…..so anyway,  that was exciting.

Unfortunately I didn’t have video rolling.  The winch worked fine.  However my operation of it left quite a bit to be desired.  I decided to proceed without using the dynanometer.   When the winching became harder I automatically assumed the new winch was not up to the task.    A pop, a ping,  followed by a final rending Kraackk!,  and in an instant, the port limb was a goner. The winch was working fine and I had forgotten that the tensioning on the bundles was already fairly extreme from when I had worked on her last.

I believe the heavy winching actually indicated that the springs had hit a wall and the pressure was spiking in  a way I had never seen before.  It would be easy to clobber myself endlessly for not monitoring the draw weight more closely, but  I have a better idea.

There is no reason to suppose that breaking  one of these limbs is actually a bad thing.   On a superficial level it may be true that limb breakage is never good, but there are factors that actually make it a worthwhile experience.  The whole system: the field frames, the kamarion, the trigger assembly, the stock, the winch, all of it,  sustained what in essence was a proof test load with flying colors.  Even the precious springs looked fine after I unwrapped them for inspection.   This means, that apart from the limbs,  the rest of the machine seems robust and well balanced in terms of overall strength.  I am guessing that my draw weight actually topped 7,000 lbs.  It doesn’t take long for poundages to spike after hitting the wall.   It may be that the springs need to be a couple of inches longer.   That would confer less of an exponential curve on the draw weight.   However, there is no need to go down that road yet.   Some new limbs and a solemn oath to never again venture into the unknown without my trusty dynanometer, will do for now.

Besides, I’m glad they broke.  They always were flawed little piggies.   Destruction provides manure for new growth.  We just hold our nose and smile.  It’s all good.

8 Responses to “The art of the better idea.”

  1. Captn Harpoon says:

    AHA – got ya. This means you actually dont read all the comments or take them seriously. Dont blame ya as I do ramble. Yes, more rotation equals longer bundles as we discussed a year or so ago I think.

    Ya gotta get with the zero point energy system, the benefits are the extra rotation. Again,no pretensioning the string to any degree, no pretension rope bundles to any great degree. Dump a few layers of rope to lesson friction,and keept it to 5000lbs at full rotation of a full 120.

    If you want pretension, keep total rotation to a limit of 105. Drilling holes through arm is accident waiting to happen. Reminds me that of my younger years – like staying in a bad marriage no good can ever come of it.

    Got a good one now think I’ll keep her LOL. Thanks for deleting and cleaning up all comments.

  2. Pat B says:

    Nick, what angle had you reached at the moment the arm split?

  3. Captn Harpoon says:

    Hey Pat B.

    Glad you are still around. I copied and pasted the results of the arm/string velocities onto my blog for further study and data capture. I will probly re-post it to romanarmytalk.com for other fellow ballista freaks. That way the info. wont be lost to all and out to the greater community.

    Nick, did some retesting yesterday of the impact accel tech. I developed for Firefly a gooooddd while back. At that time the arms were still in the orig. 12 oclock rest positon.

    The impact system works well enough for THAT particular design, giving it the SAME capabilities as the extra rotation assigned to her more recently.

    Applying the impact tech to the extra rotation capabilities is pretty much a mute point without also assigning extra draw length to attain previous or increased rotation capabilities. ONe cancels out the other without introducing a variable.

    Bottom line is that adding the extra rotation capability seems the best and easiest way to go (simplistic).

    I have had my doubts for some time that the impact accel. tech. would work as designed on the shorter/lighter arms I use, as it was originally designed for those arms that just broke, which leave only the width of the tiller or rail between the two points.

    The most useful conclusion I came was that pretensioning the string and spring had NO discernable performance improvement over my zero point energy theory.

    The only thing it really did was to increase stress and component shock, which became more and more noticeable as tension increased. I would not want to dry-fire it that way as damage is bound to occur somewhere and over time.

    So, although the impact accel tech did work as designed for the old Firefly arm position, but the new arm position negates the velocity increase element (equal to), so thats its highest use remains as a brake for the arms, replacing the stanchions or stops in useage.

    The zero point energy systems for the most part eliminates the need for stachions or stops altogether, so again I have rendered it a mute point for Fireflies current design.

    This is where I again study Pat B’s math and get put on the tin foil cap again.

    Well, time for internet seminar again, so catch ya later.

