Pneumatic Spud Gun / Potato Cannon 1.0

Have you gotten tired of using potatoes like everyone else does? Are you fed up with using conventional planting methods that require way too much time and effort? Don't you just hate it when you have a friend a quarter mile away who says they want a lime but the last time they asked for one that far away they changed their mind when you finally walked over??? If you said yes to all of those things, then you're just like me. And just like me, you need a spud gun in your life!

Seriousness aside, building a pneumatic potato launching device is a remarkably fun yet simple project that can be the source of many great times with friends and family. The pneumatic spud gun design brings in the blasting excitement of the classic, hair-spray-powered spud gun, while also providing much more reliable and consistent performance. It demonstrates how human energy can be transferred into compressing air using a bike pump, stored in the form of pressurized air, and release suddenly to propel a projectile at great speeds and distances that are seemingly superhuman. They can be used to launch a number of things other than potatoes, such as limes or clementines. If it fits, it ships!

So grab you purple primer, valve stems, and steal a few potatoes and limes from the kitchen, because it's time to make an EPIC Spud Gun!

Side note: V2.0 coming out soon!

Before you go out and buy a bunch of parts, it's always a good idea to sketch out a simple design that includes all the parts you'll need. You can get as sophisticated and make a design using a CAD program, or just have a rough sketch on a napkin, whatever works for you. There are literally endless ways to design your spud gun. So by all means, use this instructable as a suggested way of doing things and get creative!

Key Design Element: How ever you design your spud gun, ensure that the ratio of the volume of the barrel to the volume of the pressure chamber is at least 1:1. This is the general consensus on various forums, and worked well for my design. The ratio can always be bigger on the pressure chamber side, but the gained performance starts to drop off as the pressure chamber volume increases above 1:1, according to forums. And keep in mind, the larger the pressure chamber, the more time and energy it will take to pump up.

The picture shows the design drawing I made for my spud gun. It's a simple folded over U-shape with a pressure chamber, modified sprinkler valve, and an interchangeable barrel. There are some key differences from the design drawing and the actual spud gun that I decided to make while building it. They are as follows:

• The sprinkler valve is flipped around the line of the barrel, with the blow gun valve facing inwards, as opposed to upwards. This in the end made it much more natural to shoot, and kept the blow gun valve from getting hit and beat up as the spud gun was transported.
• The length of 2" PVC pipe directly behind the sprinkler valve, originally dimensioned to be 2 inches long, should be at least 6-1/2 inches long. It must be long enough to allow spacing for the new sprinkler valve orientation.

The heart of the pneumatic spud gun is the valve. Its performance greatly dictates the overall performance of the spud gun. The ideal valve would hold pressure, release it all in an instant, and create very little resistance to the flow of air after it is released. After searching the interwebs, the consensus is that a modified sprinkler valve works the best. But how does it work, you ask?

A sprinkler valve has several main parts: a spring, diaphragm, and solenoid valve. Within the sprinkler valve, flow enters and is diverted around the center outlet port. When not under pressure, the diaphragm is pushed down onto the center outlet port. There is a small hole in the diaphragm, which allows the pressure of the entering fluid to bleed into the upper region of the valve, above the diaphragm. When the entering fluid is pressurized from the inlet of the valve (usually water), the fluid bleeds through the small hole in the diaphragm and creates a pressure differential between the top of the diaphragm and the center outlet port, which is at outside ambient pressure, creating a tight seal that prevents any flow from escaping. Above the diaphragm is a small pressure relief port which is controlled from a solenoid. When 24 volts (depending on the model) is run through the solenoid, it opens, and the fluid above the diaphragm can depressurize. This removes the pressure differential between the diaphragm and the outlet port. The pressure of the incoming fluid can then overcome the force of the diaphragm spring, and push the diaphragm up and out of the way. With the diaphragm out of the way the fluid can then flow through the outlet port. If this is at all confusing, refer to the GIF I found animating the process.

