Guillotine Pie

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Guillotine Pie

Postby Ripcord » Thu Mar 20, 2014 1:26 pm

Readers of the old CATMESS forum may recall the Guillotine design I created. To my knowledge it has never been built but I still consider design changes in response to my mental image of its operation. This has brought to mind a major problem. Pie.

Briefly, the Guillotine allows for nearly maximum Potential Energy (PE) to be generated by a given Counter Weight (CW) mass. This energy is then used to rotate the Throwing Arm (TA) at a very high rpm rate. Additionally, the TA falls along with the CW, until the TA is forced to switch over from a purely vertical motion to one that is horizontal and it is during the start of that transition the the TA begins to rotate.

Given that 'punkins' are a common payload, the problem is the very high acceleration forces created during the TA rotation. Punkins squish when too much pressure is applied to them. There are two solutions to this problem, discounting freezing the punkin or other modifications. The first is to recognize the maximum acceleration obtainable by the punkin without destroying it, then dialing in the tuning of the machine to stay under that limit. Although a general number can be obtained, there are such vast differences between species of punkins and even individual ones within a species, that this method leaves much uncertainty about the possible outcome. Still, it is very possible and if one has a generous supply of punkins available, I would expect great things to happen. If, however, one is attempting a maximum range then almost certainly the acceleration rates will exceed the limits of the punkin. Pie.

The second method requires a re-design of the machine itself. In order to reduce the acceleration rate while still obtaining the highest escape velocity possible, one needs to lengthen the acceleration time. For the Guillotine, this means starting the TA rotation at an earlier time and making the vertical to horizontal transition take place over a longer period as well. Both of those elements would require a major redesign effort. The longer rotation period will also require a longer Sling, further complicating the redesign process.

After a while I began to realize that the redesigned Guillotine was running into another problem, air drag. Having the TA begin rotation earlier than with the original design, air drag will begin earlier as well and that will probably mean a notably reduced rate of CW fall. That means less energy for putting punkins through the air.

I've also been envisioning various scenarios, one where the TA rotates only once and another where it rotates multiple times. I've also been considering methods where the rotation rate can be modified during the entire (or even partial) sequence of motion. It gets complex!
When one is dealing with a gravity powered machine, where the PE can be easily modified, then the design process must take into account numerous factors that are usually not found or are at least fixed variables in other types of catapults. I am NOT saying that the others are easy to design! (Been there-done that, even got a few T-shirts out of the deal!) Only that gravity machines have their own idiosyncrasies.

It's maddening, but fun.
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Re: Guillotine Pie

Postby Wheelman » Sun Mar 23, 2014 9:10 pm

The main problem with trying to design around the exact limitations of the pumpkin is that pumpkins are not at all equal. Even if you could find the exact limits of one, the next could be less than half the strength.
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Re: Guillotine Pie

Postby Ripcord » Mon Mar 24, 2014 9:36 am

Precisely. This is one of the reasons competitions can be rather sporting, one can never be sure of the outcome, despite all the efforts put into designing, building and even testing. I would say that most spectators don't appreciate this particular aspect of the sport but Hurlers sure do!
For non-competitive hurls, I would favor 10 pound bowling balls. Having a payload with a very consistent size, shape and weight would allow for more precise tuning and a good chance to find out what a given machine is capable of doing.
But the uncertainty of 'punkins' does add an element of suspense to competition that makes it a better spectator sport.

I would point out that bowling balls are not immune to damage or total failure. If you think getting showered by fragments and juices from an impacting 'punkin' is funny, try an exploding bowling ball. They can be lethal.
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Re: Guillotine Pie

Postby madmattd » Wed Mar 26, 2014 12:27 pm

I'm curious what circumstances led to exploding bowling balls for you Ripcord. I can see hitting a rock on landing might cause a problem, but surely not launch-time pie?

I ask because we've talked for a number of years now about launching bowling balls, and I anticipate actually doing so this year.

Also, I would love to make a Whistler like Capt. Bob did: http://www.youtube.com/watch?v=1BPa_nOiK4A


On another note, I'm still intrigued by this Guillotine concept that you've been playing with. So very similar to something a couple of us on the team were looking at pre-NASAW, but we never figured out how to deal with some of the wierdnesses that you have (plus the NASAW design concept came up and took over very quickly).
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Re: Guillotine Pie

Postby Ripcord » Sat Mar 29, 2014 2:34 pm

Sorry Matt, I've been away all week.

Bowling balls suffer weathering fairly badly, more or less depending on what material is used, plastic, rubber, etc. Keep your bowling balls in a protected area until use.

On firm ground, even a smallish rock can chip or crack a BB, even though the impact my drive the rock several inches into the ground. I've not seen a BB suffer large damage from hitting a tree or tree limb, but that may be due to the whimpish trees of my experience. (Popular for the most part.)
The most catastrophic results came from bedrock or simply large rock encounters. The rubber BB's usually developed large cracks but the plastic ones could explode, sending razor sharp fragments flying. Regular bombs going off.

