At home, we built a tennis ball mortar, following directions in Gerstelle's Backyard Ballistics book. The construction really wasn't difficult, and all the components were easy to obtain. It's made from potato chip canisters, duct tape, and a PVC pipe.
We tested it at home and after a lot of failed attempts (we had a tough time getting the firing holes to line up at get the lighter or match to the touch point properly) we finally thought we had a working model to take to co-op...
However.... (I hate when there's a "however"!) on our last test fire, the cardboard actually started to smolder, but there was a delay until smoke started coming out of the blast tube, so we didn't notice right away. And this was the result:
Of course, this happened less than an hour before I had to leave for co-op. *sigh* So...
We learned that there is a difference between a mortar and a cannon. Mortars are loaded through the muzzle, with the ammunition allowed to fall to the breech (back end) and fires the projectile at a relatively low velocity and high angle of trajectory. A cannon is a more generic term, but refers to a weapon which has the ammunition loaded through the breech and fires at a much higher velocity and a fairly flat trajectory.
The lesson was a brief one on ballistics - the "science of shooting" with an explanation of how the tennis ball mortar is supposed to work. The fuel/air mixture in the barrel undergoes a rapid chemical change when put to a flame, and it exerts a force on the inside of the barrel. Since the bottom and sides are rigid, the force is channeled against the tennis ball sitting against the top baffle, and forces it out of the tube at a high velocity.
Needless to say, although I attempted a repair of the damaged barrel with more duct tape, it was very hard to keep it rigid and to keep a proper firing hole. We did give it a try outside, and it was an epic fail. We did, however, take the opportunity to discuss some of the possible causes for the problem and suggested some improvements and adjustments that could be made. One thing we've discovered during the course of the co-op is that these students have excellent ideas for design modifications, and we've told them over and over that those ideas are their physics knowledge at work. And when something doesn't work the way we expect it to, we examine why. We concluded that the baffles, especially the top one on which the tennis ball rests, may not have been smooth enough, meaning that there wasn't a good seal at the top keeping the fuel/air mixture inside the barrel. The compromised firing hole was also a factor. Another theory is that lining up the firing holes on the barrel and the blast tube was time-consuming, there was additional leaking of the fuel/air mixture. A suggested solution was to set it up so that the barrel extended below the the blast tube so that there was only one firing hole instead of needing to line up two of them.
This was a simple enough project that I think we may try it again sometime - at home!
This is the eleventh (and final) post in a series -
- Physics Week One (gravity and friction)
- Physics Week Two (structural engineering)
- Physics Week Three (suspension bridges)
- Physics Week Four (constructing catapults)
- Physics Week Five (catapults)
- Physics Week Six (constructing trebuchets)
- Physics Week Seven (trebuchets)
- Physics Week Eight (Viking catapult and hand slings)
- Physics Week Nine (match rockets)
- Physics Week Ten (rockets)
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