The tennis ball mortar is a great project for many reasons: all the materials are cheap and readily available, its construction is extremely simple, and it gives off a nice loud *pop* which the video doesn't do justice.
Quick and dirty explanation on how it works: The mortar is composed of three sections of Pringles cans duct taped together. The metal bottoms of the top two cans are cut to form baffles, which will focus the force of the expanding hot gas created by the burnt vaporized naptha, onto the center of the tennis ball. Thanks to the near exact match in diameter of the ball and can, the force of the compressed gases is trapped behind the tennis ball for the whole length of the top can.
As you can probably guess this design has room for improvements. For example, if the length of the top can were extended, the ball would have a longer time to accelerate and thus achieve a faster speed, but at what point does the downward air pressure on the ball overpower the structural integrity of cardboard and duct tape? How does increasing the volume of the cans affect overall performance? What effect would adding a few more baffled cans have? I'm sure the answer is only a simple math problem or two away, but you look bored and like your enjoying a few too many fingers. (More after the video)
Today (June 1st) I am beginning a contest (Shhh! Its not an official contest so don't tell my bosses Eric and Christy) to see who can come up with the best tennis ball mortar design. No one characteristic defines "best", it is a combination of performance, creativity, adn 1's aBiLiTy 2 not tlak LIEK Dis!!!1!1 I am judge and jury of this contest, and my decision is final.
How to enter: Join the group Tennis Ball Mortars (Its moderated) If you feel your design is truly innovative, and that you are making great advances in the field of tennis ball mortarology, write a new instructable and post it in the group. If you just have some pictures, or if the only difference between your mortar and someone else's is the color of the duct tape, post it in the forums, and then add it to the group.
Now for the good part *THE PRIZE*. I'm sure your dieing to know what sacrificing several fingers, gaining numerous pounds, and clogging all your arteries will win? ::Drum roll:: The ULTIMATE SUPER GRAND PRIZE is ...
a cookie, thats right you win a cookie (Specifically: (1) Entenmann's chocolate chip cookie mailed in plain white envelope to any address in the world ( Its internationally delicious!! )). I know what your thinking "Damn Tetranitrate you cheapskate, even Fred, the guy who sits in his van by the playground is more generous with sweets then you", to which all I reply is "After all those Pringles you don't need anymore junk food, and you should probably tell a trusted adult about Fred."
**DISCLAIMER: By entering this contest, thinking about entering this contest, or even just reading this, you retain all liability should you happen to blow up or injure yourself in an attempt to build this.
(edit: I ate the cookie)