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
I’d try to find animations or videos, not pictures.
The energy storage is not the same. The storage mechanism is not the string held back. In a felxion based system, it’s the limbs. In a torsional based one, it’s ropes at the base of the limb.
In a flexion based system, the energy is stored by the flexing of the limbs. So when you pull on a bow or crossbow string, the limbs bend. The base of the limb stays perpendicular to the base and the arm deflects so it is no longer straight.
In the torsional system like the second picture, the limbs don’t bend. You’ll notice that the limbs are embedded in rope. When you pull back, the limb stays straight, and the rope twists. And the base of the limb rotates while the limb stays straight.
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