Tuesday, January 5, 2010

Cannon Fodder - A Brief History

It all started around September. There I was relaxing one evening watching Mythbusters, when an idea popped into my head. The episode was about duck tape. And they were doing all sorts of things with duck tape. Lifting cars, making sailboats, and naturally, building a cannon.

The building of the cannon intrigued me. Their goal was to make a cannon out of duck tape that performed as well as their steel cannon they had used in other episodes. Their steel cannon shot a 18 pound ball, 500+ yards on three shotgun shells worth of gunpowder.

They took the right approach by figuring out how thick the duck tape needed to be. What was amazing was that a 1 inch thick slab of duck tape was able to withstand the pressure. They then proceeded to wrap a post with ducktape into a cannon shape and removed the post, giving them a cannon. One of the major downfalls to using 100% duck tape is that even with 1+ inch thickness, the duck tape is still not rigid. Not only that but it is highly susceptible to self compression - in other words, as the duck tape is wrapped tightly around the 3" post, the post provides a force back on the duck tape such that the post is not compressed. However, once the post is removed, that counteracting force is no longer there, forcing the barrel to collapse some (so now it is only 2.88").

How could this be bad? Well, in a steel or metal cannon, the cannon ball will just fit into the barrel, it should slide down though with minimal forcing. For the duck tape cannon, the ball literally had to be shoved and pounded in. This created an interesting dilemma, which they did discuss. Do they have a cannon, or a bomb?

So let me give a quick intro in how cannon's work. It is not exactly advanced technology. It has been around for at least 1000 years, and the general principles have been known for many millenia before that.

A cannon has three main components: the barrel, the combustion chamber, and the touch hole. The touch hole or fuse hole or vent hole is a small hole where the ignition source is introduced to ignite the main charge. This hole should be relatively small (<1/8") which I'll explain in a moment.

Next is the combustion chamber. It sounds somewhat high tech, but in actuality it is just a cavity where the charge is burned. Once the charge is ignited through the touch hole, it creates a whole bunch (that is a technical term for you non-engineers) of gases. Since the combustion chamber is completely enclosed at one end and covered by the cannon ball at the other end, at the beginning of the reaction it is a constant volume closed space. So as the charge rapidly burns, the amount of gases increase. Simple physics tells us that for a constant volume, as the amount of gases increase, the pressure will increase. Additionally, since the burning of the charge releases heat, the temperature of these gases goes up also forcing pressure higher. Therefore, the combustion chamber needs to be the strongest part of the cannon. More on that in a second.

Finally, the barrel is a long tube. It holds the the cannon ball. As the gases expand in the combustion chamber, they push on the cannon ball. The barrel helps to direct the trajectory of the cannonball and also allows the further expansion of gases transferring the force of the burning charge, into kinetic energy of the cannon ball.

So those are the basic parts, now to tie up the loose ends. The touch hole needs to be small so that there is not a large pathway for combustion gases to escape. Otherwise, much of the energy would be lost as gases leaked out of the touch hole rather than acting on the cannon ball.

The combustion chamber is made stronger usually by having thicker material around the combustion chamber. This is done in two ways. First, the combustion chamber can be made smaller diameter than the barrel. For a constant outer diameter (say 4"), a 3" bore (1/2" thickness), the combustion chamber may only be 2" in diameter (1" thick). Another way of making the material around the combustion chamber thicker is to taper the metal. This gives cannons their their classic bulbous shape at the end. One or both of these methods could be used.

The cannon ball needs to be matched to the size of the barrel. There needs to be a tight fit. A loose fit will allow gases to leak by, thereby wasting the charge and in extreme cases, not shooting the cannon ball out of the cannon (which would defeat the purpose). A tight fitting ball is going to have increased friction as it travels along the barrel, this could be mitigated by lubricating the barrel. Too tight of a fit and bad things can happen. First, the charge might not be able to overcome the force of friction, thereby sticking the ball in the cannon. A stuck ball can be extracted with much difficulty. Second, the possibility is there that the ball could be friction welded to the cannon, in other words, depending on the metals of the barrel and the ball, the friction might cause enough heat to weld the ball to the cannon. At this point, you cannon is pretty much scrap metal. Third, the worst thing that could happen is your barrel or combustion chamber could fail (i.e. break apart) from the force of the expanding gases, thereby creating a bomb rather than a cannon.

Basically, a good cannon is a fine balance between friction and expansion. A "perfectly" balanced cannon would have a complete burn of the charge (i.e. expansion) at the very instance that the cannon ball left the cannon (i.e. no more friction to overcome). However, that would also eliminate the puff of flame and smoke that makes cannons so fun (mostly the flame).

So back to Mythbusters. With the extremely tight fit of the cannon ball, they surmised that they might have a bomb on their hands. Of course, in the pursuit of science, there is only one thing to do. Light it off and find out (applied science is a lot more fun than the theoretical stuff). They took appropriate safety precautions and lit the fuse. I sat on the edge of my seat as the fuse burned down. Then BOOM! I could see a definite trajectory of smoke and what looked like an intact cannon. Sure enough it worked. The cannon was in one piece, and the cannon ball flew 175 yards. Not quite the 500 of the steel cannon, but my guess is the majority of that can be attributed to the increased friction of the tight fit in the barrel. However, it is not repeatable, then you can't really call it science. So they shot it again. I was in heaven. When my kids watched it, they were in heaven.

And that is when I had my idea: I am going to make a duck tape cannon!
(More coming in Cannon Fodder - Building a Cannon)

No comments:

Post a Comment