The standard sprinkler valve design works well in applications where the valve-opening time isn't important, like water irrigation. But for the pneumatic spud gun, reducing the valve-opening time improves performance, so some modifications are needed. I have seen several builds where people leave the sprinkler valve the way it is with the solenoid. They use a series of batteries to produce 24 volts to trigger the solenoid and fire their spud gun. This setup works, but not as well as an improved sprinkler valve, one where the solenoid is replaced with a faster pressure-relieving valve. In this Instrucable, I'll detail how to replace the solenoid with a blow gun valve, drastically improving the time for the sprinkler valve to open.

The following is the parts list for my design:

• 1x 10 feet of 2" PVC
• 1x 10 feet of 1" PVC
• 2x 2" 90 degree elbows
• 1x 2" coupling (female to female)
• 2x 2" to 1" bushing
• 2x 1" threaded adapter with male threaded end
• 1x 2" cap
• 1x 1" inline sprinkler valve (I bought one made by Orbit)
• 1x 1/4" MIP Hex Brass Nipple (could be different size depending on the blow gun handle valve)
• 1x Valve stem with 1/2" rubber housing
• Plumbing tape rated for gas
• JB weld / epoxy
• Standard blow gun handle valve
• PVC Cement and Purple Primer

Everything was bought from home depot, except the valve stem, which I acquired from a Discount Tire by just asking if they had some (they gave me 3 for free when I asked for 2). Otherwise, you can find them at Walmart in the auto section or any auto store.

When you're buying PVC parts, it's very, VERY important that you get parts that have the code NSF-pw on them, or the box which they are sold in will say "Pressure" on it. This means that the parts are made to handle pressure and will work fine for this design intent. It's also critical that you use unexpired PVC cement and purple primer. If you don't double check these two things, you'll become famous in one of those "Ultimate Fail Compilation" videos on YouTube.

Here are the tools you'll need:

• Bare Minimum:
• Power drill + drill bits (1/4 & 1/2)
• Screw driver
• Hack saw
• Channel locking wrench
• Tape Measure
• Sharpie
• Preferred
• Drill press
• Miter saw
• 3D printer

Square cuts created from a miter saw will make it easier for everything to line up straight. However, sometimes you have to use what you have on hand. I made my spud gun with a hack saw and a hand drill, and it didn't turn out so bad! As you'll see later, I used a 3D printer to make the handle out of PETG filament. An alternative handle could be made using a small length of PVC or a wooden pole with through holes drilled on the ends and hose clamps attaching them to the barrel and pressure chamber, more on that later.

The first step to this build is modifying the sprinkler valve. If you're OK with a wimpy spud gun and don't want to modify the sprinkler valve and instead just trigger it with the solenoid it comes with, shame on you and just skip this step.

Important: I originally modified my sprinkler valve following a method which was feasible with older sprinkler valve designs. Newer designs have less plastic in the top lid where the blow gun valve is typically installed per the old modification methods. The thinner plastic doesn't hold in the threads as well as the old designs, and in my experience, caused the blow gun valve to pop out of the sprinkler valve after several shots. I eventually gave up on epoxying the blow gun valve back in, and instead rebuilt the valve according to this video.

The valve worked perfectly after modifying it in this way and has yet to quit after hundreds, maybe thousands, of shots. The last pictures show this final setup with the original hole epoxied close. I don't recommend modifying the sprinkler valve in the way which I originally did. However, I documented the process, and some of the steps are also included in the newer method shown in the video. But again, I highly recommend following the video.

