Introduction: Pocket-Sized Ballista

By 30+ STEM Projects for Kids
About: I'm a writer, maker, and educator who's on a mission to better the world through hands-on engineering projects. Check out my work: www.stem-inventions.com/

Long before rubber bands were invented, clever engineers powered their war machines with bundles of twisted rope, known as torsion bundles. One of the most iconic uses of this engineering technique is manifested in the ballista, which employed twin torsion bundles to power a pair of throwing arms that hurl bolts.

The key to a successful pocket-sized ballista is to create durable torsion bundles that can be tightly wound-up without breaking. Cotton string is too thick and frays too easily at such a tiny scale, so you’ll be using a clever alternative… dental floss!

This Instructable is a full excerpt from my book, Launchers, Lobbers and Rockets Engineer. Enjoy!

Step 1: Gather Materials

* It's critical to use waxed dental floss. Floss has extremely high tensile strength, which means that it can hold many pounds of force before breaking. The waxy coating is also important because it makes it much easier to thread into the tiny holes, and it protects the floss from fraying.

**Bamboo skewers can't be substituted with wood dowels or similar materials. The fibrous structure of the bamboo is resistant to being crushed or snapped, which makes it ideal for holding up against the crushing forces of the torsion bundles. It's also very thin yet inexpensive!

When you make a purchase through these links, I may earn an affiliate commission at zero cost to you.

Step 2: Build the Frame

Hot glue the edges of two pairs of craft sticks together as shown (picture 1).

Overlap the glued pairs of sticks by about 2" (5cm). The bottom pair of sticks will be the front end of the ballista (picture 2).

Glue a single stick onto the underside of the front of the frame (picture 3).

Step 3: Create the Front of the Frame

Use a pair of pliers to grip and snap off sixteen pieces of craft sticks that are about 3/8" (1cm) square (pictures 1 and 2).

Glue the square pieces of sticks in stacks of 4 as shown, then glue another stick on top (pictures 3 and 4).

If you have a more convenient material than bits of craft sticks, like corks or wooden cubes, you can use that instead.

Step 4: Prepare the Torsion Mechanism

First drill two 1/8" (3.2mm) holes, and four 1/16" (1.6mm) as shown*. Make sure to space all the holes about ¼" (6.4mm) apart (picture 1).

Cut and straighten a paperclip into two 1.25 (3.2cm) pieces, and cut one of the bamboo skewers into four 1" (2.5cm) and two 3.5" (8.9cm) pieces (picture 2). The paperclips will be pegs that hold the bundles in place, the short bamboo pieces will be the winding pegs, and the long bamboo pieces will be the throwing arms.

*Pro tip - Here's how to drill holes in craft sticks without splitting the wood:

  1. Put the ballista on top of a disposable piece of wood.
  2. Get the drill in position, and rev it up to the highest speed.
  3. Slowly rest the weight of the drill onto the craft stick. Once the hole starts forming, don't increase the weight, but keep the drill spinning as fast as possible. If you drill into the stick too fast, the wood might split.

The wood is already unlikely to split because it's strengthened by the stacks of stick pieces from the previous step. If it does break, no big deal! Cut off the broken stick with a pair of wire cutters, and glue on a new one.

Step 5: Create the Torsion Bundles

Cut a scrap of cardboard or other material to about 1" (2.5cm) wide (picture 1). Wrap the waxed dental floss around it ≈10 times. Repeat, and tie the ends together to create two loops (picture 2).

  • Creating a thick bundle of floss makes the torsion bundle more durable, which in turn, allows you to wind it up more and generate more power.

This is where the waxy floss begins to show its value! Any other material - like sewing thread - would be very difficult to work with.

Pinch the waxed loop of floss together on one end, and insert it into the 1/8" (3.2mm) holes (picture 3). Once through, separate the strands again (picture 4).

