we all know that its the one moron that ruins things for the countless safe ones out there. I'm not calling YOU a moron, i'm talking about the other guy who may see your post and fill his ABS gun with 150 PSI of compressed air and send a valve handle into his eye socket. This is why is it always a good idea to use PVC no matter what. ABS may work for the lowest level of combustions, but people tend to like to take their ideas and advance with them. Using PVC from the start sets the right tone and gives plenty of room to expand into far more powerful cannons (within reason) safely. It hits a soft spot for me when I see/hear people discussing these things in any way other than the established fail-safe method that I use religiously. (And yes, in a way, I am saying that my way is the best way and any other way is the wrong way).

And I guarantee that someday the feds will ban these things and it will be because some idiot kid built one and shot a screwdriver through his little brother's skull, ruining a hobby that others have been able to enjoy safely for years.

No worries man, I feel the same way about breathing. What if someone ruins it for us all?

Its always cooler when you can die from it.

ADMIT IT.

Really though, how many cool hobbies AREN'T illegal? How many of them don't involve possible death and bodily harm?

One.

Posting here.

Wait...

Nevermind.

To test my guns I cap both ends, creating a sealed chamber and ignite the mix. I admit not a very good idea, although I fire it remotely and would not be in danger if it burst. The threads hiss a little bit, but the combustion is perfectly contained within the PVC and NOTHING happens. I have tried that on every gun I have built to test it before I go around shooting it in my hands, and I have had no problems. I would put down money that ABS would burst.

DO NOT try this.

yeah I know.. I was just quoting a movie

Dash, have you tried forcing it so all the air moves through the fan. maybe it would compress the air better..?

What do you mean?

I dont think you quite understand how it works, all the fan does is mix up the propane and air so the mix is more consistant throughout the chamber. Also, between shots it doubles as a fan to vent out the chamber with fresh air so the next detonation is good. There is always a 4" male cap screwed into the rear of the gun, I only take it off for a few seconds as I said to vent between shots.

Wow, nice work! But won't the CPU fan get damaged from repeated explosions? I'm surprised the force from the combustion doesn't at least break off the fan blades.

Being that the combustion is forced out the other side, I doubt it would hurt it.

That is one of the coolest things ever.

Get good with it. Practice alot, and find a way to make home made explosive shells. Then buy a junk car.

Quite interesting. I never thought these things are made of plastic. I assumed people always use steel. But I suppose it makes sense when the material is pressure rated, like you said.

When are you going to make a coil gun?

I've made a few of those, the best could fire a screw through a 2mm sheet iron plate, and the only sound you could hear was the sound of the capacitors loading.. i also shot a bird down with it

Wow, fancy.

I solved the air/mixture problem a little bit differently. It seems to me that the fan won't last very long, considering what exactly goes on in there...

The way I took care of the problem was quite simple. I used that nice hose setup like you've got, only I used two, then had one of them for propane (coleman camping barbecue fuel) or butane and the other for oxygen (for a welding torch, Home Depot, $8 dollars). Then I open the propane and the oxygen until I get the desired amount of propellant then close them both. It takes a few tries to figure out a god mixture, but it isn't hard.

I wish I had a stun gun coil, though. I am still stuck with an old flint from a lantern. I tried a piezo-electric starter (for a gas barbecue), but it wouldn't spark consistently enough to get the desired explosion. The flint works well, though. My combustion hcamber is a 4 inch pipe, 12 inches long. I use a 2.5 inch pipe, 5 feet long for the barrel. Unfortunately, the way they're urbanizing my area more and more, I find little opportunity to shoot it.

Fun things to shoot:
Tennis Balls
Golf balls (wrap in a cloth)
Soda Cans
Glass Bottles (open end down)
Styrofoam/wax cups filled with stuff
At night, Glowsticks (put them in a potato)
Frozen water (this one is great! get a piece of your barrel pipe, cap one end, fill with water, put in freezer)
A missile (we made a missile out of some leftover piping, gets the longest distance of any)
Model Rocket Engines (stuff in a potato for max distance)
Eggs (use some sort of wadding)
Sponge soaked in gasoline (use some sort of wadding, I forgot what I used)
Fireworks of any kind

Have fun! Be sure to take picture of it in action.

