In order to maximize the "up time" of one's pneumatic cannon, multi-shot capability is essential and must be incorporated into the design of the cannon from the cannon's inception - carrying around a bulky air compressor or a large secondary tank for an air supply is far too cumbersome for me.
When I started drawing up plans for a new pneumatic cannon, the limitations I described above really turned me off. I wanted to make a "do everything" small bore (1.5" to 0.5") cannon with good power, low maintenance, and high portability. On top of that, as a full-time student, I had a fixed budget and limited facilities.
Since I don't have an air compressor other than a bike pump, I researched possible sources of pressurized gas for such a cannon. A few stood out: high pressure air (HPA), CO2, or... well that was it. So I looked up the average prices of CO2 and HPA tanks. I ended up buying a 5lb CO2 tank with a dual gauge regulator for about $100 total (Amazon). To me this made the most sense. CO2 is easy and inexpensive to refill plus one can pack a bunch of it (since it's stored as a liquid) in a 5lb tank. HPA, while a good clean source, probably would have been harder to fill and would have yielded less medium power shots (for the same amount of money).
A 5lb CO2 tank and regulator is the "base" I used for this cannon. The other essential parts sort of "flowed" out of that. We'll get to the required parts in the next steps.
I recommend that you read all of the steps prior to building rather than jumping right in. A better understanding of what is happening will help when it comes time to build (I promise!).
Step 1: Safety First!
This project in particular requires extra concern since lethal muzzle energies and extremely high pressures can be produced.
- Use appropriate Personal Protective Equipment when working with tools, especially power tools.
- Only use pressure rated materials
- Do not, under any circumstances, exceed the pressure rating of the weakest component. The lowest pressure rating on any component in my cannon is 125psi.
- Take great care in following the proper procedures for installation and use, as provided by each part's manufacturer, for all of the materials used.
- Tighten/torque all connections to specifications. Use pipe sealant on all pipe connections.
Projectiles And Cannon Use:
- Wear impact glasses with use of any projectile shooting device, whether it be a firearm or a homemade cannon.
- I recommend wearing gloves. The parts can get very cold very fast.
- Treat the cannon like a loaded firearm at all times. Point the barrel in a safe direction. Be aware of your surroundings.
- Unless the muzzle energy of the cannon is so low that it can't possibly harm someone, don't even point the cannon at someone, much less shoot someone. (One of my intended uses is for Nerf, and in this case the calculated muzzle energy is less than half of those of paintball marker. Basically just be aware of how powerful your cannon is.)
What you do with your cannon is up to you. Just remember the limits of your design and the force that your cannon produces.
Use common sense
I am not responsible for any harm resulting from building or operating such a cannon. Build at your own risk.
Step 2: Theory - How Is Something Like This Going to Work?!
1. Get a "charge"
How can you get a uniform charge to shoot something? In our case, we have a large CO2 tank that produces pressures upwards of 800 psi. Yikes. We need to be able to break that down to about 20 to 100 psi. This is where a regulator comes into play. With a regulator, one can adjust the output pressure of gas from a source (the CO2 tank) to a manageable level. The one I got for this project has two pressure gauges - one to show the pressure in the tank and one to show the output pressure. I recommend this type since it gives the user a lot more precision with adjustments (plus it tells you when you're going to run out of gas). So now we have a steady supply of CO2. Now what?
2. Store that "charge"
With a consistent stream of propellant, we can trap some of that gas in another smaller chamber. We'll call this the secondary chamber. If we can direct the flow of the gas into such a chamber, we will have a set volume of CO2 at a set pressure - that of the regulator. It is this gas in the secondary chamber that is used to "shoot" the projectile. Therefore, we are not directly using gas from the large CO2 tank. It goes into a isolated (secondary) chamber so that we can shut off the flow of CO2 once the secondary chamber is filled. But still, we only have a smaller secondary chamber full of CO2. How does that propel an object?
