Here are the plans for a small desktop catapult ideal for ping pong ball or marshmallow fights. The spring force is provided by a twisted string mechanism, so it lasts and lasts, in contrast to rubber band-powered catapults. The cool thing is my kids can safely battle each other (so long as the missiles are limited to marshmallows).
This design includes a variable stop, allowing you to adjust the trajectory of your missile by arresting rotation of the throwing arm at 3 different angles. See the video on this page.
To complete this you'll need a length of 1x6" wood, some strong cord, and a dowel rod (?1/4"?). The plans can be printed from the freeware drawing program Inkscape, or viewed in 3-d form in the freeware Sketchup.
Tool-wise, you'll need a drill (press would be ideal, but hand drill would do) and a detailing saw of some sort (band saw and scroll saw would be ideal, but a jig saw would do). You'll also want a can of spray adhesive to stick the plans to your wood.
Step 1: Plans
Open Inkscape and print out the plans attached here (the .svg files). You'll need 1 base (likely need to print over 2 pages), 1 crank sheet, 1 crossbeam, 1 arm, but 2 sides. Using spray glue, attach these to your 1x6" board for drilling and cutting. When done, use a hair dryer or heat gun to remove the plans from your wood.
The cup to hold your missile can be anything, from a cut-off plastic cup to the lid for a can of spray paint. I made mine from scrap wood by first drilling out the center with a forstner bit, then rounding the outside with a bandsaw and disk sander.
Step 2: Tensioning the Arm
The moving parts are all held together by the string. Thread it around one crank, through the vertical arms, around the throwing arm, through the other vertical arm, around the other crank, and back, several times. Tie it off firmly then you are done.
Now you need to crank up the tension. Look carefully and you will see that dowel rods with angled ends are in place to arrest backward movement of the cranks. This allows them to be wound forward without slipping. Once you have wound them as tightly as you are able, you are ready to load and release your catapult.
Step 3: Loosing the Catapult
I like to dullen a nail and insert it into the holes in the base and throwing arm to serve as a release. See the video on this page. Kids usually don't bother with this, and just crank 'n fire. Either way, you will want to experiment with placement of the stop, to get the combination of height and distance you desire.
If you enjoy this project please vote for it in the String contest.
Thanks for reading!