Second, they are great teaching tools. Lessons in Materials science, Chemistry, Physics, Mathematics, etc. can all be taught in a hands on way. The two lessons that I remember most from high school physics were when we made dry ice bombs (try getting that into schools these days) and ignited methane bubbles. Sure many of the formulas and principles have been ingrained in my mind through later courses in college and hands on experience, but if you ask me what I remember about high school physics those are the stories I would tell you. Since my wife and I homeschool our children, we have a lot of latitude in how they are taught. And since my wife says that I am in charge of science, it is my duty as a father and a teacher to provide the best education possible. Why play the cannon game where you lob shells at an opponent on the computer, when you can do it in real life. So in essence, its for the children!
Third, cannons are FUN!. Fireworks are glorified cannons, and we all like them. The last thing we need is to teach our children to be afraid of smoke and flames coming from a barrel. They need to be taught to respect the power of them (just like they should respect the power in a 100hp engine attached to a one and a half ton metal box). But they should also learn that the proper application can bring joy and happiness. (Not to mention it can be a great stress relief - I have never been stressed after blowing something up).
So I was going to build a cannon. Not an air powered or even hairspray/lighter fluid cannon. I wanted a black powder cannon. Something that would have smoke and flames! I wanted to build it out of everyday materials that could be found in any decent sized town in America. I did not want to use any specialized tools or specialized skills. In other words, I wanted to do it so that any adult capable of getting a driver's license could do the same thing. As a final result, I wanted a cannon capable of launching a 5 lb shell, 100 yards.
Now, let me allay any fears that some may have. In showing in excruciating detail how to build a cannon of one's own, am I aiding terrorists or criminals? Uh, no. As I mentioned in Cannon Fodder - A Brief History, the technology I am explaining is 1000 years old and the principles have been known for almost all of recorded history. If terrorists or criminals are coming to my blog to learn how to make destructive weapons, then we don't have too much to worry about.
So, let's start with a list of tools that you will need:
Hacksaw
Drill w/ 1/4" bit
Materials and where to get them:
PVC pipe (home improvement store)
Duck tape (home improvement store or Walmart)
PVC joining glue (home improvement store)
Small copper tubing (home improvement store)
Concrete filler (home improvement store or Walmart)
High temperature grill paint (home improvement store or Walmart)
Various sized cans (from your wife or mom after she makes dinner)
Large band clamp (auto parts store or Walmart)
Shelf tracks (home improvement store)
small nuts and bolts (somewhere in your garage or basement)
So on to the assembly!
First, the barrel is made from PVC pipe. I used 4" sch 40 because it snuggly fit a can of peaches (which became my outermost layer of the combustion chamber). A cap is solvent welded onto one end.
Now, PVC has the nice quality of being rigid (hence less risk of compression holding the shell too tightly and making a bomb). However, it is also a brittle material that will be subjected to many stresses that are going to crack/shatter it in a cannon. Hence, I wrapped it with duck tape. The cloth tape will provide a protective barrier to prevent the PVC from breaking apart, and by wrapping it very tightly, I am introducing some compressive forces on the PVC which will partially counteract the hoop stress from expanding gases.
Second, the combustion chamber is made by nesting various size cans inside of one another. I started with a big 25 oz peach can for the outer base and ended with a 4 oz tomato paste can on the inside. I used concrete filler (the rubbery stuff) to fill in the space between the cans. This does two things. It increases the mass of the combustion chamber (providing strength) and being a compressible material, it will mitigate the amount of hoop stress transfered to the barrel (the highest pressures are going to be in the combustion chamber).
I then sprayed the inside of the cannon with the high temperature grill paint. Not so much to make it heat resistant, as to give it a uniform color. The heat resistance is a bonus. To assemble the cannon, I simply dropped the combustion chamber into the barrel. Then I drilled a hole through the wall and into the combustion chamber. A short piece of copper tubing was inserted to act as the touch hole.
Finally, after scrounging around my garage, I found a 4" hose clamp and a couple of old shelving tracks. I used these with some nuts and bolts to create some legs for the cannon, and viola! The cannon was completed!
Next up, what good is a cannon if you don't use it. See Cannon Fodder - Applied Science.
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