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Basilisk120
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    "The Entropy Slayer 2000": Revolutionary Sling-X-Bow

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    Post by JoergS Mon Jan 23, 2012 3:08 am

    As you may know, rubber suffers from the "hysteresis" effect. Means, it looses power quickly when you keep it stretched. This is a big disadvantage if you want to use it as a source for a crossbow.

    I decided to find out what is going on. So I rented a thermal camera. Turns out heat is the key! Rubber stores the energy thermally. So the bands warm up very much when you draw them out (about 15 centigrade difference, in a fraction of a second). They cool down just as quickly when you release.



    Wikipedia confirms these findings. It is an entropy thing. A basic law of physics says that if you bring order into the chaos, heat accumulates, and when things swing back to chaos, things cool down. That is what happens, by pulling out the bands you align the rubber molecules, moving them out of their chaotic relaxed condition.

    The scientific information I gathered using the thermal heat camera lead to more experiments: What happens if you heat a stretched band, one that already cooled down to room temperature?

    It regains the power it lost during the cooldown phase. That is right, this video has the proof.

    Now I had to use this critical information in order to build a slingshot crossbow that can heat the stretched rubber with the
    flick of a switch. The result looks bizarre and very uncenventional, but it does work very well.

    Three heating films heat up the interior of an encapsulated band box. Using a strong 3300 mAh rechargeable battery, the bands can make the bands about 20 centigrade warmer than the environment, adding (or, better, regaining) about 40%
    energy.

    This weapon can be left cocked for a long time, and then - minutes before action is expected - be brought to top performance.

    A nice side effect: The compartment also works as a silencer, it traps the sound as well as the heat. Very important for effective Anti Zombie sniper jobs!

    Two new slingshots are also presented as a bonus.

    mac
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    Post by mac Mon Jan 23, 2012 2:46 pm

    Joerg,

    You are a genius. It is always a pleasure to see what you are up to.

    Mac

    (edit for spelling)


    Last edited by mac on Mon Jan 23, 2012 8:02 pm; edited 2 times in total
    Basilisk120
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    Post by Basilisk120 Mon Jan 23, 2012 7:18 pm

    Glad to see you back Joerg.

    Those are some nice videos that do a good job of supporting your theory. I'm going to have to investigate this further. Going to to have to borrow one of the thermal cameras from work and some rubber bands.



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    Post by Ivo Mon Jan 23, 2012 7:30 pm

    Joerg,

    As always...bravo! Smile

    I've wondered about that bit myself, but never to such extent as to getting the lost energy back. Definitely one of my favorite vids now...by the way...just realized I haven't subbed to your channel yet. cyclops

    Keep your camera rolling Joerg and good luck with that Doctorate. Wink

    Ivo



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    Post by arthurcnoll Tue Jan 24, 2012 2:18 am

    Very interesting, Joerg. One thing on theory, though. There is a quiet revolution in physics going on with ideas about order-disorder and entropy. Very briefly, you haven't increased the order in the bands by stretching them. It seems that way unless you think about it more closely. You have long molecules in rubber that are crosslinked at random positions. In a resting state these long molecules are coiled and bent on themselves in a random way, true, and when you stretch them out it sounds like that would be more orderly- but in fact what you have done is make them start vibrating and jerking and affecting each other like demented violin strings- there is a lot of chaos going on. That is why the rubber heats up. More chaos than before, order has not increased. Where a particular element in a particular molecular chain might be in relationship to the others, which is how you look at order-disorder, has a greatly increased number of probabilities with all this motion. And if you imagine holding on to a rope tied to something- like a jump rope- and then imagine someone swinging that rope, that swinging will increase the tension, right? Same thing for all the molecular chains in the rubber. The cooling comes from these vibrations reducing, energy dissipating, and the overall tension reduces. Heat it back up, the vibrations increase, and the tension returns. It is actually a simpler way of understanding what is going on... Unfortunately it flies in the face of what has previously been taught, the wiki article being an example, but concepts about entropy-order disorder are already being taught differently in some places. Changes in thought like this are seldom neat, as people previously taught one way can have a very hard time letting go of it. But it is happening. There is a professor of chemistry named Lambert who has been quite influential about spearheading this change of thought. He says at least twenty new textbooks have been changed... If you want you can do a search on him and possibly get more information than you really wanted. ;-) I am an engineer, but wasn't taught this, I was taught the old way about order-disorder and entropy. I started seriously questioning relationships between order-disorder and entropy on my own back in 1990, but with regard to other topics, not rubber, not chemistry so much, either. I was interested in how order-disorder related to questions about life... I don't know if Lambert talks specifically about rubber, either, though he might. Last I knew, he wasn't at all interested in relating it to life, either. But for me it is all similar, just a matter of understanding basic concepts of how things move.

    I know this is really going off topic and I'll understand if the moderator doesn't want it to stay. I am interested in crossbows, have made a bunch of them over the years with some unique designs, but never really satisfied with them. Constantly changing things. My last design seems to have solved a number of previous issues and if that proves out, I hope to post something about it eventually. I've been busy with the implications of this understanding about order-disorder and entropy in most of my free time since 1990 and don't spend as much time on crossbows or other projects as I might otherwise do. The word "entropy", obviously grabs my attention because of that. I'm also not much into rubber slingshots, but you certainly take them down some interesting paths, Joerg.

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    Post by stoneagebowyer Tue Jan 24, 2012 2:21 am

    Joerg, this new topic and the video is immensely interesting, as all your projects are. I am grateful you are sharing them with us. The Slayer looks like a very, very fun weapon. Zombies beware!

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    Post by jeep Tue Jan 31, 2012 2:02 am

    Hello Joerg
    Brilliant demo lot more efficient then a long description. Your conclusions about the power returning whit heat is striking !!! I will have to deal whit this when I'll complete my in swinger crossbow . Thank for the research work.
    Jeep
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    Post by mac Tue Jan 31, 2012 5:38 pm

    It just occurred to me that this same phenomenon would apply to those hurling engines that used pistons to store energy as compressed air. I'll bet those things lose power as they sit with the pistons cooling. Likewise, warming them back up might bring them back to full power.

    I both cases (rubber and compressed gas) I wonder if heating them up beyond the amount that they heat themselves would add usable energy to the system. Obviously, there is a limit to how hot you can make rubber before it breaks down. Likewise there is a limit to how hot you can heat your pistons before the packing degrades.

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