This is a simple 10 minute catapult that I built using the 521 piece K'NEX set. Using a strong rubber band I have been able to fire a marble about 20 feet.
Step 1: Parts List
Here's a parts list.
4 red rods
3 yellow rods
3 bendable orange rods
2 white rods
11 green rods
6 grey 45 degree connectors
6 red 90 degree connectors
4 green 135 degree connector
4 orange connectors
2 grey sliding connectors
2 blue sliding connectors
2 purple clips
2 tan connector clips
2 back rod clips
1 small gear
Step 2: The Main Body
first connect 2 red connectors with a green rod (X3)
then attach the two grey sliding connectors, with the two blue connectors.
next connect both of the blue grey connectors with a green rod (as shown below)
Step 3: Connect Main Body Parts
connect the two red assemblies with the blue green assembly as shown in the picture.
Step 4: Connect Body Parts 2
then create the vertical boom piece of the catapult with two yellow rods, and a red assembly.
then attach two purple pieces with green connectors too the center piece, as shown below.
Step 5: The Arm
put two red pieces, with two straight orange snaps. then, attach the two pieces with two light grey connectors.
the bottom grey piece is the one that one side of the rubber band will attach too.
the top one is the bottom of the bucket. you can slide it up or down for different size projectiles.
Step 6: Attaching the Arm to the Body
slide a yellow rod through the two purple pieces and attach the arm.
Then add two tan, two black, and a small gear as show in the picture.
Step 7: Add Strength to the Body
now to make the pieces to help strengthen the body.
attach two green connectors with a green rod, and two more light grey pieces to that with 2 more green connectors.
now (X2) connect a green connector with a green rod to a orange straight connector, and an orange bendy rod on the other side of the green connector.
Step 8: Attach Pieces
Attach the green piece tho the two main red rods of the body, and add two light grey connector on the inside of the two red rods. (see picture)
Attach the other two assemblies with the orange bendy rods to the two upright yellow rods, and the two orange rods too the light grey pieces on the inside of the two red rods. Slide the two grey pieces to make the uprights perpendicular too the bottom of the body.
Step 9: The Final Piece
After this add the final orange bendy rod to the back assembly. (see picture)
Step 10: Attach the Rubber Band
Now look at the included pictures to see where to attach the rubberbands.