Dogapult

Everyone knows about Catapults. I have built about five of them in my life. As far as I know, I am the ONLY person to have built a Dogapult!

Around 1973, my two dogs, Phobos and Deimos, were each around five years old. They were very sturdy hunting dogs, Phobos being a Black and Tan Coonhound and Deimos being a Redbone Coonhound. Deimos absolutely hated water and even rain, being somewhat of a sissy in that regard. I have been around several dozen dogs during my life, and all but one of them loved getting wet. Deimos was the exception, and she was serious about it! If there were puddles on the ground, all the other dogs would run through them, but Deimos would carefully walk around them! If there was any rain when I let the housefull of nine dogs out for a run from our farmhouse, Deimos would screech to a stop as soon as she saw the rain, and she would apparently decide to "hold it" for a few hours until the next time they got to go out! She was very well-behaved, so an occasional bath in the bathtub was tolerated, but her expression showed that she was NOT pleased! But she LIKED all the attention she got with a towel afterward. But Phobos was not remotely like that and he loved water and rain and swimming.

I had felt that cats had gotten far too much Press than they deserved (as I have never been a cat-person) and I decided to build a Dogapult to see if dogs might get some media attention!

It actually was NOT a traditional Catapult (which has the more correct name of Trebuchet). Instead, I made a rather crude teeter-totter, right next to a pond. Phobos was always extremely well behaved, and so when I had him SIT on his end of the teeter totter (facing outward, toward the pond, which was only a foot away), he did. And when I jumped on MY end of the teeter-totter, Phobos got sort of launched (maybe one foot upward as I did not jump hard and he DID weigh 62 pounds!) and he (safely) landed in the water. Later, and on succeeding days, I sometimes saw him sitting on the teeter-totter and realized that he wanted another ride! And this time, he even JUMPED to land farther out into the pond!

The thing seemed to have become FAR too important to him and I felt it necessary to dismantle the device a few days later. I sort of wondered if he had somehow known about the dog that the Soviets launched into orbit in 1957! And yes, he HAD previously had a bicycle helmet and sunglasses on as I took some photos of him SITTING on my small motorcycle, so he was used to humoring me!

As far as I am concerned, that WAS a Dogapult!


As to actual Trebuchets, the last and best two I ever made were in 1991. One was actually designed to Set Volleyballs, and so I designed a system that was GRAVITY-DRIVEN and not spring-based or such. Where all the Trebuchets which have ever been made have always had rather variable strength (meaning they were hard to aim), my designing of using GRAVITY to power mine made them amazingly consistent launchers.

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In fact, to confirm to myself that my Volleyball Setter was as consistent as I wanted, I set it up so it could launch volleyballs toward a basketball basket. As long as the ball was positioned properly (regarding the seams and backspin), it made thirty-in-a-row of perfect freethrows, all of which were perfect swishes! Not one even hit the rim in going through! My guess is that it could have made a thousand freethrows in a row!

So that proved to me that it would provide the PERFECT Setter for a practice hitting line for a volleyball team. It could set 1's, middle 2's, 33's, and even 11's and 12's (which few human setters rarely can set consistently). Unfortunately, my prototype of the device (my only defense!) was incredibly ugly looking and large and very heavy. I had welded it up in my factory, where the main part of it weighed close to 200 pounds, but then it also needed another 300 pounds of weights, to fall with enough speed and power to throw the volleyball. In the Conservation of Energy, there had to be a great enough weight drop a sufficient distance to give up the necessary amount of Potential Energy, to provide the Kinetic Energy of the launched volleyball. STILL, I considered that to have been a very successful test of my Robo-Setter!

That success inspired a very similar device to be welded up a few months later. It was of similar size but had even more weight to drive it! Instead of a volleyball, this one was initially designed to launch a standard baseball, but I found that there was a problem! The position of the seams on the baseball turned out to be unexpectedly important in the precision of where it would land. In just a 90-foot throw, a pitcher can make a baseball do a variety of effect, such as curve balls. Since my intended range of my experiments was to be about a city block or 500 feet, I found that the baseballs were landing over a substantial area. To improve that accuracy, I abandoned the baseballs and instead decided to try to use a standard can of green beans (frozen so the weight could not shift around inside it)! I arranged for my trebuchet to cause the can to spiral-spin the way a rifle causes bullets to spin (called rifling there), so the can would not tumble during flight and be more consistent in flight. I had an empty 55-gallon drum that was often used as a garbage can, which I placed in a field around 460 feet from the Trebuchet (about a city block away). I had to wait a number of days until there was a day when a brief period near sunrise had dead calm wind. It took several launches to learn where the 55-gallon drum needed to be placed, but when I decided to start counting, four cans of beans went INTO the distant garbage can (each of which knocked it over on impact with the inner back wall!) and the fifth hit the can but did not go in. I did this at a time when there was absolutely no noticeable wind, which I knew would have affected the flights.

The experiment was very time consuming since I had to walk to the garbage can after each time it got knocked over to stand it back up, so my attention span faded fairly quickly.

