Project: Rat Trap Catapult


A BYU-I Engineering Project


Posted 18 Nov 2017    Edited 07 Oct 2018


I took a product design class last semester, and the first assignment was a team build contest. The challenge was to create a catapult that could launch a golf ball using a rat trap. The only rules were:
  • the only energy storage method allowed is the rat trap distributed to each team at the beginning of the assignment
  • the resting position of the golf ball before launch can be no more than six vertical inches from the ground
  • the set-up time for an individual launch cannot exceed two or three minutes
And the three categories for judgment were:
  • longest distance
  • accuracy: four discrete distances between ten and twenty-five feet would be announced a few minutes prior to the contest, and the ball had to hit the ground as close as possible to each distance
  • aesthetic design: this was a pretty bullshit part of the contest this semester because the class voted on the aesthetic score and everyone just voted full points for every design
My group tried a few different designs with very limited success. We tried a design where a weight attached to the end of the mouse trap knocked the golf ball off a platform. It like literally "golfed" the ball. The problem was that getting a hammer beefy enough to knock against the ball without bending the rat trap wire made the snap really, really slow and the ball didn't go far at all.

We also tried attaching an arm with a basket to the end of the trap wire but this also slowed the snap motion so much that the ball didn't fly more than a few feet from the catapult. So we looked for inspiration.


We got this from another BYU-I student Nate Maughan. He posted a blog article about the same project from a few years ago. We were inspired by the middle hinge design that seemed to give a lot more control over our previous designs and helped us get more oomph from the spring. Nate says that his team won the accuracy competition, but fell short in the maximum distance category.

Instead of using different chain lengths to achieve different distances, we just used angle settings to adjust the path from short to long shots. We also changed the release mechanism from chains on hooks to a solid wire that released off of a sliding bolt lock.


We spent a solid hour calibrating the catapult from its minimum distance angle to the max distance angle, and put little tick marks where the bottom edge of the rat trap would rest against the body for each foot from fourteen to twenty-seven (we couldn't get the trap to the lower distances from ten to fourteen very well, so we just kinda hoped short distances wouldn't be called out in the contest - and they weren't).

One of the team members kenneth took the catapult home and gave it a paint job to score points in the aesthetic design category. What he cranked out was awesome. Here it is at rest...


...and in "cocked" position


The rope was used to keep the center bar (made from a wire coat hanger) from popping out of the body. The wire needed to be able to rotate up with the changing angle, but also needed to have some force applied to keep it attached. the rope was an effective and easy solution.


Above you can see some detail on the release mechanics. This provided us really consistent results once we got it calibrated. The wire that ran from the trap to the sliding release was flexible enough to adjust but stiff enough to not change shape between launches.

The little seat for the golf ball was cut from an aluminum can. It was curved a little so the golf ball would stay in and would rest in the same place before every launch, and it was positioned behind the trap wire so that it wasn't deformed when the trap snapped shut.

At the actual contest, strips of masking tape were placed on the floor at fourteen feet, eighteen feet, twenty-one feet, and twenty-three feet (if I remember right... something like that). We went first on the eighteen foot launch and the golf ball actually hit the tape. It left a little mark on it.

With all the launches we were never more than about three inches from the target distance. Our total error was like eight inches. The next closest to us was two-and-a-half feet, I think. At the end we went for max distance and ours beat the next closest by about a foot-and-a-half.