Incredibly Powerful Barrel-Sealing T-Shirt Cannon

As of publishing this Instructable, I am an incoming Freshman to the University of Washington majoring in Mechanical Engineering.

For the last six or so years, I've held an interest in PVC cannons. Not only are they just plain fun to use, but they're an accessible way of getting a hands-on approach to gaining an intuitive understanding of many big ideas in physics, like projectile motion, Newtonian mechanics, and aerodynamics.

In the past, I've made a handful of physical and digital designs for PVC cannons and other air-powered projectile launchers, but what I present here is my most recent design, and one I'm finally proud to share with others.

Because this is a long Instructable, I recommend setting aside 15-30 minutes if you want to thoroughly read through this article, and I estimate it would take you about 4-6 hours to build and tune this cannon (not including 3D printing time). It took me around 10 hours to finally land on this design in CAD, another 6 hours to build the physical cannon, and at least 25 hours to write this Instructable!

THROUGH READING THIS INSTRUCTABLE, YOU WILL FIND:

• Tools and supplies required to build this cannon from scratch
• The basic operating principles and physics of this cannon
• The design process behind this cannon - using Fusion 360 (of course!)
• How to prepare components for assembly
• How to assemble and operate the cannon
• My ideas for variations or future improvements

Lastly, I am experimenting with writing more advanced Intructables using the possibilities available from editing the direct webpage code, which allows me to embed media, control styles, and do other neat things. Please let me know if you think this benefits the quality of this presentation or if there are ways I could improve my work. Don't forget to favorite this Instructable, comment your thoughts and criticisms, and follow my account so I can work to write better Instructables in the future. And lastly, if you're a judge for the 2023 Make it Fly Student Design Contest for which this is entered, I appreciate the time you're spending to read through this. Thanks!

CONTEST-SPECIFIC DISCLAIMER:

Any device that utilizes a form of stored energy, whether in compressed air, propellants, fuels, batteries, mechanical springs, or chemicals, can be dangerous if used carelessly or in a reckless manner, regardless of how safe they are in normal use. This cannon falls under this warning, as it can potentially be dangerous if over-pressurized or used incorrectly. However, it is not dangerous when used correctly. The cannon is specifically designed to operate at as low of pressure as needed, and is meant for launching soft objects such as t-shirts for events with large crowds of people. Of course, follow my instructions at your own risk, and take safety precautions when possible.

PVC cannons, more commonly called "potato cannons" or "spud guns" for their popular use being to propel potatoes, have a rather long history through which many variants have been made. While they all function by launching a projectile, like a potato, t-shirt, ping pong ball, or soda can from a sudden release of pressure, designs widely vary in what provides that energy. Some use compressed air (like this Instructable), but others use combustible fuels, dry ice, or even the pressure differential of a vacuum. If you're interested in the history, uses, and other designs that have been made, I have linked the Wikipedia page for potato cannonshere. For the sake of your sanity, I'll be strictly covering the designs that use compressed air, as they're the safest, most accessible, and most popular form of potato cannon.

I designed this cannon using a mechanism formally named a piston valve for advantages it has over the standard design of using a ball or electric valve, such as something like this. I will explain later in this step the physics that make the piston valve ideal for this type of cannon, but because it is also inherently more complex, I've used Fusion 360's section view tool and an online photoshop program to make a step-by-step model of how the piston valve in this cannon operates:

Stage 1:

When the cannon is being charged, pressurized air from an air compressor or bicycle pump enters through the back of the cannon near the sprinkler valve, which cannot be seen on this model. The valve remains closed without an electrical signal, and the air is added through a tank valve, which prevents the air from escaping the system. As this high pressure air fills the available space behind the piston, it has nowhere to go, so it exerts an outwards force to the walls of the cannon. This force causes the piston, which is limiting the air from entering the rest of the cannon, to move forward.

Stage 2:

The piston is pushed forwards enough that it hits a rubber gasket (made from the pipe coupling) around the 1-1/2" pipe leading to the barrel. On the side of the piston is a small hole which allows air to slowly leak to the main chamber of the cannon which extends below the image. This air cannot escape through the barrel because of the piston forming a seal with the gasket.

Stage 3:

Eventually, enough air will make its way through the piston such that the entire cannon reaches an equilibrium as shown. The piston is still held firmly in place because the atmospheric pressure pushing down the barrel towards the piston is less than that of the high pressure air behind the piston pushing it towards the barrel.

Stage 4:

Upon pulling the trigger, current is sent to the solenoid sprinkler valve in the back of the cannon, which allows all of the pressure behind the piston to quickly escape. The volume of this air is minimized so that this can happen as quickly as possible without a significant amount of air from the main pressure chamber leaking through the hole in the piston. Without the pressure keeping the piston in place, it now is also propelled backwards.