In any case, I felt that I had proved that MY Trebuchet was essentially five for five, to a target smaller in size than a hula hoop, from a LONG distance away! Around 2008, many years later, the PBS program NOVA decided to try to present Trebuchets. They seemed to collect a bunch of people who had NO IDEA regarding how to build one, or how to aim one, or even the very basics of ballistics! I thought that NOVA program on Trebuchets was quite amusing, as they constantly displayed total incompetence as they tried and failed many times to build anything that might even work, and then they clearly had NO idea of where the projectile might land! It was quite laughable as they kept making bad guesses about nearly everything! An actual scientist would have spent a lot of time in a good Library first (like I did) in order to LEARN the important factors in building one and also to learn the Physics of Ballistics (which I already knew as a Physicist).

Where THEY seemed thrilled when they even occasionally HIT the large wall that was their designated target, I would have certainly have marked A PARTICULAR BRICK in that wall as the target I would be aiming at!

In any case, I felt that my 1991 experiments proved that a gravity-driven Trebuchet could have remarkable accuracy, not remotely like the randomness that the PBS NOVA group seemed to consider acceptable.

But my Phobos would NEVER have gotten "launched" like that! He was too important to me to possibly hurt him, so a one-foot range was fine for the Dogapult!

There are some other web-pages which are dog-centered:

Animal Intelligence. Does It Exist?
Animal Insight, Intelligence, Logic

Dogapult
My Dogs - Their Chores
Birthday Calendar for Dogs
Dogs Are Being Exterminated in the United States
Dog on Snow Skis - My Dog Meatball

The Math

Both my final Trebuchets were Engineered to launch a specific weight to fly a specific distance. A lot of relatively simple mathematics had to be done first. As an outline: First, the standard Newtonian formulas for the Physics of Ballistics were calculated. If we chose a launch angle of 45°, the math is simpler still. The initial horizontal and vertical launch velocities must have the same value for that angle. so we have V0h * t = Vdistance or 150 meters. Vv = V0h - g * t

The parabolic path is symmetric so the ending vertical velocity must be -V0h. Solving these two simple simultaneous equations with simple High School Physics, we see that the object needs to be in the air for 5.53 seconds, having been launched with an angled velocity of 38.34 meters/second which is about 85.8 mph.

The Kinetic Energy it contained as it was launched is the usual 1/2 * mass * velocity-squared, and so it is then easy to calculate that needed amount of kinetic energy for a specific weight can.

The Trebuchet needs to accelerate not only the can but the sturdy arm which supported and guided the can during that acceleration. In my case, I made such a strong and rigid launching arm that most of the needed Potential Energy was actually used up accelerating the arm, but again the math is really not that difficult to do.

But after these calculations, it is then possible to calculate the weight of the large descending weight in the Trebuchet which would need to supply those two Kinetic Energy amounts.

It turns out that there is a small amount of air friction that is energy which gets wasted by the fairly long Trebuchet arm swinging around while it was accelerating the projectile, but the outer end of the arm is going at over 85 mph just before the projectile is released, and so air friction is great enough that it needs to be calculated and planned for. For the volleyball-launcher Trebuchet the airspeeds are much slower and so air friction turned out to be small enough that it could have been ignored.

In both of my final Trebuchets, an additional problem showed up! When a strong steel arm has gotten up to over 85 mph just prior to launching the can of beans, it needs to then stop! I spent more time in Designing various ways to stop or eventually, slow down, the throwing arm, than I did for the entire rest of the Trebuchets! Some of the failed attempts had the throwing arm get bent as it hit a 'stopper'; some had that heavy throwing arm break the object that had been meant to stop it; and I encountered many other unexpected complications there! I eventually wound up installing a number of car shock absorbers, which were able to SLOW the rapidly moving throwing arm, so it could then be stopped.

That aspect was actually important, as I needed to be able to schedule an exact time for the throwing arm to stop accelerating the projectile (by suddenly slowing down). I could adjust the launch angle by adjusting the exact location where the shock absorbers were!

Using shock absorbers also eliminated a VERY loud metal to metal impact that had earlier been stopping the throwing arm!

The precise landing point of the projectile was always slightly short of the calculated distance because the can of beans was not very aerodynamic and there was significant air friction when flying at over 85 mph. The theoretical parabolic shape of the path was therefore slightly distorted due to the projectile slowing down during the 5.5 second flight. The result was that the landing point was around 30 feet short of the calculated distance of 150 meters (or 492 feet). But that effect was the same for every launch, so once I knew where to place the target garbage can, the launches seemed to be consistently within a foot or two in both distance and azimuth, so they successfully attacked the garbage can! The landing speed was very close to 76 mph in those experiments.


I realize that no one actually CARES about these math matters, but I wanted to provide a simple example of how a Physicist or Engineer can use High School Physics skills to Design a device. The same approach applies to every other device or mechanism, where Physics has provided you a LOT of things that you never think twice about!

Too bad the NOVA program did not hire Physicists or Mathematicians who knew how to do such things, to possibly make a program which then might have demonstrated 'actual science!'



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C Johnson, Theoretical Physicist, Physics Degree from Univ of Chicago