  4. Pat B says:

    I’m not up to speed on “impact accl tech”. What’s that?

    Post my results by all means, but do be careful to point out that those are not absolute velocities. They are velocity ratios. If the figure is “200”, it means the missile is going 200 times as fast as the arm-tip. But we don’t know how fast the arm-tip is going. It could be slowing down at that point – quite possible. In fact, with the most efficient machine, that’s inevitable. Any velocities left over in the system after the missile takes off is wasted energy, so in the most efficient system, the acceleration of the missile will bring the rest of it to a standstill. You’ll never see a machine that actually achieves it, but that’s the ideal scenario.

  5. Captn Harpoon says:

    Nick – not completely sure what the mirror post really means, but I do know the broke arm means that we cannot conclusively defeat Mr. Harts assumption the performance gain is or was due strickly to a longer draw, or that the extra rotation caused the potential for a longer draw.

    I was pretty sure you used the same amount of draw for both tests, the only exception was the intial rest postion. We got a 22% gain, and Firefly was not even cranked up to full rotation.

    Since it appears the arms will retain original length thats okay too. By June or July and several rounds of testing perhaps you will come to the same conclusion about the arms as you did for the frame. Metal is inherently stronger.

    In just about all depictions on stone, we seem to have much longer spring bundles, and much shorter and stouter arms. Like a couple of thick stout paddles about four inches in diameter, round and with NO taper. I’ve yet to see a picture of a stone carving showing tapered arms.

    5000lbs pull is a lot, and it sounds like you are gonna bump it up some.

    Pat B: – the impact accel. system is something I came up to juice up Firefly performance using the original rest position of the big long arms (the broke one). A lot of energy is tied up in getting those arms up to speed and at the end of the stroke is just thrown away. If you convert the inertia (transfer movement)to the projectile at the end of the stroke acceleration should occur.

    IT is the same effect as placing a lever over a fulcrum, putting a projectile on one end, and then dropping a heavy object on the other end.

    HOwever, since Nick wasnt hip to that, I had to devise an alternative way. In studying the Orsova frame at some length I came to some conclusions, and the new rest positions of the arms is the result. The extra rotation has the same effect as the impact acceleration has, so unless increases the terminal speed of the arms, there is no need to use it. He should be able to attain well over 400fps with the planned set up and with the current length arms.

    I have pointed out to Nick the benefits of a ‘zero point energy system” meaning as much energy as possible is utilized, with zero potential left (remaining amount of torque). This means starting off with zero energy as well – meaning little to no TWISTING or torque in springs at rest position. Any tension in spring should be vertical only.

    This is also where the impact accel would occur, making use of the remaining energy in the arms inertia or momentum.

    I have devised a theory based on past observations and theory that a loose string (not spring) not under any great tension at rest, at the end of the stroke causes the stop or stanchion to behave as a simple fulcrum (arm inertia), which in turn forces the spring to act as an arm brake,helping to arrest movement, and restore it to its original rest position (memory).

    When this occurs at the very end of stroke and for just the briefest of a microsecond, the arms axis shifts position to the stanchion. At this time the arm SHOULD act like it suddenly lost a portion of its length. If this is so, then almost instantanious acceleration has to occur according to the amount of energy left, and stretch of the string.

    When you combine the above with your findings on the last few degrees of movement before string is completed stretched (infinite acceleration), a fine tuning and combination should be able to enhance current expectations regarding performance.

    If Nick opts to use the extra rotation capabilities, it also means that the string undergoes a reversal in directions under power. However 15 degree is not a lot, BUT, it should be expected that this system will attempt to generate a sine wave in the string at end of stoke.

    It basically duplicates the actions of one “cracking” a whip although any great movement is restricted, the action is none the less there.

    The impact acceleration tech I further developed for Firefly is no longer nec. with the new arm positon, but for a crossbow sized machine width is a concern, and its use allows for a narrower config. providing the same effect as the extra rotation.

    Since I have ways to accelerate the arms a fair bit, it is more useful to me at this point. If Nicks new proposed arms are to last any time whatsoever, then he needs to consider the zero point energy stuff, where pretensioning is not as important as the amount of rotation Firefly can achieve with the same bundle, as it appears he is not gonna change that either.

    In conclusion, it may never be we will never see a machine that is able to use 100% of potential energies generated by a single pulling and release action, BUT, I aim to come as close as I can get.