Stage 5:

With nothing stopping the remaining air from escaping, everything in the main pressure chamber can freely travel through the pipe leading to the barrel at once. This creates a massive pressure wave, allowing all the energy stored in the compressed air to be put towards accelerating the projectile through the barrel.

You may be wondering why a piston valve is so much better than simply firing the cannon with an inline ball valve or electric sprinkler valve (which are other, more common options). The answer? Cross-sectional area:

To simplify a lot of complex multivariate physics, we can represent the air as an ideal, compressible, and unmoving gas, which follows this equation:

Pressure * Volume = Moles of gas * Gas constant (8.314 J / mol) * Temperature

OR

PV = nRT

If you've taken a high school physics or chemistry class, you may recognize this formula, but if you don't, it represents the mathematical relationship between the listed variables in an ideal gas; by defining some, you can derive others or compare their relationship, which is what we're doing here. Since, for the most part, the cannon generally operates under a fixed volume and a constant temperature, our two main variables here are pressure and moles of gas, with moles meaning the amount of matter that makes up the gas held in the pressure chamber. As we add air to the cannon, the number of moles of gas increases, and according to the equation, so does pressure.

A unit of pressure can have many different values, such as a PSI, atm, Pascal, or mm Hg, but they all represent the same physical meaning. Let's take the Pascal for example, as it's the SI (System International) unit. 1 Pascal is equivalent to 1 N/m^2, or one Newton, a unit of force, per square meter, a unit of area. PSI, or pounds per square inch, is a more obvious representation of this, though it uses imperial units (which are objectively worse). Given this meaning of pressure, we can represent it in an equation like this:

Pressure = Force / Area

OR

P = F/A

It can also be rearranged like this:

F = P * A

If we consider pressure to be constant, this equation tells us that as a given area increases, the force that area experiences is greater. If we then apply this to the cannon (and ignore the complexity of compressible fluid dynamics), we could interpret it to mean that a greater propulsive force can be gained by having a larger pipe, which has a greater cross-sectional area (given by the equation (Internal Diameter / 2)^2 * pi) for the compressed air to move through. Force is translated to acceleration given the projectiles mass (m) and Newton's second law, F = ma, and a greater acceleration through the barrel of the cannon will cause the exit velocity of the projectile to be greater.

Now that we've shown that a larger cross-sectional area leads to better airflow, and therefore better projectile motion, we can see why the piston value reigns supreme. If we look back at the different valve types, the ball valve raises the first issue. Even though ball valves with large diameters aren't hard to find, they have a fundamental issue. Because the valve must be manually twisted 90 degrees to be fully open, some air will inevitably pass through before the valve is fully opened, meaning the first bit of force that's applied to the projectile is much lower because the cross-sectional area is initially small. With how fast pressure can move compared to the speed of a human hand, the projectile will have inevitably moved up through the barrel before the bulk of the air passes through the valve, meaning there is less barrel length for the air to accelerate the projectile, and the exit velocity will be sub-optimal. The manual nature also poses another issue as well; the speed at which the ball valve can be opened is unreliable, and highly dependent on the strength of the person using the cannon which decreases its accessibility.

An electric solenoid valve addresses both of these issues, but also has its own problem. A 3/4" sprinkler valve, for example, most often does not have a constant cross-sectional area equal or greater than that of a 3/4" pipe. Often the 3/4" part of the valve is just an adapter for that size of threads, and in actuality, the diameter is reduced to something like 1/2" or 3/8". Another issue is that these valves do not scale much larger than a 1" opening, meaning a 2" or 3" barrel would still be limited to a significantly proportionally smaller airflow than what it could handle. While this valve does open much faster thanks to the solenoid, it's still held back by the cross-sectional area.

A piston valve addresses all of these issues by being fast-acting, having a wide cross-sectional area, and being able to be triggered electrically. In essence, a piston valve is an amplifier of another valve. It is triggered by a sudden drop in air pressure behind the piston caused by a small ball or electric valve, which is able to happen quickly because of the low volume of air held behind the piston compared to what's in the cannon's pressure chamber. The piston can also be wider than the diameter of the valve that triggers it, and in this case, allows air to flow freely through a 1-1/2" pipe to the chamber rather than the 3/4" that the electric valve I use to trigger it.