3. The Main Valve
In order to be able to fill a secondary chamber and then release that pressure into a barrel on cue, we will need some special valves (Sorry, no ball valve cannons in this project). What we need is a valve that will direct a gas into a chamber/cylinder and then dump all of that gas into a different port when triggered. It just so happens that such a valve exists: a quick exhaust valve, or QEV. A QEV has three ports; an inlet port, a cylinder port, and an exhaust port. When a high pressure gas flows into the inlet port, a diaphragm inside the valve seals off the exhaust port. This allows all of the gas to flow through the cylinder port, or into the secondary chamber. Once the secondary chamber has reached its maximum pressure (as set by the CO2 regulator), the gas between the diaphragm and the inlet port can be vented to the atmosphere (when it's time to fire the cannon). Because of the pressure differences around the diaphragm, it will have a net force acting upon it and move in the direction of the venting gas. This exposes the exhaust port. Now, since the diaphragm is blocking the path to the atmosphere through the inlet port, the gas will rapidly "exhaust" through the exhaust port and the secondary chamber will empty. This happens in a fraction of a second.
4. The Fill Valve
So now with a QEV we can "fire" from what's in the secondary chamber. The only problem now is that we have to vent the inlet port of the QEV. We'll need another special valve for this. This valve needs to let gas pass through it into the QEV and then, after a trigger from an electrical current or push-button or something, it opens up a new pathway from the inlet port to the atmosphere. Luckily, such a thing also exists. This valve is called a directional control valve (DCV), or, in this specific instance, a 3-way 2-position directional control valve. This type of valve also has 3 ports. They are typically oriented with 1 port on one side and 2 on the other. The lone port on the first side will connect to the inlet of the QEV since it is this lone port on the DCV that will "connect" to either of the two other ports, depending on the position of the solenoid or valve internals (depending on your particular type). Think of a DCV as a momentary, normally closed single pole double throw electrical switch. Just replace the electricity with a flow of CO2. The normally closed position will allow for an automatic connection to the main CO2 tank for filling after each shot. Once the valve is energized or toggled, the secondary chamber will be effectively isolated from the main chamber. Toggling the DCV will also "trigger" the QEV, dumping all of the CO2 from the secondary chamber straight into the barrel. This is how you will fire the cannon. Pretty awesome, right?
For more information, click here.
Credits to http://www.valvehydraulic.com for the QEV diagram and http://www.pirate4x4.com for the DCV diagram.
Step 3: Get the Materials
On to the parts:
Tank and connections:
- Main CO2 tank. I used a 5lb CO2 tank from Catalina Cylinders.
- Dual gauge CO2 regulator with 3/8" barb fitting
- About 6' pressure rated hose - 3/8" ID
- (2) Hose clamp; 1/4" to 9/16"
- 3/8" hose to1/4" male NPT
- Hose quick connect set: 1/4" female NPT to quick connect fittings to 1/4" male NPT
- (2) 1/4" x 3/8" NPT bushing
- 3/8" NPT tee
- (2) 3/8" NPT close nipple
- 1/4" inlet pressure gauge; 0 - 200 PSI
- 3/8" x 3/4" NPT bushing
- 3/4" NPT QEV
- 3/8" NPT 3-way 2-position solenoid valve
- 3/4" NPT street elbow
- 3/4" NPT close nipple
- 3/4" x 2" NPT reducer coupling
- 2" x 10" NPT nipple
- 2" NPT cap
- 3/4" NPT close nipple
- 3/4" x 1 1/2" NPT bushing
- 1 1/2" female NPT to female cam lock hose fitting
- (however many you want, one per barrel) 1 1/2" male cam lock hose fitting to 1 1/2" male NPT
- Barrel material. I am using 1/2" PVC and 1.5" PVC
- 24V Battery
- Project box/enclosure
- SPDT 24VDC relay
- N.O. momentary pushbutton switch
- SPST ON-OFF toggle switch
- SPST ON-OFF key switch
- (3) 2.1mm DC CO-AX connector sets - panel mount (a set as in both the socket and male plug)
- Approx. 6ft split-loom tubing
- Wire/Connectors as needed
- Teflon tape (as needed)
- Electrical tape
- Pipe wrenches
- Wire strippers/crimp tool
- Soldering equipment
Some notes on material choice:
You may have noticed that I do not have any plastic PVC pipe on the pressurized side of the cannon. I chose to use all metal fittings for a few reasons:
First, PVC isn't really that safe with high pressure gasses. Yes, they may hold up under relatively high pressures, but if the PVC breaks for any reason, it will be dangerously explosive. Metal fittings have more of a tendency to develop "gashes" when fatigued in such a way. This is good for their ability to be replaced and for safety.
