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Picture of Pocket-Sized Ballista
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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

Picture of 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!

Chrystalkay5 months ago
I love little gadgets... especially ones that do not require tools that not everyone has. This little creation is make-able by everyone! Congratulations on a well made instructable!