Background

The first air-powered PVC cannon I built closely followed this very well-made tutorial by NightHawkInLight on Instructables here. Like my cannon design, it operates using a piston valve, however unlike my design, it's coaxial. This means that the barrel runs through the inside of the pressure chamber. This is quite a brilliant design in its simplicity, but I found it difficult to maintain and the length and barrel diameter were fixed which prevented customization. The use of a disc-shaped piston also proved to be an issue. Because it had a wider diameter than it was deep, I had difficulty in ensuring it formed a good seal against the barrel and prevented air from slipping around the edges (It also had no O-ring to seal the piston against the inside of the pipe). I eventually got some success from the design, so when I approached the task of designing this cannon I took inspiration from an improved piston valve again made by NightHawkInLight hereon YouTube.

You can also view my design here to reference as I describe the design process, and you can also download the entire assembly file at the end of this step if you want to import and modify it yourself.

After first visiting my local hardware store to learn what fittings it had in stock, my second step was to sketch out a design on some regular graph paper. I've found it helps me get a better grasp of my designs before I invest the time to CAD them. Dimensions and relative sizes can be approximate, as this doesn't have to be perfect, and I almost always make significant design changes between sketching them out and finishing the CAD model.

Once I had a general idea of the layout, most of the CAD process in Fusion 360 was fairly simple. McMaster-Carr, an industrial distribution company, proved vital in modelling the core design of this cannon. Their website, found here, allows users to access a catalogue of tens of thousands of different components in various materials, most of which also have free download links of 3D models in a variety of file formats. I was able to search through their PVC section, and recalling what fittings were in stock at the hardware store, imported the matching fitting models into my Fusion 360 document. I also used GrabCAD, another website, to get some more difficult-to-find models, like the microswitch for the trigger. I also created a cylindrical model of a PVC pipe with derivable configurations that could control the length and standardized diameters of pipe.

From there I was able to play around with small variations of the core design, and supplemented the downloaded parts with some more custom ones made in Fusion 360, such as a configurable O-ring for the piston and structural components to allow for the external frame. The back grip and trigger mechanism proved to be somewhat challenging, as I had to design a grip from scratch, which needs to be fairly organic to be able to be held comfortably. In addition, I implemented a compliant spring which could be 3D printed for the trigger, and optimized both sides to print entirely supportless. With Fusion 360's render tool, I was also able to create a few nice renders for use in this Instructable.

While the core functionality of this cannon doesn't rely on 3D printing, I've utilized it for the outer structure and grip/trigger mechanisms. All of these components are printed on 1.2mm walls, 30% gyroid infill, and 0.2mm layer height with a standard 0.4mm nozzle. Supports are not absolutely necessary but may reduce required post-processing on the Lower Structural Brace and the Upper Structural Brace. Despite the use of machine screws in the assembly of the trigger mechanism, I opted to have the screws directly thread into the printed plastic rather than using any heat-inserted nuts or nuts inserted mid-print. This slightly impacts the strength of their connections, but simplifies assembly and saves cost.

.step files are also available for download at the end of this step if you feel the need to modify these components to fit your project better. This type of file supports smooth geometry, and can be opened and modified easier than .stl files in parametric CAD software like Fusion 360, Autodesk Inventor, or Solidworks, whereas rougher mesh-based .stl files can be edited in programs like Blender.

The main structural parts are printed in black PLA:

Upper Structural Brace.stl

DownloadView in 3D

Lower Structural Brace.stl

DownloadView in 3D

Trigger Housing Part A.stl

DownloadView in 3D

Trigger Housing Part B.stl

DownloadView in 3D

Battery Mount.stl

DownloadView in 3D

The Front Grip is printed using black TPU, but this is for the marginally increased grip and flexibility it gives over PLA.

Front Grip.stl

DownloadView in 3D

To stand out from the other black components, the Trigger and Trigger Spring are printed using red PLA.

Trigger Spring.stl

DownloadView in 3D

Trigger.stl

DownloadView in 3D

Because I designed this cannon entirely through CAD rather than a sketch, I had the luxury of being able to measure and pre-cut all the sections of PVC pipe before assembly knowing they would fit together as planned. Here is a table of all the required quantities and lengths so you can do this as well. Note that to allow for tolerances when finally assembling, all measurements can be cut about 1/8" shorter than described unless otherwise specified.

Dimensions	Quantity	Image
3" X 24"	1
3" X 18"	1
2" x 4-1/2" Keep Precise	1
2" x 3"	4
1-1/2" X 7" Keep Precise	1
1-1/2" X 2-5/8"	1
1-1/4" X 2-1/2" Keep Precise	1
1/2" X 6-1/2"	2
1/2" X 5"	3
1/2" X 3-1/4"	2
1/2" X 3"	3
1/2" X 2-3/4"	2
1/2" X 1-1/2"	2

I cut all these pieces out, other than the 3" pipe, on a vertical bandsaw and cleaned the edges with a craft knife and sandpaper. These can all be cut fairly easily using a normal handsaw and something to secure the pipe, such as a vise or set of table clamps. A bandsaw allows for precise cuts, but is not as accessible as a regular saw.