Secondly, when CO2 goes from a liquid to a gaseous state, it absorbs a lot of heat. Therefore, most of the parts on the pressurized side of the launcher may very well become extremely cold. PVC is really brittle with cold temperatures. Using PVC would greatly increase the risk of user injury. It'd be an accident just waiting to happen.
Third, metal pipe can withstand abuse, transport, and high pressure. It's just generally more stable. The only problem with metal is that it's a little more expensive and its heavy.
If you want to use PVC, go for it. I won't stop you. Just be aware of the associated risks.
Step 4: General Assembly
The chamber consists of the 2" cap, 2" x 10" nipple, 2" x 3/4" reducer, 3/4" close nipple, and 3/4" street elbow. This one's pretty easy... it basically has only one way of being assembled. Be sure to use Teflon or PTFE tape on all of the joints.
After you get the secondary chamber all tightened up, assemble the fill station and trigger system.
Start with the QEV. The QEV, if you're using a 3/4" one, should be labeled with IN, CYL, and EXH on the three ports. The port labeled IN is the one to which the trigger and fill will be attached. See the pictures for the layout that I used. If you don't like the layout, you are welcome to change it to whatever you need. As long as you have the fill BEHIND the DCV you should be fine (by behind, I mean that the fill is isolated from the QEV by the DCV).
For this part, I also used a 1/4" quick connect/disconnect for the gas connection. By using this, I'm able to disconnect (and seal) the gas supply from the launcher for transportation or for situations when one needs improved mobility.
It's also a good idea to put a pressure gauge somewhere in here as well. Without pressure gauges, the user would have absolutely no idea if the cannon is "armed" or not. I put the gauge on the fill side instead of the pilot side (the area between the diaphragm in the QEV and the DCV) since we want to keep the pilot volume as low as possible. This will greatly improve efficiency and trigger responsiveness. You won't need a gauge on the other side of the valve since the pressure through the QEV will be at or lower than the pressure on the fill side of the DCV.
With the fill and the secondary chamber built, all that is left is to thread the street elbow from the secondary chamber into the QEV. Note which port you thread the secondary chamber into - it should be the one marked cylinder. See the included picture for more.
I chose to orient the secondary chamber parallel to the fill portion of the cannon so that I can double the secondary chamber as a stock of sorts. The chamber can be oriented the other way as well. It's orientation won't affect anything.
Step 5: CO2 Tank and Supporting Devices
Putting together the "fill" isn't very hard. If your tank comes with a threaded valve already (most should), make sure you purchase the matching regulator. The cylinder/regulator combo that I used is commonly used for home brewing for pressurizing kegs. After filling the tank, just thread on the regulator, attach the hose, clamps, and quick connect and you're good to go.
You can get larger tanks filled (like these 5lb ones) at welding or medical supplies stores.
Step 6: Electronics
The DCV operates off of a 24VDC power source. For this project, I'm using a Black and Decker 24 volt battery that came out of a string trimmer. While testing out the DCV to make sure that it worked, I realized that the valve is normally closed - that is, it fills when energized and fires when in it's neutral state. This is the opposite of what I had expected, but no worries, a relay and a few switches can easily rectify the situation.
From the 24 volt trimmer battery, we need to run 2 wires to a console of sorts. For the console, I bought a black ABS plastic box (4.7" x 2.6" x 1.55") into which I can feed all of the wires and make the appearance of the launcher cleaner in general. Since we still want to keep the cannon as portable and functional as possible, I suggest using coax power plugs/sockets. This will let you plug or unplug whenever you want.
This may seem like a trivial observation, but I assure you that it is not. Since the finished product, even without electronics, will be a bit awkward, every little bit that will aid portability or functionality will greatly increase the fun factor of the launcher. After making several launchers of this style, I can guarantee you that this is the case.
You can see from pictures of the finished project that I have 3 coax jacks in the console enclosure. The wires from the battery are routed through the first coax junction. From there, the power supply is immediately interrupted by 2 switches: a key switch and a toggle switch. The key switch is for safety. I wanted to make sure that only I (or an authorized person) could operate the launcher. The toggle is a simple on-off for the rest of the circuit.
The logic for the circuit is simple: we need to get power to the DCV when the normally open trigger is closed.
