Funding for this project was canceled by the project creator on January 7, 2012.
About this project
This project has relaunched at http://www.kickstarter.com/projects/jfehr/magnetapult-the-magnetic-catapult-0
Thanks to all my backers!
This Kickstarter project is for the final design and production of a laser-cut desktop Magnetic Catapult kit. The Magnetapult is capable of accelerating a projectile at over 16 g's to a distance of about 40 times the length of the firing arm. (See FAQ to find how we came up with these values.)
The kit will come with everything needed to build and use the Magnetapult, including laser cut wood parts, supermagnets, bolts/nuts to hold it together, and a number of projectiles. You supply the (hopefully moving) targets!
The Magnetapult, locked and loaded with a foam golf ball:
Another, with a ping pong ball:
Before and after launch shot, which took many attempts, since the ball is traveling at 25 feet/second. You can see my finger depressing the hook trigger at the back:
Strong Magnets Warning!
This project uses two very powerful magnets to accelerate the launching arm and projectile. You should avoid this project if you wear a pacemaker or are in any way negatively affected by powerful magnets. Be super careful assembling this kit because if your fingers are caught between the two magnets, they will pinch, and it does hurt if they do! Do NOT hold each magnet with just your fingers and see how close you can get them, as they WILL fly out of your grip and collide, possibly sending off shards of magnet at high speed! By backing this project and assembling the kit you are acknowledging responsibility for any personal or property damage that may occur! Not suitable for unsupervised children!
What seemed like a fairly simple concept turned into a few months of almost daily prototypes. After a number of attempts trying to get the correct angles , magnet distances from the axis, size of the catapult and arm, and combinations of smaller (and less expensive!) magnets, I finally came to the realization that it just wasn't possible to launch anything a considerable distance (say, 20 feet) using anything but larger, more expensive magnets.
A number of supermagnet orders later I found one that was about as expensive as I wanted to get. (That is, inexpensive enough so that I could launch the project without having to make the minimum Magnetapult award amount too large.) The magnets I'm using in the video and pictures shown are about $20 with shipping.
Over the next month I'll be ordering a variety of magnets (round and square) to find the perfect price/performance combination that meets the launch distance criteria. The catapult design itself should remain very similar to the current version, pending brilliant suggestions from kickstarters. I'll also be looking for the perfect projectile, and the best fittings to use to keep it together properly. Finally, I'll be stress testing the Magnetapult to ensure the strong acceleration and deceleration don't damage the structure, and making any changes required.
Your Input Wanted!
I'm still undecided about what kind of projectiles to use. If anyone has information on where I can get bulk foam balls, around 1 inch in diameter, please let me know! If you're the first one to suggest the source I use, you'll get a free Magnetapult! I'd like to keep it the approximate size of a ping pong ball/golf ball so that my backers can easily get more ammo if they lose theirs.
We calculated this from the launch picture and verified with distance:
The shutter is 1/25th of a second, and the streak that is the ball is a foot long. Interpolating shows the ball to be traveling at 25 feet/second, which is 7.62 m/s. The distance from the ball at rest until the launch point is about 7 inches, or .18 m. As we all know, velocity squared = 2*acceleration*distance, and solving for acceleration, we get acceleration=161.3 m/second squared. Gravity is 9.8 m/second squared, so we're accelerating at an average of over 16 g's!
Measuring the angle of the ball's initial trajectory, you can see that I've nailed it at 45 degrees, the optimal angle for distance. (There's also stops for higher angles.) The vertical velocity is calculated as sin(45) * velocity, or about 5.4 m/s. Since t=2v/a, we substitute to get t=2*5.4/9.8=1.1 seconds in the air.
We can calculate the total distance from the horizontal velocity as cos(45) * velocity, or again, about 5.4 m/s, or 17.7 feet/second. Since d=v*t, we substitute to get d=17.7*1.1, or 19.47 feet. We've been getting about 20 feet in our tests so we can see our calculations are correct.
As I continue to update the design for the Magnetapult, I may add more rewards. I plan, for example, to release a version build out of Baltic Birch plywood, once I can figure out the solution to the variable thickness problem it is prone to. A custom limited edition version is also a possibility.
If you want multiple rewards, simply back the project with the totals for each reward you're interested in, and select the highest/most limited reward. Also please send me a message to let me know which rewards you want, and I'll make a note.
For limited, numbered rewards, the order will be based on when you initially backed the project, not when you changed your reward. Please remember that you can change your backing level and rewards at any time, so your best bet would be to back the project as early as possible.
- (35 days)