Guillotine Pie
Posted: 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.
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.