The fan is 100% unarmed (I even leave it one while I fire the gun) because when detonation occurs there is equal pressure on all sides of the fan so there is basically no unbalanced force on it at all. Things break because of unbalanced forces. There is an unbalanced force on the pipe wall and the projectile, and on the screws and propane pipe if you want to get really technical, however those things can take the pressure 4 times over. The fan is not a barrier between the combustion pressure and atmospheric pressure so it experiences nothing.

Take a DVD and put it on a tile floor then run it over with a car. It will be unharmed because equal pressure is on both sides, the earth puts as much pressure on the bottom of the DVD as the car puts on the top of it and the force is balanced.

I believe this falls under Newtons 3rd law....?

Are you certain? It seems like it can't be equal on both sides, since it located at the rear of the chamber. It also seems it will be unequal when you have a large object blocking the exit out the barrel.Besides, it is plastic, and things get pretty hot inside. It does look to be mounted pretty solidly, though, so it may work. I dunno. I will go ahead and take your word for it on this one.

If you get the chance, try the oxygen mixing technique, and see how it works. You are able to use more propellant gas if you add oxygen, than if you try mix it with the fan, because you still only have so much "combustible" air in the chamber.

sounds like a way to scratch the hell out of the DVD though.


Unfortunately, Potato guns are illegal around here (subject to seizure if caught) so I'll never get the chance to legally make one.

I have used the exact same fan on every single one of my guns for hundreds and hundreds of shots, fan running during combustion, and it has kept on ticking.


Build a tank out of LEGO bricks. Fill it to 200 PSI. Will it hold? Dont think so.

Now put those same LEGO pieces completely INSIDE of a larger metal tank. Fill the tank to 200 PSI. The tank will hold the pressure, and the LEGO pieces will be unharmed because there is no pressure difference acting upon them. There is an equal 200 PSI acting on them from ALL directions (see Pascal's law of fluid mechanics) but because they are not a barrier between the 200 PSI tank and atmospheric pressure they dont even move. I could LITERALLY build an oragami figure out of aluminum foil and stick it inside my chamber during combustion and it will be structurely unchanged because of this. Its not a very complex concept, just think about it a little bit.


Directly injecting propane and oxygen would not solve the problem is the reason for me having a fan in there. Once the propane is injected regardless if you inject oxygen as well, it will balance itself out as far as pressure goes, however there will tend to be more concentrated in certain parts of the chamber.




A fan simply mixes the propane thuroughly with the air (or oxygen, whatever you are using as an oxidizer)



I however dont see the point of using oxygen because the formulas I am using are the optimal combusting-propane percanteges for AIR, not OXYGEN, and you would have to re-work those. Surely you would get more power from it, however it would be just as easy to create a hybrid cannon (described above) as it would be to build a direct propane/oxygen cannon, and you would get infinitely more power from that.

Any of you RIT goons want to back me up on this?

The point is that I can increase the amount of propellent used. I can inject oxygen, allowing me to also inject more propellant, where with another way, my mixture is limited to what air is already in the chamber. Also, I inject the mixture from the rear, and the ignition is toward the center, so the even if the mixture is too heavy or lean, the center is just right. And once a little bit of it ignites, it all ignites.

It works very, very well for me. I have never misfired, and get ignition every time. Each time we up the fuel content, it continues to work (we dare not go much higher than we do now, though, the glue might not hold)

I am thinking about building an adjustable bi-pod so we can aim a little better, but I really don't know how. Suggestions?

I was thinking about the heat melting the fan or weakening it due to thermal cyclic stresses.

That is impressive, Dash. Good description w/ pix.

hmm... ever thought of using a car coil and a spark plug? I mean, that's EXACTLY what it was designed for

I made a few potato guns a few years ago, the first version did fire the potato, but the potatoes exploded after like 10 metres... shouldn't compress 18bar of air into a pepsi bottle
the second version worked too, it fired the potatos, and did it damn well... a few times.. then the bottle (yes, i used a pepsi bottle for this one too) blew up. this one used butane for fuel, and coil+spark plug+car battery -combination.

The third one used gunpowder, but that's another story... as you may guess, the potato did NOT survive. RIP potato