Launch It: How to Trebuchet!

Learn how to make a firing trebuchet in this project. Our public library https://www.librarypoint.org/ has an ongoing competition called "Launch It!" where the community is invited to make smallish trebuchets and then launch them together in competitions based on distance, accuracy and decorative style! What is a trebuchet? Take a look here at all the differences between catapults and trebuchets. The key difference is that trebuchets use weight rather than tension to launch their payloads. It also makes for a great project that combines physics with woodworking, design and creativity. We will be building a working trebuchet using about $15 worth of materials here to get ready for our "Launch It!" competition.

See video for more guidance!

Supplies

Tools:

Drill and Driver

Saw and Miter Box

Hacksaw

Stapler or Needle and Thread

Hammer

Materials:

(2) 1x2x8s (about 9 feet of wood needed)

Wood screws

One trim nail

Wood glue

Scrap fabric (cotton or felt)

Rope or paracord (3 ft)

Metal Rod 5/16ths (something like this)

Metal weight (2 and 1/2 pounds or more)

Step 1: Design and Video

Get started with your build with this planning document and step-by-step video!

Step 2: Cutting Your Wood

Using a saw and planning document, plan all your cuts. Remember, you can scale up or down the size of your trebuchet based on the guidance of the planning document. For this trebuchet, we will need:

2 x 12 inches (length of base)

2 x 6 inches (width of base)

4 x 11 inches (uprights)

4 x 3 inches (supports, cut at 45 degree angles)

1 x 16 inches (swing arm)

After all your cuts, be sure to sand any rough edges.

See video for more guidance - 7:35

Step 3: Building the Base

Use the two 12 inch pieces and two 6 inch pieces to build the base of the trebuchet. Place the length pieces on top of the width pieces. Predrill and screw two screws into each corner of the base.

See video for more guidance - 9:40

Step 4: Planning the Uprights

The pivot point of the frame (where the uprights will be placed) isn't in the middle, but at 1/4 of the length of the frame. Measure 4 inches from one end of the frame and mark it. Line up the uprights at the line so they are doubled with the seam on the inside (see photo). Glue and screw each pair of uprights together before installing them on the base.

See video for more guidance - 11:00

Step 5: Installing the Uprights and Supports

With the placement of the uprights in mind, drill holes through the bottom of the base, then use wood glue to mount them. Next, line up the four support pieces. Glue and screw them into place to support the uprights.

See video for more guidance - 11:00

Step 6: Installing the Pivot Point

With the frame completed, get ready to add the pivot point and swing arm. Measure one inch down from the top of the upright and drill a hole the width of the pivot rod that you will use. (Use a 5/16s bit if you are using the rod suggested for this project). Prepare both uprights this way. Next, measure the length of the rod needed (6 inches) and cut with a hacksaw.

See video for more guidance - 15:45

Step 7: Adding the Rod and Swing Arm

With the rod cut and the holes drilled in the frame, mark where your pivot will go through the swing arm. The ratio is 1:3.75, so for our 16 inch swing arm, the pivot point will be drilled at 3.25 inches. Drill this hole, then line up the swing arm with the frame and add the rod so that it fit snugly through all three layers. Tap into place with a hammer so that the short side of the swing arm is on the short side of the frame. Test that the arm rotates from the front to the back of the frame.

See video for more guidance - 15:45

Step 8: Adding a Counterweight

With the swing arm in place we need to add a counterweight to the short end of the arm. There are lots of options here for how to add weight. Ensure that whatever you use can pass through the gap between the uprights. If you choose a larger or wider weight, you may need to widen the width of your trebuchet. For this project, we will use a large zip tie and a 2 and 1/2 pound round metal weight for the counterweight. Remember that it is suggested that your counterweight be 100x to 133x the weight of the object you wish to throw. So if a marble weighs 5.5 grams, then 1 to 2 pounds for a counterweight would be just right.

Drill a hole the size of the zip tie 3/4s of an inch from the end of the short side of the counterweight. Feed the zip tie through the wood and around the counterweight. Pull it snug with a pair of pliers so that the weight rests firmly on the end of the arm.

See video for more guidance - 19:40

Step 9: Making the Sling

Drill a hole on the long side of the swing arm about 3/4s of an inch from the end. Cut a length of cord that is double the length of the swing arm. In this example, the cord was 32 inches long. Feed it through the drilled hole and tie the rope so that the loop of cord extends to the pivot point of the swing arm. Slide the knot so that it is in the middle of the loop where the pouch will be installed.

Cut a piece of fabric (cotton or felt) about 3 inches by 10 inches. After the cord is attached to the swing arm, fold the fabric over the middle of the loop to create a pouch with the two lengths of the cord on the outside edges. Attach the fabric to the cord using a needle and thread or staples. Test to ensure that the pouch is in the middle of the loop of cord.

See video for more guidance - 23:50

Step 10: Test the Sling and Add a Launch Pin

Test that the pouch sits in the middle of the loop of string by bringing the bottom of the loop up to the top. This will be how the sling rests right before a launch. Use a trim nail as the launch pin. This will be placed on the top end of the swing arm. Hammer the nail half way into the wood. Place it close to the rear side of the board (see photo). Check it's placement by placing the end of the sling on the nail. The pouch should rest balanced and even when the swing arm is parallel to the ground. It is does not, adjust the pouch as needed.

See video for more guidance - 23:50

Step 11: Ready to Launch!

Time to test the trebuchet! Gather something to launch, like a ping pong ball, marble or golf ball. Start with something soft or light if possible. Please note that a trebuchet can fire its payload in nearly any directly, so be aware of your surrounding. Use two hands to prepare your trebuchet. With one hand, pinch the payload in its pouch so that it is centered and even. With your other hand push down on your swing arm until it is as low as it will go. Follow this motion with your other hand still pinching the payload and pouch. Place your body and arms on the side of the trebuchet and toward the back, out of the swing arm's path! When you are ready to launch, release the swing arm and let the pouch slide through your other hand. Be sure to keep your face and hands out of the reach of the swing arm and launch pin. Do keep some weight on the rear end of the trebuchet as it launches, so that it doesn't tip over.

See video for more guidance - 32:14

Step 12: Direct Hit!

How did it go? I hope your launch was a success. If not, check out some tips for adjusting the angle of the launch using the launch pin in the video - 34:45

Be sure you take some time to decorate your trebuchet after it is firing well. Keep the middle path clear for the swing arm, but the outside of the trebuchet have lots of room for paint, paper, glitter or more! Get creative. Also if you are ready for some Trebuchet 2.0 tips, consider wheels, more weight, larger sizes, or a firing pin to make launching easier. These could all make for some great upgrades!

Good luck!