I built this over either Thanksgiving Holiday or Spring Break in 2007. The only thing I might have bought for it were the metal brackets holding the trigger mechanism onto the rest of the assembly, however I believe these were laying around as well. There were no plans for this, other than what I had in my head, and I built it all from the image of a ballista I had in my mind.
Made entirely from 2x4 treated pine, the frame uses no glue, nails, or other fastening devices. There are concealed mortise and tenon joints about 2x2x2 inches, cubic to prevent rotation. The tension provided by the torsion ropes is enough to prevent any of the assembly form shifting around so it is very sturdy. It has withstood some fairly high falls, and doesn't give at all while firing. In fact, if I was going to build it again, I would have built the frame much larger to allow for more rope and larger arms.
The mortise and tenon joints were probably the hardest part to do, seeings as I don't have a tool designed to do that. I basically used a table saw and jigsaw to cut the tenons and a dremel/router to cut the mortises. The fit was not always perfect, so I had to hammer them together, however they lined up quite well, and when tension was applied from the ropes the whole frame pulled surprisingly straight.
About 13 loops of 1/4" braided nylon rope provides the torque for the ballista's arms. The rope passes through a 1" hole in the the upper and lower beam on the frame, and into a square 4x4 inch block of 2x4 with a 1" hole cut in it and a 1/2" copper lightening rod to loop the rope around. Because the wood on wood friction between the beam and the block is so great, no other fastener is needed to hold the blocks in place while tightened, however I've found that wetting the wood will lock it down once it is tight enough.
The braided nylon is perfect for use as the torsion rope. It is slightly stretchy, and holds it shape very well. In addition to that, its very high tinsel strength makes it almost impossible for me to tighten it enough to break it. Breaking that rope would be catastrophic due to the amount of energy stored in it. I have measured about 100 pounds of angular force at the end of the throwing arm, if you got in the way of that moving, it would easily break anything it hit, including arms and legs.
Broomsticks would not quite cut it, sadly. I had to sacrifice a banister rail which was almost circular and quite strong. The brass rod in the end came out of my arc welder, brazing rods if you are familiar with them. They are not too strong, but they just need to be able to hold the rope in place, almost all of the stress is put on the wood the way I have it tied.
On the flip side of the arms, I added two small sections of PVC electrical conduit to increase the distance between the rope contacts on the arms and increase the torque and speed provided by the torsion engine. I ran some tests and found that the natural tendency of the rope to bunch up was causing a lot of the energy in the ropes to be wasted, adding the space got it to almost the perfect position for the best possible force/speed ratio.
Trigger and Pouch
The pouch is made of layered packaging tape and black electrical tape for stretchiness. The packaging tape is surprisingly strong, but is not as flexible as I would like. It does, however, provide plenty of stress to keep the ropes from getting tangled. On a ballista, the two rope system is almost required, not only does it keep the arms from wildly going up and down, it ensures the projectile will leave the ballista without getting caught or backfiring.
The trigger is a masterpiece as far as I am concerned. It is mounted on a 2x6 approximately six feet long in the current version, four feet in the one photographed here, which is fastened onto the frame with four staggered bolts because I was too lazy to create a more age appropriate mount strong enough to hold back 200+ lbs of force. The two upright 2x6 blocks are mounted onto the long board with steel brackets that I honestly don't remember buying but don't remember finding them laying around either. Two more brass brazing rods are used to fasten the two components of the trigger, a lever and a notched wheel. The wheel is shaped something like a mushroom, the lever pushes on one notch, and the rope pulls on the other, when the lever is tilted, the wheel can spin freely and the rope is allowed to gently slip off. The wheel is made of a 2x6 block cut to the described shape, the lever is a piece of decking board cut to 2x6x1/4 block. See the diagram to the left for a visual representation of what I described.
Range and Power
Since I do not have a good measurement device for distances like 100 yards, and the projectiles tend to bounce quite a distance, I don't have any precise distances to give you. However, you can tell by the image of the test that this thing packs quite a punch. From this distance, the objects were landing consistently near the farthest set of playground equipment, which as you can see is quite a distance. If you look closely, you can see that the pouch is blurred in the image, e.g. it was still moving forward when the picture was taken. However, the ball is already that far away from the ballista and still climbing. Just to give you some idea of how fast the parts move, that picture had a 1/200 second exposure, but still blurred the pouch. And, this is a picture of the original design, not incorporating any of the new design modifications. The projectile in this circumstance was a hard rubber ball about the size of a tennis ball, so it had not made it very far judging by the size of it in the image.
There is so much force stored in the torsion engines that now have to use a obtusely large wrench, nicknamed "the persuader," in order to pull the rope onto the trigger to fire. Using an industrial scale I measured just under 200 pounds of force straight back at the full draw length, between 5 and 6 feet. That means there is approximately 80 to 100 pounds of angular force at the rope attachment to the arms. Doing some crude math, the speed of the projectile (discounting the mass of the projectile as negligible, tennis/golf ball sized) as it exits the ballista is between 200 and 300 feet per second, which of course dramatically slows with drag, unless it is a golf ball.
The original design is the one photographed here, and I have made some modifications since then. Nothing drastic, just some tweaks here and there.
The distance between the contacts on the throwing arms made by the rope coils was so small that most of the energy stored in the torsion engine was lost when firing the ballista. To fix this, I added some 3/4" PVC electrical conduit above and below the throwing arms within the rope coils. These served to increase the distance just enough to provide a significant increase in power, while loosing very little speed.
I added about two feet onto the long board connecting the trigger to the frame. This addition in turn increased the draw length of the ballista, which increased the energy put into the projectile during each shot. This almost doubled the range from what it was to what it is now, but has the downside of me having to use a wrench to prise the pouch's ropes onto the trigger. The last foot and a half of the draw is well over 200 lbs of combined force from the torsion engines. In fact, I am pretty sure that the pouch can not keep up with the projectile after the first two feet of draw because so much more energy is expended in those two feet than in the last few. However, I do not have a high speed camera to test this with at the moment, and the performance is excellent as it is.
Thoughts and Considerations
I built this mostly to say that I could, and because I saw that I had enough spare parts to finish it. I had very few goals in mind, besides being able to shoot stuff a long way. One major design issue was transportation was being able to move it, something I completely ignored in my Trebuchet. This ballista will fit in the trunk of my car by letting the seats down in the back and retracting the arms after releasing the pressure on them. In addition, it is light enough to pick up and carry around without straining yourself.
One downside to the compact construction is that it has no base. Its not a major issue, but obviously it needs to sit at about a 45 degree incline to fire properly. To make up for this, I usually balance it on top of whatever bucket I'm carrying around with me to hold projectiles.