Step 1: Watch the Video!
Step 2: Rocket Rifle & Candy Cannon
My favorite for launching water balloons is the a modification of the "Rocket Rifle".
You can see how to build that step-by-step in this video:
This will also work with the "Candy Cannon" and no modification is necessary. It's ready "as is" and you can see how to build that in this video:
Step 3: Modify the Rocket Rifle With the Cannon Adaptor
The Cannon Adaptor is simply a 2' length of 2" PVC pipe, a 2" coupling, and some reducer bushings to bring the threads down to 1". Lastly, a 1", Close (short), PVC riser so it will screw into the sprinkler valve on the rifle.
I added some camouflage paint to match the gun.
Step 4: Shooting Water Balloons
There are 4 steps you can take to get a successful launch.
You need something that will cushion the pressure on the the balloon as it's launched. I made some wads with plastic cups. They are firm, but cushion the balloon enough during the acceleration to keep it intact. You could also use cloth, or even as something as simple as a moistened rolled up sock.
You can use the barrel from your rocket rifle as a ram-rod to push the wad to the bottom of the barrel. It should be as close to an air-tight fit as you can get.
If the barrel is dry, the balloon will pop when it's fired. This is because of the friction during payload acceleration.
I found that using a bit of vegetable oil to coat the sides of the barrel eliminated this problem. It also helps lubricate the diaphragm in the sprinkler valve to prevent it from locking up.
Water works as lubrication as well, but over time, the minerals from the water will dry inside the valve and may make it lock up at higher pressures, to the point where the 9V battery isn't strong enough to open it.
3. Size of balloon
The smaller the balloon, the stronger it will be. I had great results with balloons that had a slightly smaller diameter than the barrel. If they are wider than the barrel, you can still make them fit, but there will be more friction as they exit, and a greater chance of failure.
4. Using the right pressure
I found that 65-70 PSI works great for a single water balloon. At this pressure, you can expect a balloon to fly around 300 feet!
You can fit multiple balloons in the barrel, and will need to adjust the pressure up slightly with each one you add.
I tested all the way up to 17 balloons at 90 PSI with great results!
Step 5: 17 Water Balloons at Once!
There is quite a bit of kick as the balloons are fired out, so using the Candy Cannon as a Water Balloon Mortar is the easiest option, and the barrel can be tilted for trajectory.
However, using the Rocket Rifle to shoot multiple balloons, has a great and powerful feeling to it that you can't get with the Candy Cannon.
For portability, the manual ball valve near the fill port can be closed, maintaining the pressure charge in your water weapon. It will probably only be good for 1 shot, but will give you the option to disconnect from the air hose and go anywhere.
The pressure charge should hold indefinitely.
If you haven't seen the video yet, you can still see it here:
If you like this project perhaps you'll like some of my others. Check them out at www.thekingofrandom.com