----Old Valve Modification Steps----

1. Disassemble the sprinkler valve. The solenoid screws out, and removing the four screws allows the lid to pop off. The diaphragm can be pried off using a flat head screw driver. Be careful that the spring behind the diaphragm doesn't shoot out across your room.
2. Drill a 1/2 inch hole in the center of the sprinkler valve lid.
3. Apply 3-4 wraps of PTFE thread seal tape on the threads of the brass nipple and patiently screw into the blow gun valve. It's best to put the blow gun valve in a vice and screw in the nipple.
4. Forcibly screw on the blow gun valve-brass nipple assembly into the 1/2 inch hole drilled into the sprinkler valve lid. A NPS tap would be ideal to use to create the threads in the plastic, however I found that forcibly screwing the treads in and epoxying them later is sufficient. Note that I originally wrapped PTFE thread seal tape on the nipple threads that interface with the sprinkler valve. I later found that this wasn't necessary, and don't recommend doing it.
5. Epoxy everything! Note that I originally used JB Weld, but later found that regular 2-part epoxy worked better. Apply epoxy to the outside of the threads interfacing with the sprinkler valve. Fill up the solenoid ports with epoxy, and be careful that it doesn't seep into the internal diaphragm area.
6. Let the epoxy dry and reassemble the sprinkler valve.

Grab your tape measure, sharpie, and hack saw and go to an area that's OK to get a little messy. Follow the dimensions you decided on and don't forget to measure twice. It's also important to cut a length of 1 inch PVC that is slightly longer than the barrel. It will be used as the ram rod. A pole or stick also works. After cutting the PVC pieces, test fit everything before gluing. That way you don't permanently attach something together and then realize that it won't fit or line up correctly.

Also, take the 2 inch end cap and drill a 1/2 inch hole though the middle. Use some pliers and pull the valve stem through the hole until it pops into place. Drip some PVC cement around the valve stem inside of the 2 inch end cap where the rubber of the valve stem interfaces with the PVC. Be careful not to drip any cement into the valve stem. I'm sure there is some better adhesive to use between the rubber and PVC, but in my experience, the PVC cement works fine. Set aside to dry.

It is very important to glue everything somewhere that has constant airflow, and can get messy. If you're like me, you'll have a Pablo Picasso moment and accidentally spill the purple primer everywhere at some point. It's not a mess, it's modern art.

As I'm sure you've figured out by now, a potato isn't going to slide very easily into the 2 inch diameter barrel. To solve this issue, you must make a Potato Carver 9000! While it sounds amazing, it's just a 6 inch long piece of 2 inch PVC pipe that had one end "sharpened." I used a dremel with a sanding tip to whittle down the plastic. A belt or hand sander would be great to use instead. If you have to, sanding by hand can be a solution, or literally carving away the plastic with a knife.

It's important to widen the end of the of the ram rod so that it doesn't punch holes in the potato as it pushes it down the barrel. I 3D printed this widened end cap that fits on 1 inch PVC. Attached is the STL of this design. A regular PVC end cap should work fine as well.

You might not be able to handle this next part. But if you have a good grip on things, you'll be fine.

With this spud gun design, the barrel and pressure chamber are slightly floppy without some kind of linkage between them. Adding a linkage about halfway down the barrel towards the end of the pressure chamber increases stiffness and can be used as a handle, which greatly improves the ease of holding and shooting the spud gun.

There are several ways to make a handle for your spud gun. I utilized 3D printing to create several handles. Attached are the handle designs I made, including a sizing guide to see if it will fit. My handle is designed for a spacing of 5-3/4 inches between the pressure chamber and the barrel. My initial handle design utilized zip ties to connect everything. They provided limited support and were somewhat loose, no matter how hard I tightened them. My second design completely went around the pipes and was clamped down using some screws, nuts, and washers. It has worked very well as a sturdy handle. I was able to print the second handle design in one piece because I have a larger CR-10 clone 3D printer. Most Ender-3 sized printers won't be able to print something that large. The alternative is to break it up into several pieces and glue them together. Also, If you're considering 3D printing a handle, I recommend using PETG filament, as it will be more resilient to heat and will take a beating better than PLA, which just shatters.

If you don't have access to a 3D printer or CAD, I recommend cutting a length of 1 inch PVC pipe that is the length between the barrel and the pressure chamber, drilling some through-holes on both ends, and using hose clamps to secure it. The pictures depicting this method are from Spud Gun 2.0, Instructable coming soon!

