How'd I come up with it?
Most of us have played with Neodymium supermagnets, and marvelled at how strongly they attract and repel each other. After the success of my Magnetic Levitation Sculpture, I had a ton of magnets kicking around, and one day while playing with them, I thought, how can I make this strong force launch a projectile for me? After a bunch of prototypes, each using bigger and bigger magnets, I came up with The Magnetapult. and launched it on Kickstarter. It has since been successful, so now I'm sharing the instructions on how to build it here. Total cost is between $20, if you have a laser cutter and use the weaker magnets, and $50+ if you don't. Could be more if you live outside of the US due to shipping costs.
If you don't cut the parts right, assemble it right, etc, etc, you may get hurt by the supermagets, including pinching, shattering, taking an eye out, etc. Be super careful and follow directions exactly!
Step 1: Cutting pattern and parts list
2 - 3/4 x 3/8" thick disc magnets, either the N42, or for more power, the N52. These are EXTREMELY strong so follow directions exactly to avoid pinching/smashing/shattering/injury. This will cost you from $15 to $20 and up with shipping, depending on where you live.
1 6-32 x 1" bolt. I use brass bolts because I had a bunch from my levitator sculpture project and they won't interfere with the magnets.
3 - 6-32 x 1.5" bolts.
4 - 6-32" nuts
2 - small spacers with 6-32 holes
1 - 1 square foot of 1/8" MDF material
Access to a laser cutter. You can probably get an online laser cutting place to do it for $30 or so. (Or save a bunch of money by backing the kickstarter project or buying it retail after the kickstarter project is over.)
You can also try cutting this by hand but most likely you won't be impressed with the results. The pieces fit together very nicely when laser cut. The holes especially should be an exact tight fit or you'll end up with shattered magnets.
Below you'll see the complete parts after you get the mdf back from the laser cutters.