BB's are all the same diameter, per regulations, but vary in weight. The result is that the lighter 10-12 pound BB's will float (barely) while the heavier ones will slowly sink. If your hurling into ponds, rivers or lakes, do not expect to recover your ammo.

I have never seen a BB suffer from launch acceleration nor from a accidental impact with the machine frame. Bear in mind, most of my frames were wood and not reaching nearly the velocities of NASAW and the like. I believe you would not be able to cause a BB structural failure during acceleration.

Used BB's typically come with 2 or 3 finger holes. Even one is sufficient for the whistling effect. This is nice in that it gives you an audio cue as to the rotation rate of the ball.
A BB with numerous holes spread around the surface certainly sounds wicked but detracts from the human ears ability to detect the rotation rate.
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Re: Guillotine Pie

Postby madmattd » Sat Mar 29, 2014 6:37 pm

Ripcord, I dug out the old Working Model sims I've hinted at and grabbed a couple screen caps for you, since we apparently had very similar ideas a year or two apart (I don't remember when you first posted about Guillotine on TheHurl. Man I wish that data was still online and available!). These are dated late 2009/early 2010 and started life as an experiment to make an infinite drop Merlin-esque design without the re-climb of the J-track at the end. ChrisG had his reasons for what he did, but we wanted to try anyway. The transition element was never fully developed, though we messed with ideas for a few days. We did try some rather cooky ideas with compound curves at the outlet of the main curve and stuff along those lines, but a trap door never occurred to us that I can recall. We only messed with this for a couple weeks on the side, and pretty quickly we latched onto the vertical tracks for a floating whipper, and NASAW was conceptually born.
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Pre-launch. Towers could be as high as one could build, independent of arm length.
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Firing action trace
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Thanks for confirming my suspicions about bowling ball launching. They'll live inside mostly, and I wasn't figuring they'd pie on launch, but I am concerned about rocks on landing. We will see what happens!
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Re: Guillotine Pie

Postby Ripcord » Sat Mar 29, 2014 7:50 pm

A quick look at some of my notes indicates around 2009, for my first drawings. I'll have to dig deeper to find anything prior, not that it really matters I suppose.

The drop off the end of the curve is something I thought of has well but quickly gave it up and too much of a stress multiplier and there was a timing element involved as well.
If the TA axle did NOT drop to the lower track in a timely fashion, things could get very ugly.
I then went to a dual-axle system, where the TA translated from the vertical/curve transitioning axle to a horizontal transition axle. That was a pain to figure out but I did get it eventually.
I gave up on it though for a variety of reasons, complexity among them.

I finally settled on the gated method, taking the relative slow motion of the TA axle at that time to advantage.
If you wish, I can try to pull together some details and pass them along to you. Must admit, it has been a while since I've played with it and I'll have to recall some of the reasons for certain aspects of the design.
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Re: Guillotine Pie

Postby madmattd » Sun Mar 30, 2014 1:36 pm

Apparently my memory is bad then. It doesn't matter though, like you said, and you've done the heavy lifting conceptually anyway.

The drop off the curve is definitely not a great way to go for all the reasons you mentioned: a major stress hit, too much potential for disaster if there's something slightly out of whack during the drop, etc etc. The hinged gate should allow you a fairly smooth transition. It would be interesting to see one of these made, but I don't see us doing it, we need another guinea pig :)
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Re: Guillotine Pie

Postby Ripcord » Mon Mar 31, 2014 8:14 am

More likely a fatted pig.

I keep my eyes open for possible materials for at least a small scale version. Something in the 2-5 foot height range for the horizontal rails would be nice.
Any smaller then that and I believe the action would be too fast for my naked eye-balls to follow. That and the payload selection would get a little weird.
Spring is peaking around the corner (despite the snow in the yard) and the itch to build is growing.
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Re: Guillotine Pie

Postby Ripcord » Tue Apr 01, 2014 10:04 am

One design element with the Guillotine that I wish to change concerns the TA carriage. The current design has a solid carriage 2 trucks, one left and one right for each rail. Each truck carries 4 wheels, in the current configuration at least. 2 problems arise with this arrangement.

1. In the passage through the curved section of the track, only the leading and trailing wheels provide support. (Think of a line tangent at both ends to a circle.) This at a time when the carriage is starting to endure significant loading. Not good.

2. Once the leading edge of the carriage triggers the trap, the trap is trying to rotate upward. Meanwhile, the rest of the carriage is still trying to pass through that same area, with the trailing wheels attempting to push the trap back down again. Very not good.

The solution that comes to mind is to include hinged trucks. A pair a trucks, each with 2 wheels, with each truck hinged at one end of the carriage assembly. This would keep the loading distributed over all the wheels and keep a load on the curve trap at all times during the carriage passage. I'll try to draw this up at some point and post it in the Guillotine details gallery.
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