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    Of Bows and Torsion Engines

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    OnlyHuman
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    Of Bows and Torsion Engines

    Post by OnlyHuman on Mon Jun 12, 2017 8:22 am

    I didn't know where to post this, and this part of the forum seemed the most fitting. I apologise if you think this is unrelated to this particular board.

     After trying to understand how to maximize a crossbows effectiveness I've came to the conclusion that torsion engines are the way to go, assuming you can call whathever it is you made a crossbow after that. 

     A short search had me stumble onto two very impressive desings from very seperate timelines: 

     Talisman's Mantis and the manuballista.

     Seeing both are effectively reliant on torsion engines I've got convinced that I was on the right path. However I questioned the use of the static limbs used on the tradional and handheld ballistas and apperantly on Mantis. My reasoning is that if you can store enough energy in the engine itself it will be very hard to turn due to the stress. Becoming some sort of an anchor -a riser, if you will-  that stores useable energy and allowing the bow limbs to bend and store even more energy. 

     The obvious problem could be the torsion engine breaking apart due to extreme stress. However I think that can be managed with the modern materials. What truly worries me is that the possibility of the forces cancelling eachother out if the force vectors are not aligned, effectively making the mechanism useless and ineffective.

     I realize that this isn't anything traditional, but as I've said: I didn't know where to posts this. Do you think it is possible? Have you seen anything like I've described? Am I overthinking it and it would simply not work due to logical reasons?

     TL;DR: Can you combine torsion engines with bow limbs to shoot faster projectiles?
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    Re: Of Bows and Torsion Engines

    Post by kenh on Tue Jun 13, 2017 5:05 am

    The Mantis is not IMHO a torsion engine --- there is no cordage being put under tension (the basic definition of a torsion engine).  It is a crossbow implementation of the common modern compound bow technology, which allows shorter, more rigid limbs and gives relatively high draw weights without as heavy of  full draw/cocked holding tension.  The Mantis has weight minimizing open strut limbs which bend to store energy, not cordage under tension.  There is a separate section here for compound crossbows.

    A Manuballista is the hand-held version of the late-Roman torsion engine called the ballista.  It had short, non-bending arms, and depends solely on the energy stored in twisted cordage for its power.   There is a separate section here for torsion engine crossbows

    I don't believe that, even with modern technology and materials, you would gain any significant amount of energy storage (say 10% or more)  by trying to combine bending limbs and torsion power.
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    Re: Of Bows and Torsion Engines

    Post by OnlyHuman on Tue Jun 13, 2017 5:15 am

    I've noticed that Mantis' limbs do not bend. I reckon that it employs thick, tightly coiled springs as torsion engines. That is why I called it that.

     As for your answer, I thought that might be the case. However, I still wonder how insignificant is insignificant? Perhaps it is enough to give it an edge? 

     I guess I'll try and see. Though I am guessing that will take time.

     Thank you for your answer.

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    Re: Of Bows and Torsion Engines

    Post by jocky on Tue Jun 13, 2017 2:46 pm

    The Talisman crossbows seem to use compressed gas as a spring (they don't give a lot of detail about it) : http://www.talismancrossbows.net/promise.html

    The limbs are apparently completely rigid.
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    Re: Of Bows and Torsion Engines

    Post by Anatine Duo on Tue Jun 13, 2017 5:55 pm

    I agree the Talisman appears to have rigid limbs.  I guess in a loose interpretation of "torsion" one could say that because the limbs rotate around pivot points and the force applied by them is torque then it is a torsion engine, BUT the gas spring providing the force is very linear so that part is definitely not a torsion engine.

    Can a torsion-style launcher benefit from working limbs as the arms?  Maybe... but let me ask another question... can a conventional bow benefit from a stretchy string which might help store energy?  The answer to the latter seems to be "no" which is why strings and cables get made of materials with higher modulus of elasticity (less stretch) ... but I can't really explain why this is so.  Maybe the mass of a working string robs energy from the projectile?  Would the mass of a working-limb-style arm on a scorpio be more than a deep, rigid beam style?  I think so... see 2 by 8 rule of beam theory.

    ...but I'd love to see it work.  Seems like it might require another level of tiller, fun
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    Re: Of Bows and Torsion Engines

    Post by Geezer on Tue Jun 13, 2017 8:14 pm

    I am under the impression that a stretchy string reduces the amount of bend you get from the prod... and in most cases, one gets less power from a stretch string than from an equivalent movement of the prod.  This may not be the case in all instances. Given the substantial power and efficiency the slingshot guy gets out of latex bands, there may be cases where a stretchy string and immoble prod is the best way to go.  Geezer

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    Re: Of Bows and Torsion Engines

    Post by 8fingers on Tue Jun 13, 2017 11:15 pm

    In 'The Traditional Bowyers Bible' they cover the effects of different tillering, and having a flexible arm in a manuballista is like whip tillering a bow, reduces power, and can lead to premature failure of the limb, would require both to be tillered, and would add another variable in tuning an already complicated mechanism. Stretchy string can also rob power as you loose energy in internal string friction and more bizarre phenomena.
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    Re: Of Bows and Torsion Engines

    Post by OnlyHuman on Thu Jun 15, 2017 2:22 am

    Thank you all for your answers. Quite a lot to take into consideration. Especially the strechy string point. 

    I'll continue drawing on possible ideas. In the hopes that I can get a reliable plan.

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    Re: Of Bows and Torsion Engines

    Post by c sitas on Fri Jun 16, 2017 2:55 pm

    If you take the time to check , the talisman is not considered a "Crossbow". It shoots an arrow or bolt, but is not accepted as a bow by any organization that I'm currantly aware of.It's fast ,it's very accurate. There is a lot to be said for it but, being a bow is not one of them.

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