Two fittings must be modified so that the piston valve can be constructed; the rubber part of the 1-1/2" Pipe Coupling must be removed and cut so that it can form a gasket between the piston and the barrel, and the 2" x 1-1/2" Reducer needs the inner flange removed. The 3/4" MPT x 3" PVC Riser also must be modified to fit the 1/8" NPT Tank Valve

To prepare the gasket for the piston valve, remove the rubber fitting from the metal sheath of the 1-1/2" Pipe Coupling, and using a knife or pair of scissors, cut one side down to about 1/8" away from the center flange as shown in the images. Begin further away, but cut incrementally closer to 1/8" ensuring that the 1-1/4" End Cap used later for the piston will fit snugly. (In the images, I went to only 1/4", which proved to be an issue as it "hugged" the side of the piston too well and make it difficult to fire)

To remove the inner flange of the 2" x 1-1/2" Reducer, I used a Dremel handheld tool with a sanding attachment. All that needs to be done is to remove enough material that a 1-1/2" PVC pipe could slide through the entire fitting. The sanding doesn't need to be particularly clean or nice-looking, and mine certainly wasn't. You could also use a drill press with a sanding bit like what NightHawkInLight did in his build tutorial of this type of piston valve I linked in step 3, or use a hand file.

To prepare the 3/4" MPT x 3" PVC Riser, drill a 3/8" hole in the middle of the side of the pipe on one side. Ensure that the 1/8" NPT Tank Valve can be threaded into the hole, and then apply a small amount of PVC cement around the tank valve and screw it in place. PTFE thread tape can be wrapped around both ends of the 3" Riser so that it'll later be able to form a seal when installed.

Make sure before assembly that you wash all your fittings and pipe segments with water and optionally soap. Ensuring a clean fit is of upmost importance to make sure the cannon is airtight where it needs to be and has as low a risk of failure as possible.

Optionally, and if you're seeking a cleaner-looking end product, you can use a solvent like acetone (commonly sold as nail polish remover) or medium-grit sandpaper to remove the ink on the PVC fittings and pipe segments. some fittings also have defects from their injection-molding manufacturing process which can be sanded down and smoothed. This serves no functional purpose, but improves the visual quality of the finished cannon. I opted not to do this so I could display the lengths on each segment of pipe through assembly.

At this point, you should have printed all the required parts for the cannon, so the trigger mechanism can be assembled. This is done before building the rest of the cannon so it can be easily installed as a single component at the end.

Step 1:

Prepare the Trigger Housing Part B, Trigger, Trigger Spring, and the M5-16mm Screw.

Step 2:

Screw the Trigger into the Trigger Housing Part B with the Socket Head M5-16mm Screw. Ensure the screw is loose enough that the Trigger is able to pivot freely. Press-fit the Trigger Spring in the housing so that it applies force on the Trigger. It should now take some effort to pull the trigger, and upon releasing, the trigger should return to its original position

Step 3:

Take a length of double stranded wire and strip both of the wires on one end. Twist these around the contacts of the microswitch so that when pressed, the switch will allow a current to pass between the two wires. Though I didn't you could optionally solder these connections to make them stronger. The microswitch can be screwed into the Trigger Housing Part B using two Socket Head M3-16mm screws so that the button of the switch lines up with the bump protruding from the back of the Trigger. The length of wire can be fed through the path down and out the back of the trigger housing. It can optionally be secured with a small amount of hot glue.

Step 4:

Trigger Housing Part A can be fit on top of Trigger Housing Part B, ensuring the heads of the socket head screws fit into the other half of the housing and that both parts sit flush with no gap in between them. The entire assembly can be flipped over, and four Socket Head M3-16mm screws can be used to hold both housing sides together.

Keep in mind that because the Trigger Spring is printed in PLA, it will wear down with excessive pulling of the trigger. This might be solved by using a stronger material like PETG or ABS, or it may be worth entirely redesigning the trigger to use the steel spring from a pen instead. For now, I would recommend not permanently fusing the two sides of the trigger housing so that this spring can be replaced with a new printed version or a different design in the future.

Assembling the piston is fairly simple, but is vital to the functionality of the cannon. Because of this, a dry assembly (before adding glue) is important to do. Assuming you have the right O-ring, this process should be fairly easy, but if not, will require a bit of trial-and-error.