When the trigger is closed, that current actuates a double throw relay (as the DCV is powered in the relay's neutral state), switching the DCV from on to off (not from off to on). Therefore, the power coming from the battery through the coax connection will go to both the actuation and actuated sides of the relay by means of parallelism. See my crudely drawn circuit diagram for the layout that I used.
The third coax connection is for future improvements. Since it is in parallel with the push button trigger, this can be used for remote firing or relocation of the trigger.
Also, with the extra space in the project box, one could easily add led indicator lights for on, charging, ready, and fire states.
To mount the project box to the cannon, I drilled four 1/2" holes in a rectangular into the back of the box that would allow hose clamps to go through and around a section of the box. See the picture for what the holes and spacing looked like. I free handed my holes since they didn't really need to be that exact. Just be sure that they form a rectangle when finished (or else the box will sit crooked when attached to the launcher). You need to make the holes far apart enough to fit the "natural" curvature of the hose clamps, but the clamps still need to be under a little bit of tension once in place. The hose clamps used are 2 1/2" clamps. They fit very nicely around the 2" nipple (secondary chamber) and this console.
Step 7: Barrels
Each barrel needs to have its own loading mechanism. Since I do not have adequate access to a shop, I have left the barrels with this cannon in their simplest form. With some snug fitting PVC pipe, one could easily create some cool breech loading or semi-auto uppers. It is for this reason that the cannon itself is not a true semi-auto cannon - it requires you to load it (unless you've deviated from this design). My original plans called for a blow-back bolt system for Nerf, but my plans had to be scaled back because of budgeting issues and lack of work space here at college.
With the cam lock feature, there is no real limit to how many barrels or what kind of barrels you need. All that is required is a 1 1/2" male cam-lock fitting to attach the barrel.
Step 8: Portability
To make things a little bit easier to transport, I recommend getting a duffel bag and a backpack. A long duffel bag is perfect for carrying the long, interchangeable barrels along with the main section of the cannon wherever you go. For a backpack, try to get one with a nice deep pocket for the CO2 tank. With this, you can then put the tank in the bag and carry that on your back during cannon use.
If you decide to go this route, you'll need some split-loom tubing. I put both the battery and the CO2 tank in the backpack and used 3/4" split-loom tubing to run both the air hose and the battery wires to the main launcher assembly. This helps prevent tangles and kinking (plus I think it gives the launcher a cleaner and more professional appearance). Just run the positive and negative leads from your battery through the tubing and connect a coax plug at the launcher end.
Step 9: Operation and Performance
On my launcher, the minimum operating pressure is about 20 PSI. Keep this in mind; if your supply of gas is less than your operating pressure, the gas can leak out. This would be really dangerous if you used a gas like CO2 in a closed space, not to mention frustrating as all of your compressed gas would be gone.
As far as performance goes, there are a lot of factors that come into play when dealing with distance, speed, etc. With the launcher itself, performance is really maximized by eliminating dead space - the space "in between" critical elements of the launcher. For example, the more space you have between your projectile and the main valve, the more your cannon's performance suffers. Moving the projectile as close as possible to the valve will result in greater power and acceleration. Since we used cam-lock fittings on this launcher, the performance will indeed suffer a little bit. However, I think the added functionality of the cam-lock fittings far outweighs the decrease in performance. Likewise, one needs to eliminate space between the main valve and the pilot valve. In this case, we did about the best we could, since the QEV and the DCV are back to back.
If you think you want more power, you could always increase your operating pressure or get a bigger secondary chamber. I would recommend this if you were planning on launching large objects (larger than golf balls) over far distances. Just remember that you won't get as many shots out of the same CO2 tank as a launcher with a smaller secondary chamber. My choice of 2"x10" was "middle of the road" in that I can shoot golf ball sized objects as well as smaller things without penalizing myself in CO2 usage.
Step 10: Fire Away!
Be creative with your cannon. You are more than welcome to use the simple design that I have made. However, I encourage you to branch out and try different things. There is a lot of flexibility in how you do the barrels. You could add a grip to the cam-lock, make a magazine for golf balls, make a blow-back system with a few more valves and a linear actuator.. there are so many things you could do. I myself am going to give crispyjones's net gun a shot. Just take his launcher assembly and slap on a cam-lock fitting.
Anyway, be safe, use common sense, and have fun.