Bigger is cool looking... But not always better. I have found smaller more robust designs to be far more fun that the gigantic heavy cannons. I can recharge (with a foot pump) and reload in less than 30 seconds and hold this thing much easier than its heavy breathing counterparts. Other than this design, I no longer build using PVC as it does fail catastrophically.
I made this particular cannon to be used for physic's demos for middle and high school physics students. 2l bottle fail at surprisingly high pressures (relatively) and have served as a reservoir very well.
Step 1: Parts List
1 2l bottle of soda - minus the soda and washed out
1 .75" sprinkler valve with manual switch
1 45 elbow
2 female/male 90 elbow
1 female/threaded adapter
1 end cap
1 .5" thread-> .25" threaded bushing (mine is galvanized)
3 1.5" long pieces of pipe
2 2-2.5" long pieces of pipe
2 .5" female - .75" threaded adapter
1 4 wayfitting
1 Tire Stem with schrader valve (walmart or autoparts store) - home depot did not have this item
1 male threaded cap OR 1 gauge (might be listed as manometer)
Optional Add on (for bypass and a later project)
3 more 1.5" tubes
1 ball valve
1 45 elbow
1 3 way Tee
Step 2: Prepping components
While the paint is drying (if you choose to paint), now is a good time to cut your tubes if you have not done so already. PVC saws (looks like a metal string) are cheap and effective. I personally use a band saw to zip though everything. Guitar strings also work, just be very careful - it will eventually snap and the filament will be extremely hot (I've had burns from this). Guitar strings also have a tendancy to wrap themselves around when they snap.
Now is also a good time to sand off any burrs and lightly sand the mating edges of the fittings and tubes.
Step 3: Make a Filler Valve Fitting
Tire stems are a press (or pull :P) fit. So make the hole as small as you possibly can. .5" caps are a tight fit, but it fits nontheless.
When you think you're ready -- push the stem through the back of the cap. Then pull it through as hard as you can. Pull one way and then another direction until you notice an edge pop out and set itself on the outside of the cap.
Step 4: Fitting it all together
The longer lengths of tube are for the reservoir (2l bottle) and for the filler stem post. Ideally, everything should be in the same plane except for the gauge post. Therefore, I recommend you make attaching the reservoir the last thing you cement together. I find an angle of about 45 degrees is ideal for the gauge post - this allows your hand to naturally hold it and allows easy viewing of the gauge while refilling.
If a part has threads, do not cement. Instead - wait until after you paint (if you choose to do so) and use the Teflon tape to seal everything.
The last picture is with the optional add on. Everything AFTER the ball valve was not cemented in place. It will not be holding pressure and allows me to change my mind on what should go there (ie another 2l bottle). If anyone needs a better description, let me know and I'll take the time to go more in detail.
Step 5: Final Assembly, Safety and Firing
Keep tightening and make sure you stop when the solenoid is facing downward. This makes a nice little point to grab on and puts the manual bypass switch at your fingertip.
I currently use a 3' barrel. Honestly, this is too short for loaded projectiles, but it is almost too big for water bottle rockets.
I always pressure test my guns above what their normal operating pressure is. I tend to stay around 60psi and I tested at 90psi. I attach my air line to the filler valve and place the whole thing in a wooden box. I then slowly bring the pressure up to 90psi and I stay here for about an hour.
I purposely leave the teflon tape off of adapter on the barrel side. This allows me to unscrew and make the whole thing easier to transport and load projectiles. I find that ketchup packets will fit if coaxed in there slowly while twisting. But, don't fire at higher pressures - otherwise the packet rips open inside the barrel and a rectangular foil wrapper shoots falls about 3 feet away.
Firing water bottle rockets: Fill a 2L (or even a smaller 20oz bottle) a third to halfway full and set it on the ground. Charge the reservoir to your firing pressure. When you are ready to fire, make sure your firing range is clear and push the barrel into the full length of the bottle.. Pick up the whole cannon/rocket and point upwards. Let everyone know you are about to fire and pull the trigger.
You will get wet -- I tend to hold it out the side a little bit.