Begin by fitting the 1-1/4" x 2-1/2" pipe segment into the 1-1/4" Straight Coupling. Slide the 326 O-ring onto the pipe followed by the 1-1/4" End Cap. Press the entire assembly together so that the O-ring cannot move.

You can now test this piston by sliding it into one of the pre-cut segments of 2" pipe. It should have a fair amount of friction between the O-ring and the walls of the 2" pipe, but after lubricating it with vegetable oil when it gets installed, it should move much easier. Take the piston in the pipe to the sink and fill one side with water. If nothing, or very little, leaks through, the piston can be glued together. If there is significant leaking, then it means the gape between the O-ring and 2" pipe is too small and a spacer must be made between the 1-1/4" pipe segment in the piston and the inside of the O-ring. If the gap is small, you can simply use a little PTFE thread tape under the O-ring, but if the gap is significant, you may need something more substantial, like electrical tape or even a 3D printed inner spacer ring made from the same TPU filament used for the Front Grip.

When you are ready for the final piston assembly, Follow the same steps outlined in the dry assembly, but with primer and cement. If you want to follow along visually, here's the link to the video NightHawkInLight made for this exact piston design (I had nothing to improve on this design, so I followed it myself). The last step (not shown in the video) is to drill a hole in the side of the 1-1/4" End Cap with the 1/16" or 1mm drill bit which will allow air to travel from behind the piston to the pressure chamber at a slow rate, like I explained in step 2.

Like with the piston, doing a dry assembly before gluing anything together helps prevent mistakes in assembly and ensures all your parts fit together as planned, especially since you may not be following my exact design due to local fitting availability or personal preference. I didn't fully press everything together so that I could have an easier time disassembling the cannon for gluing, but this did cause some things to not align perfectly, which is expected

I've found that the cannon can be assembled in three overall subassemblies (excluding the trigger, piston, and solenoid mechanism:

• Core (contains the piston valve and connects to every other part of the cannon)
• Barrel and Pressure Chamber
• External Structure (I broke this into smaller parts so it could be assembled separately from the core

Assembling the Core:

1. Starting with a 2" Tee connector with the center hole facing down, insert two 2" x 3" pipe segments into the center and right connectors. Insert the 2" x 4-1/2" pipe segment on the left hole.
2. On the 2" x 4-1/2" segment of pipe, slide on the 3D printed Upper Structural Brace and then connect one of the 2" Female Threaded Adapters. Screw the complimenting 2" Male Threaded Adapter in and fit a 2" x 3/4" Male to Female Threaded Reducer in the back. To this, the modified 3/4" MPT x 3" PVC Riser with the glued-in 1/8" NPT Tank Valve can be added.
3. On the 2" x 3" segment of pipe on the right side of the 2" Tee Connector, the 2" x 1-1/2" Reducer can be added. Into this, the 1-1/2" x 7" segment of pipe can be added such that it extends out from the right of the reducer by only about 1-1/4" and should extend about halfway through the 2" Tee Connector once everything is pressed together. On the right end of this segment, add a 1-1/2" Male Threaded Adapter. This will later connect to the barrel.
4. On the bottom 2" x 3" pipe segment, connect a 2" Tee Connector by the center hole such that it's oriented opposite that of the top 2" Tee Connector. On the left side of this, add the second 2" x 3/4" Male to Female Threaded Reducer and the 3/4" Male Threaded Plug. On the right side of the 2" Tee Connector, add the third 2" x 3" pipe segment, slide the 3D printed Lower Structural Brace onto the Tee Fitting, and fit the second 2" Female Threaded Adapter onto the 2" x 3" pipe segment. This will later connect to the pressure chamber.

Assembling the Barrel and Pressure Chamber

1. Starting with the barrel, fit the 1-1/2" x 2-5/8" segment of pipe into the 1-1/2" Male Threaded Adapter.
2. On the other side of the pipe segment, add the 3" x 1-1/2" Reducer and the 3" x 24" pipe segment as the barrel
3. To make the pressure chamber, the process is very similar. Fit the final 2" x 3" segment into the second 2" Male Threaded Adapter and then add the 3" x 2" Reducer to the other side.
4. Lastly, add the 3" x 18" pipe segment and the 3" End Cap to close off the end of the chamber.

Assembling the External Structure

As a preface, this should be assembled in 5 pieces before being assembled onto the core. This is to help with final assembly and so that the external structure can properly slide into the 3D printed structural braces on the core.