Step 6: Install the Throwing Arms

Insert the 3.5" (8.9cm) and two of the 1" (2.5cm) pieces of skewers in between the strands of floss as shown (picture 1). Again, the 3.5" skewer is the throwing arm, and the 1" skewers are now the pegs that will wind up the torsion spring.

Rotate the 1" (2.5cm) winding pegs to make the 3.5" (8.9cm) throwing arm press into the front of the ballista (picture 2). In this picture, the pegs are wound counterclockwise. When the throwing arm is pressed firmly against the frame, insert one of the paperclip pieces into the appropriate hole to prevent it from unwinding.

For now, don’t wind it up too much – you’ll increase the tension later.

Repeat with the second arm (picture 3).

Step 7: String the Ballista

Carefully split the tip of each throwing arm with the utility knife (picture 1).

Slip a 11" (28cm) piece of floss into the split, with about 1.5" (3.8cm) of hanging out one side (picture 2).

Wrap the short end of the floss tightly around the tip of the throwing arm to hold it in place. Importantly, this also prevents the throwing arm from splitting further (picture 3).

Repeat on the other side, and cut off the excess floss. Apply a dab of hot glue onto the wrapped floss to prevent it from unwinding (picture 4).

Step 8: Create the Trigger and Bipod

Finish the ballista by hot gluing a binder clip onto the back of the frame (picture 1).

To create a bipod, cut a craft stick in half at about a 45° angle, and glue the pieces to the front of the ballista as shown. Make sure the ends of the pieces do not protrude in front of the center gap.

Note: You can choose to forgo the bipod to make the ballista even more pocket-friendly :)

The ballista is complete! Now time to make the bolts.

Step 9: Make the Bolts

Cut another skewer to 7" (17.8cm), then carefully fold a 1" (2.5cm) piece of tape over the blunt end of the skewer, leaving about ¼" (6.4mm) exposed at the very end (picture 1).

Trim the fins into a tapered shape that protrudes no more than ¼" (6.4mm) from the skewer (picture 2). The fins must be able to easily fit through the front gap of the ballista frame.

Straighten a whole small paperclip, and then wrap it tightly around the tip of the skewer (picture 3 and 4). This creates a leading weight*

Using the tip of the utility knife, carefully carve out a small nock at the back of the skewer. Make sure the gap in the nock is aligned with the fins as shown (picture 5).

*Adding weight to the tip of the bolt is crucial. Why? Imagine trying to throw a long strip of paper. It won’t go very far and it definitely won’t go straight. Now imagine attaching a rock to one end of the paper and throwing it again. The momentum of the rock will carry the paper through the air and the paper will trail behind it in a straight line. This same idea applies to long projectiles — like a bolt. It needs weight at the tip to generate momentum that'll carry it far and straight.

Step 10: Add Tension, Load, and Fire!

Add more tension (not pictured). Remove the paperclips that are stopping the winding pegs. Rotate the winding pegs evenly to slowly increase the tension. The bundled dental floss is extremely strong, so don’t be afraid to really crank it up (the winding pegs will break before the floss does!) Make sure to rotate the top and bottom winding pegs evenly, and that the tension is even for both throwing arms. If one arm is more powerful than the other, your shot won’t be accurate. You may need to make small adjustments to the amount of tension in each torsion bundle to make them even.

Load and fire! Insert the bolt through the front of the ballista, and position the nock onto the string. With your other hand, hold the back of the bolt in place. This frees your first hand to switch positions (picture 1).

Pinch the bolt near the fins, and open the trigger (picture 2). Avoid having your hands (or anything else!) in front of the ballista as you're loading it in case of a misfire.

Push the bolt into the trigger (picture 3).

Center the bolt, aim, and simply press the trigger to fire (picture 4).

Step 11: Long Live Your Ballista

When you're not waging pocket-sized warfare, take out the tension from the torsion bundles by unwinding the pegs one full rotation. You don't need to completely undo the torsion bundle.

Decreasing the tension will improve the longevity of your ballista.

Thanks for reading this far! If you build your own mini siege machine, share a picture in the comments!