I’ve been recently reading about ballistæ (and their many relatives!), and I’ve come across the assertion that early such weapons were tension-based (like any ordinary bow) while more advanced ones would work akin to a crossbow (torsion-based).
My question is: how the heck do the two differ after all? I can see that the energy-storage mechanism is fundamentally the same (pulled elastic string held back), and I thought it was about the different ways you could pull the string itself, but then I had a look at the Wiki page of the oxybeles (supposedly tension engine), which looks EXACTLY like a torsion ballista. Besides – don’t some crossbows get their string pulled by hand? Are those torsion weapons too?
Bonus: [these](https://www.stormthecastle.com/catapult/images/oxybeles-drawing1.jpg) two [images](https://www.stormthecastle.com/catapult/images/ballista-drawing1.jpg) attempt to explain the concept; I still don’t get it. Can anyone please explain it to me?
Thank you!
In: 8
The difference is in what changes shape to store energy.
What you’re picturing would be a tension system, like a rubber band stretching to launch a projectile. The rubber band gets longer – this is tension.
A bow is a flexion system. The thing that changes shape bends. It curves more or less as it stores energy. A crossbow is the same. The string does not stretch – it does not change shape to store energy – and so this is not a tension system.
A torsion system does not see the member curve. The member stays more or less straight, but it *twists* into a spiral shape.
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