1. For the first part, fit two 1/2" 90 Degree Elbows on either side of a 1/2" x 3" segment of pipe so that the other end of each elbow is facing the same direction. To each of these, add a 1/2" x 3-1/4" segment of pipe to create a 'U' shape.
2. For the second part, again fit two 1/2" 90 Degree Elbows on either side of a 1/2" x 3" segment of pipe so that the other end of each elbow is facing the same direction, however the trigger mechanism can be slid in between the elbows and rotated such that the open end of each elbow is pointed about 45 degrees below the flat, front face of the trigger. To each of these, add a 1/2" x 6-1/2" segment of pipe.
3. For the third part, fit two 1/2" 90 Degree Elbows on either side of a 1/2" x 5" segment of pipe so that the other end of each elbow is facing the same direction, however the 3D printed Front Grip should be slid on the pipe before both elbows are added.
4. For the fourth and fifth part, they can be assembled the same, except one must be a mirror image of the other. To start, a 1/2" Tee Connector can have a 1/2" x 1-1/2" segment fit into the right side of the connector and a 1/2" x 2-3/4" segment fit into the left side.
5. A 1/2" 45 Degree Elbow can be added to both segments of pipe, and should face in the opposite direction of the 1/2" Tee Connector. One of these elbows, the one connected to the 1/2" x 1-1/2" segment, can be angled slightly outwards, and a 1/2" x 5" pipe segment can be added to the end

Assembling the Sub-Assemblies

1. Slide the first part of the external structure upwards through the Lower Structural Brace on the core sub-assembly. the fourth and fifth parts of the external structure can be fitted on top of this to hold everything in place, ensuring the 1/2" x 5" segments of pipe are pointing towards the front of the core and angled outwards. ensure these are parallel with each other and the core.
2. Slide the second part of the external structure through the Upper Structural Brace so that the flat face of the trigger mechanism lines up with the screw holes in the Upper structural Brace, and so that the second part fits into the fourth and fifth parts.
3. Fit the third part of the external structure onto the front of the forth and fifth parts. The barrel can then be screwed into the upper 1-1/2" Male Threaded Adapter, and the pressure chamber can be screwed into the lower 2" Female Threaded Adapter.

Note that the order of final assembly is vital to follow, as in some cases the specific steps cannot be completed out of order. Also ensure you are working in a very well-ventilated area or outdoors when working with PVC primer and cement, as they release fumes that are both bad for your health and highly flammable.

How to properly glue PVC:

If you've made it this far and are still considering building this cannon, you probably already know how to work with PVC. However, a leaky connection is the easiest way ruin your project, so I'll provide a brief run-through here:

To properly glue PVC together so the connection will be airtight and safe for pressure, first ensure that both the pipe and fitting are clean. Then, using PVC primer, brush around the both the pipe and fitting the entire length that will be connected in a full circle approximately 30 times. Using the PVC cement, apply a full coat to the outside of the pipe and inside of the fitting, and twist the two as you press them together to evenly distribute the cement. Hold the two together for 15 seconds to a minute, as sometimes the pipe will slide out of the fitting if they aren't held together. Lastly, excess glue can be wiped away from the connection point using a paper towel

Assembly Steps:

1. Begin by assembling the barrel: 1-1/2" Threaded Male Adapter into the 1-1/2" x 2-5/8" pipe segment into the 3" x 1-1/2" Reducer into the 3" x 24" pipe segment. Order doesn't matter, and these are good joints to test on, as they aren't under constant pressure when the cannon is in use.
2. The pressure chamber can be assembled next: 2" Threaded Female Adapter into the 2" x 3" pipe segment into the 3" x 2" Reducer into the 3" x 18" pipe segment into the 3" End Cap. Order doesn't matter on this part either.
3. You can start assembling the core now, beginning with the lower 2" Tee Connector, which can have a 2" x 3" pipe segment glued in on the top and right joint, and the 2" x 3/4" Male to Female Threaded Reducer on the left joint.
4. The Lower Structural Brace can be added over the right side of the 2" Tee Connector, and a 2" Female Threaded Adapter can also be glued in the right side.
5. Moving elsewhere, the 1-1/2" x 7" pipe segment can be glued into the 1-1/2" Male Threaded Adapter.
6. Critical Step: Apply primer to both sides of the modified 2" x 1/2" Reducer, and to the inch of exposed pipe of the 1-1/2" x 7" segment nearest to the 1-1/2" Male Threaded Adapter. Allow both to dry, and slide the primed 2" x 1-1/2" Reducer up the pipe segment so the 2" side of the reducer is pointed towards the exposed end of the pipe. Then, apply a generous amount of cement to the primed section of the pipe, and slide the reducer up to the male threaded adapter while twisting, to ensure the glue spreads evenly.
7. Apply primer and glue to one side of a 2" x 3" pipe segment, and connect it (while twisting, of course) to the 2" end of the 2" x 1-1/2" Reducer.
8. Lightly sand from the end of the 1-1/2" x 7" pipe segment about an inch inwards to scratch the surface. Using super glue, glue the modified 1-1/2" Rubber Pipe Coupling with the trimmed end outwards and allow it to fully dry
9. Now with the second 2" Tee Connector, glue the 2" x 4-1/2" pipe segment into one of the sides, slide on the Upper Structural Brace, and secure it in with a 2" Threaded Female Adapter.
10. Critical Step: Apply primer to the other side of the 2" Tee Connector and two inches inwards from the end of the 2" x 3" pipe segment on the sub-assembly with the rubber coupling. Apply glue only to the 2" x 3" pipe segment (to prevent any getting on the rubber coupling) and join the two. CHECK! make sure that the edge of the rubber coupling roughly lines up with the center of the 2" Tee Connector. If it does, you are following the steps correctly.
11. To build the firing mechanism, join a 2" x 3/4" Male to Female Threaded Reducer to a 2" Male Threaded Adapter. Then, ensuring the threads are covered in pipe tape, screw in the modified 3/4" MPT x 3" Riser with the Tank Valve installed followed by the 24V Electric Sprinkler Valve. After applying pipe tape to the 2" Male Threaded Adapter, you can temporarily install it onto the corresponding adapter on the core.
12. Before joining the two halves of the core, the external structure can be started on the lower half of the core. Glue together two 1/2" 90 Degree Elbows on either side of a 1/2" x 3" segment of pipe so that the other end of each elbow is facing the same direction. To each of these, add a 1/2" x 3-1/4" segment of pipe to create a 'U' shape and slide it through the Lower Structural Brace from the bottom. To the exposed ends of 1/2" pipe, add two 1/2" Tee Connectors pointed parallel to the 2" Tee Connector.
13. To each 1/2" Tee Connector, add a 1/2" x 1-1/2" segment of pipe on the front end of the tee, and a 1/2" x 2-3/4" segment of pipe on the back end.
14. Dry-fit the two halves of the core together (no glue!) and add a 1/2" 45 Degree Elbow to both 1/2" x 2-3/4" segments of pipe so that the exposed end of the elbow is pointed towards the holes in the Upper Structural Brace. While the cement is still wet in the joint between the elbows and pipes, align the elbows using both 1/2" x 6-1/2" segments of pipe fed through the angled holes of the Upper Structural Brace and into the exposed ends of the elbows. Allow the cement to dry, and at this point, you can also fix the Lower Structural Brace in place with hot glue, tape, super glue, or some other adhesive.
15. While both halves of the core are separate, test-fit the Piston. Remove the firing assembly (with the tank valve and sprinkler valve), apply some lubricant (I recommend vegetable oil) to the walls of the 2" x 4-1/2" segment of pipe and the O-ring of the Piston, and slide it in with the end cap facing forwards. Confirm that it fits up against the modified rubber coupling. At this point, I recommend re-installing the firing assembly, taping over the hole of the piston, and attempting to pressurize the system with a bicycle pump. Confirm there to be no significant leaks (very small leaks around the O-ring are undesirable, but acceptable).
16. Critical Step: Remove the firing assembly and piston, and glue together both halves of the core, ensuring they are pointed in the right direction and are perfectly parallel with each other. I used the seams left over from the injection molding process on the sides of the 2" Tee Connectors to align them precisely.
17. Glue in both 1/2" x 6-1/2" segments of pipe to the exposed ends of the 1/2" 45 Degree Elbows, Add the 3/4" Male Threaded Plug (with PTFE pipe tape), and test-fit the barrel and pressure chamber. The pressurized part of the air cannon is finished, and the rest is assembling the external structure and electronics.
18. Assemble the front of the structure by gluing two 1/2" 90 Degree Elbows on either side of a 1/2" x 5" segment of pipe with the 3D printed Front Grip installed so that the other end of each elbow is facing the same direction. To each elbow, add a 1/2" x 3" segment of pipe.
19. To ensure the front grip is symmetrical when glued in, dry fit together the following: two 1/2" 45 Degree Elbows can be added to the front of the 1/2" x 1-1/2" pipe segments facing upwards and angled slightly outwards. To each of those, add a 1/2" x 5" segment of pipe, then another 1/2" 45 Degree Elbow facing upwards again. Fit the front grip subassembly to this, and shift things around until everything is symmetrical like the picture.
20. Now that all the 1/2" 45 Degree Elbows are oriented correctly, remove the front grip subassembly and glue it back in to the top set of elbows. Remove the whole assembly from where the bottom 1/2" 45 Degree Elbows connect to the 1/2" x 1-1/2" pipe segments, and reattach that joint with glue. Lastly, glue in the 1/2" x 5" pipe segments on both sides.
21. Fit two 1/2" 90 Degree Elbows on either side of a 1/2" x 3" segment of pipe after sliding in the Trigger Mechanism in the middle, with the other end of each elbow is facing the same direction. Glue this to the rest of the external structure and use four M3x20mm Socket Head Cap Screws to screw the Trigger Mechanism into the Upper Structural Brace.
22. At this point, the Piston can be re-installed as well as anything else, like the Barrel, Pressure Chamber, and Firing Mechanism. The Battery Mount can be snapped in over the back 2" x Female Threaded Adapter and a 9V Battery can be inserted. Wire the system together so that pulling the trigger opens the sprinkler valve, and the assembly is finished!