    The final pieces I am working on is the energy left in the string inertia, and working with the resulting string sine waves to extract as much energy as possible from every component.

    Again thanx for the valuable input and data you provided. together it and the other research will certainly help my amateur efforts.

  6. Pat B says:

    You’re welcome. But don’t read more into those figures than they deserve. I posted them in response to a question Nick had raised in a much earlier blog, and to confirm that his arrow is bound to be accelerating relative to the arm-tip as a matter of pure geometry/trigonometry.

    About the shape of the limbs, I don’t know which stone bas-reliefs you’re referring to, but the one on Trajan’s column shows no limbs, tapering or otherwise. But those are in any case ornamental depictions, not engineering drawings. Tapering has been standard engineering practice since time immemorial. The hand-bows of the most primitive tribes are tapered. Where there are torque forces, the parts closest to the centre of rotation need to be the strongest, and thickest, because that’s where the biggest forces are. Conversely, the parts furthest from the axis are moving with the highest velocity, and need to be as light as possible. Tapering solves both problems. Here is a modern re-construction of a Roman stone-thrower:


    Quite rightly, the constructors have fashioned a tapered arm. The Romans had hand-bows long before they had ballistas, and for them not to have incorporated this standard technique would have been perverse.

    It looks like your “impact acceleration” idea is based on transfer of momentum from one part of the limb to another. The inner end is brought to a halt when it collides with the stanchion, and the outer end briefly speeds up. There might be something in the idea (though not a lot), if the inner portion of the limb was free to move, to swing suddenly inwards. But it will be decelerated instantly by the bundles. The final result is likely to be less velocity at the tip, not more. Besides, this is all taking place in that region that I blanked out in my figures with the word “indeterminate”. It’s a time-slice so small that nothing can be calculated within its borders. The timing required to catch the limb at exactly the right moment is such that the chances of doing so is remote. Similarly, your idea that a sine wave is generated in the last micro-second which might give a boost to the velocity falls victim to the same thing, only more so. A sine wave in a string that is very nearly taut is an extremely fast phenomenon, and consists of troughs as well as peaks. And a trough, of course, would have the reverse effect. The last instant of contact between arrow and string could catch either a peak or a trough, and it could even vary from shot to shot.

    These are all interesting ideas, but peering into the events of the last time-slice before the arrow leaves the string can yield up only tiny percentages. I think there are more fundamental issues to be addressed with Nick’s machine in terms of drawing more from the energy store than he is currently doing, which could yield much more.

    (We might also be in danger of flooding his brain. Information overload…)

  7. Captn Harpoon says:

    The sine wave generation is dependent upon a number of things built into the design, which is very different from NIcks,but using the same basic design.

    Nick uses a three inch diameter spring, I use 1.5″ eccentric lever which is the equivelent. Next I will replace the eccentric with a 3″ pulley or maybe cam it.

    For now there is no need for further speed. Nicks non compounding inswinger will get 400fps plus. My simple arrangement automatically doubles the string speed to arm speed ratio.

    Nick and I have gone over a few ideas together regarding the spring bundle,but I suspect he is getting tired with never ending rebuilds. His goal was 400 and it is now within his grasp. Even with those 7500 grain harpoons he uses to appease my constant harrassment. I still think he should bump it up to an even 10,000 grains to match the rise in power he will be getting.

    Well as far as getting more out of his current energy store, that is what the extra rotation is all about. YOu have done the study and have the figures.

    By advancing the rest position of the arms 15 degrees, the dynamics change. So instead of starting at the usual 90 degrees where arm tips are closest, you do the same 90 degree computations but ending up in the new arm position.

    If your figures do not tell give you a gain in velocities of at least 22% as already proven, then…???

    It appears I am the only one who is going with the short arm to width thing. Not even NIck likes it LOL. None the less I have done the testing. While some may argue the wider spacing is to allow the Orsova to be inswinging or some argue outswinging, I go alone on my arm length to width between bundle theories.

    All I know is that if I want my Scorpio to be faster, without changing arm lengths, is to either add more power, or add more draw length.

    Since I neither want to add on the length of the machine, I need to shorten the arms, and move the entire riser forward an inch. Only then will I be able to make use of 130 plus degrees of rotation, and take the machine to full draw.

    There is no other way, unless to play with the amount of energy or draw weight. NIck said 5000lbs was the limit he wanted, so thats what I design for.

