🎯 Spring Catapult: step-by-step build

Cock the arm, click the trigger, FIRE! A spring flings the ammo across the room.

Long ago, castle armies used catapults to throw things over walls. Ours throws a little grey bush — much safer! When you push the arm down, you squash a strong red spring. The spring stores your push, like a coiled snake. A little latch holds the arm down until you pull it back — then SNAP, the spring lets go and the arm whips up. The arm is a lever: the spring pushes near the pivot, so the cup at the far end flies much faster.

  • Level 2 of 5 (easy)
  • About 25 minutes
  • 14 steps, 43 parts
  • Powered by: Hand-crank
  • Series: Mechanical Wonders

What you will learn

  • Stored Energy: Energy can be saved up in a spring and let out later.
  • Levers: A bar on a pivot multiplies your push.
  • Ratchet & Pawl: Lets something turn one way but not the other.
  • Pins: Grippy or Smooth: Pins join beams together — some hold tight, some let things swing.
  • Frames: A frame is a ready-made strong rectangle.

Build these first

  • ⚖️ Balance See-Saw: A see-saw that is also a scale. Can one gear beat two? (level 1 of 5, about 12 minutes, 10 steps)

Parts you need

  • 1 × Open frame 5×11
  • 6 × Blue long pin
  • 7 × Split pin block
  • 7 × Red axle-pin
  • 2 × Beam 7
  • 2 × Axle 7
  • 4 × Bush
  • 1 × Beam 13
  • 2 × Half bush
  • 4 × Black pin (grippy)
  • 1 × L-beam 2×4
  • 1 × Hard spring
  • 1 × Grey pin (smooth)
  • 1 × Beam 5
  • 1 × Tan axle-pin
  • 1 × Thin beam 4
  • 1 × Axle 3

Steps

  1. Lay the 5×11 frame flat, long side facing you.
  2. Back post base: push blue long pins up into the 7th and 9th holes of the back side. Press two split pin blocks onto them, one on top of the other.
  3. Front post base: the same on the front side — two blue long pins and two split pin blocks.
  4. Push a red axle pin into the middle of each block, pointing out of the frame.
  5. Press a 7-hole post onto the back red pins. Push the 7-long axle through its 6th hole and add two bushes.
  6. Slide the 13-hole throwing arm on by its 11th hole, then add two more bushes on the front.
  7. Press the front post onto the front red pins, sliding the axle into its 6th hole.
  8. Stop bar: hold two half bushes between the posts and push a 7-long axle through the top holes of both posts and the bushes. Slide the half bushes out against the posts so the bar cannot slide out.
  9. Pin the black L-beam to the end of the arm as the cup.
  10. Spring base: push black pins up into the 5th and 7th holes of the front side and press a split pin block onto them, under the middle of the arm.
  11. Stand the red spring under the arm, push a red axle pin through the block into its bottom hole and pin its top to the arm with a grey pin.
  12. Trigger: push blue long pins up into the 3rd and 5th holes of the back side and press two split pin blocks onto them, one on top of the other.
  13. Push two red axle pins into the blocks, pointing out the back, and press a 5-hole post onto them.
  14. Push a tan axle-pin into the post's 4th hole, pin end first, and press the black latch onto its axle end by the latch's bottom hole. Push a 3-long axle through the latch's top hole as the tooth. (The end holes of a thin beam are cross-shaped — the latch turns with the smooth tan pin.)

Play time

Push the arm down under the latch tooth, drop a loose bush into the cup as ammo (it is not built in — it has to fly!), aim, and pull the latch back. Set up paper-cup targets! Move the spring's top pin one hole along the arm — does it throw further or shorter?

Real-world inspiration

Rebrickable "The Catapult" MOC-107421 — https://rebrickable.com/mocs/MOC-107421/EDGE%20OF%20BRICKS/the-catapult/

Build next

  • 🛠️ Ratchet Car Jack: Crank the handle — click, click, click — and lift a toy car. Let go and it stays up! (level 3 of 5, about 35 minutes, 15 steps)