As a safety precaution, use safety glasses when operating the cannon. Though this cannon isn't very loud, hearing protection may also be applicable.

Before using the cannon, be sure that all threaded fittings are screwed in securely and are using PTFE thread tape. Prior to pressurizing the cannon, load your T-shirt (or other projectile) into the barrel, and use a long piece of pipe or other thin rod to shove the projectile to the back of the barrel. You'll also want to ensure that the piston has been lubricated with vegetable oil and can slide easily. You can then begin pressurizing the cannon by connecting a bicycle pump or air compressor to the tank valve, and adding air.

You should hear a "thunk" sound of the piston moving forward and sealing against the barrel, and only a small hiss of the air filling the pressure chamber through the hole in the piston. If this doesn't happen, the piston may already be against the barrel, air leaks too easily around the O-ring, you haven't properly lubricated the piston, or you just simply aren't adding air fast enough for there to be enough force to push the piston forward. You should also not hear any air exiting the barrel. If this is the case, the piston is not properly sealing against the barrel, meaning there may be debris which is preventing sealing, or some other issue.

After pressurizing the cannon (I recommend 20-30 PSI for the first shot, and increasing afterwards if needed) and ensuring it holds air, it's ready to fire. Connect the battery if it's not already, point it upwards and away from any people or valuable property, and fire it. If it doesn't fire, the piston may be stuck to the barrel, in which case either fire with more pressure (which will give the projectile more power), or depressurize and apply a small amount of oil to the gasket. While oil can damage rubber, vegetable oil seems to not be prone to this issue. I had some trouble operating the electric sprinkler valve using only one 9V battery, so if nothing happens when pulling the trigger, you may also need to add another 9V.

As you can see, this cannon is incredibly powerful. One of my first test fires, which you can see in the GIF above, launched a T-shirt over 150 feet at only 60 PSI! Most commercial cannons use 60-120 PSI and are lucky if they get half the range this cannon has!

Because this cannon is designed with modularity in mind, I have a few ideas for design variations:

• Modified barrel length: Shorter or longer barrels can change the projectile's behavior and are suited for better purposes. Longer barrels increase accuracy and improve projectile exit velocity, but are heavier and larger. A short barrel would be better for a T-shirt cannon that doesn't require a long range.
• Modified barrel diameter: Different diameters suit different projectiles. 3" barrels are great for T-shirts, 2" barrels are great for potatoes, 1-1/2" barrels are good for ping pong balls, and 4" barrels could make a perfect toilet paper bazooka for pranks.
• Modified pressure chambers: just like barrels, different pressure chambers suit different applications. A bigger pressure chamber would be good for large barrel diameters or less aerodynamic projectiles, while smaller diameters are good for smaller projectiles or lower power.
• Built-in compressor: small air compressors are fairly cheap, and I've always wanted to fit one into an air cannon so that it could be portable and battery-powered. This would take considerable design effort but should be possible.
• Better air inlet: pressurizing the pressure chamber only through the small hole in the piston is slow. using some extra one-way valves and a bit of tubing, the 3/4" Male Threaded Plug could be replaced with an extra air inlet that, as long as the barrel is sealed, would allow faster pressurization of the cannon.

Most importantly, create your own modifications! There is a lot of fun to be had with these cannons that I haven't covered, and I highly recommend you to explore other ideas or create your own!

If you made it through this (admittedly long) Instructable, congratulations! Whether or not you follow my tutorial and build your own cannon is up to you, but I would like to thank you for reading. I hope you've been able to learn something at the very least, whether it's how to properly glue together PVC or find 3D part files for your own projects on the internet.