    BTW, when you back off the rope bundle tension, you introduce more play into it, so that it allows a greater sideways movement before bringing the arm to a halt and returning to rest position. Remember a half inch at the bundle is a lot at arm tip. Happens all at the end – magic spot. It doesnt have to last long, but it happens.

    The sine wave thing would not be noticeable at all or shouldnt be on NIcks machine and set up. If you dont think a sine wave happens on just about every crossbow or ballista, then I invite you to go to the Crossbowreview website and watch a few vids in slow motion.

    There are inswingers, recurve, and regular compounding bows. Amazing footage showing main stress points and what happens at end of stroke slow down a hundred or more times.
    Most often it travels along the string starting on one side and going to the middle. At least thats the way it looks on the videos where they mostly show only one side.

    The sine wave might be coming from both ends towards the middle which I believe it does. If so, then I should be able to employ that last remaining energy to the arrow or projectile. In this case, Nicks string might weigh half that of the arrow. So we have more than one force at play here as well.

    Anyhow, wifes due home and time to put the kettle on. NIce chatting with you. I know NIck would enjoy seeing how the arm speed to string speed ratios for the new arm position would fair side to side with that of the regular arm position. Both sets using 90 degrees of rotation.

    Dont know what your math has to say about that 22% velocity gain the new arm position coughs up?

    Bye for now.

  8. Pat B says:

    Just a couple of points. You say:

    “Well as far as getting more out of his current energy store, that is what the extra rotation is all about. YOu have done the study and have the figures.

    If your figures do not tell give you a gain in velocities of at least 22% as already proven, then…??? ”

    My figures dealt with limbs stopping at 12 o’clock. They said nothing about the consequences of a further 15 deg. But without even referring to any formulas, I could predict an improvement, at least on Nick’s machine. And the reason is one I’ve given in another comment – namely, the benefits of a longer bowstring. An extra 15 deg means a bowstring that is 25% longer. That would give a better velocity ratio in the last few degrees of movement.

    What I wouldn’t expect, though, is 22% improvement. That’s unrealistic. I hate to pour cold water over the experiment that Nick did last year, but it was a one-shot experiment at the end of a series of shots he made to test what happens with a shorter limb.

    He removed the inserts in the stanchions to allow the extra rotation, but didn’t make any adjustments to the tensioning. And in determining the cocking angle, he used his dynamometer as the guide, stopping when it was at 4,800 lbs, the same as it was on the previous shots. With the tensioning unchanged, it means the cocking angle must have been roughly what it was before. From the photo on that page it looks to be around 45 deg. What that means is that in the earlier case the arm had travelled through just 30 deg before it hit the acceleration-rich zone of the last 15 deg. In the second case, it had travelled through 45 deg – 50% further – and would have had a considerably higher velocity going into the last 15 deg. That alone probably accounts for 90% of the improvement.

    And on some other machine, I might have been tempted to say it accounts for ALL of it. Because there is no guarantee that a longer bowstring will boost the velocity at all. It depends. The higher velocity ratio you get with a longer string exacts a price in the form of a higher inertial resistance in the missile – i.e. it becomes more effective in slowing down the arm. Where the energy source is limited, that could result in a loss of performance rather than a gain. But Nick’s machine has more energy available than he’ll ever be able to use, so I’d expect a positive result in his case. Anything he can do to extract extra from the huge amounts that are being wasted at present is an improvement. It just wouldn’t be anything like 22%, that’s all.

    Your shorter-arms theory – one of Nick’s main concerns must be the integrity of his machine in the face of some giant forces. He’s already buckled his kameiron once. In another comment I predicted a 50% increase in stress during cocking if he were to adopt your idea of limbs of 2/3 the length of the present ones. That could be enough to bend the kameiron double.

    During the same experiment I’ve just discussed, Nick gets a hint of this. Having just shortened the limbs by a couple of inches, he observes: “The draw weight in this new position climbed to 4800 lbs, up 200 lbs from the 4600 lbs registered with the string in its normal position.” That should act as an alarm bell.

    The second reason I wouldn’t advocate shorter arms is that I believe it would result in performance loss. The wasted-energy situation I mentioned before would only be made worse. I think we need to go in the opposite direction.

    But the idea you came up with somewhere of having the bowstring coming round the top edge of the limb is a good one, if Nick could find a practical way of doing it. It would use the length of the limb to the max, which is all to the good.

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