Now that everything is assembled, it's important to wait for everything to cure before pressurizing. I recommend at least 24 hours, regardless of the adhesives you used. Once that passes, I recommend checking for leaks around the spud gun by pressurizing to a low pressure - 20 PSI should do. Listen for leaks and hissing around the pressure chamber and sprinkler valve. Spray soap and water on a potential leak you hear, and the escaping air will make obvious bubbles where it's coming from. Patch up any leaks with epoxy/ PVC cement and repeat the low pressure test after it's cured.

Once it's established that there are no obvious leaks, begin pressurized dry fire tests that increase the pressure by 20 PSI each time. Fire the spud gun by intentionally squeezing the blow gun valve. Slowly squeezing it will cause the sprinkler valve to open slowly. Listen for any more leaks that show up and keep some distance initially. I would recommend stopping at 120 PSI. The PVC should withstand pressure up to around 10 times what this spud gun design operates at. If you used proper PVC primer and cement and applied it correctly, the bonds should fail after the PVC does. A fun video testing the actual strength of PVC can be found here, made by TheBackyardScientist.

Once you've successfully pressure tested the spud gun, it's time to do some test shots! But first, let's talk safely. Essentially, you should treat this spud gun like it's a firearm and follow standard firearm safety. There are 4 main rules of firearm safety that in no way limit the amount of fun to be had with your spud gun, when followed.

1. Treat all guns as if they are loaded. This is easier to track with the spud gun, because you have to ram a potato down the barrel and pump it up each time to shoot it. There's always the chance if you're in a large group of friends that someone already loaded it without your knowing, and squeezing the blow gun valve would fire a potato into someone's unsuspecting stomach. Don't look down the barrel, but if for some reason you do need to look down the barrel to see that there isn't any obstruction, double check that the pressure chamber is depressurized by squeezing the blow gun valve and opening the valve stem with a pen or screwdriver.
2. Keep the gun pointed in the safest direction. This is a DIY project, and depending on your skill level or mistake-proneness, you might have done something wrong, and it could lead to the valve malfunctioning and opening when unintended. So when pressurizing, keep the barrel pointed in a safe direction, where if the spud gun were to go off, it wouldn't hurt anybody or anything.
3. Keep your finger off the trigger until you're ready to shoot. This is easy to follow, and very important because there is no safety mechanism. Additionally, shooting potatoes will cause a significant amount of recoil. If the spud gun is fired when not held firmly, it could fly out of your hands or knock you over if you're not bracing for the recoil in your stance.
4. Know the target, its surroundings, and beyond. If not followed, this can really get you into trouble. You will likely be surprised by how far your spud gun shoots, and you need to be ready for any distance, especially when first firing it. Additionally, potatoes spin and curve all over the place, and will likely not hit where you're aiming at. Go to a big open field where there is nothing that can break, or no one that can possibly be hit down range when shooting, and you know this for a fact.

1. Ensure that the pressure chamber is unpressurized.
2. Carve a potato using the Potato Carver 9000 and shove it down the barrel using the ram rod. DO NOT insert the potato while the pressure chamber is pressurized, this is like looking down the barrel of a loaded gun.
3. Point the spud gun in a safe direction and begin pressurizing the pressure chamber. A bike pump with a pressure gauge or an air compressor works well.
4. Once pressurized, pick up the spud gun, being careful to keep it pointed in a safe direction and your hand off the trigger until you are aimed and ready to fire.
5. Brace for some recoil and intentionally squeeze the blow gun valve to fire the potato.
6. Repeat and have fun!

If you get bored of potatoes, try using limes or clementines. When they are too small to make a good seal in the barrel, use a paper towel as a wadding.

As mentioned before, your imagination is the limiting factor to the craziness of your spud gun. There are all kinds of cool spud gun designs online where people have made quick-reloading designs, multiple barrels, all the works. Some ideas I've had that might get you inspired are:

• Cool paint scheme
• Muzzle break
• Iron sights/ scope mount/ laser mount
• Bolt action/ break action
• Paint ball gun pressure tank + regulator for quick repressurization
• Potato magazine
• Big barrel for bigger projectiles (remember how I said the barrel is interchangeable?)

I encourage you to go crazy with your design and make something awesome!