Making GT ballista bolts Care and feeding Stuffing the tubes Building up the nocks Swivel mechanisms for ballistas Design Idea Discussions: Arm and Rail Length

Ballista Bolts     The simplest ballista bolt is the golf tube bolt. The parts are: a golf tube for the body, a tennis ball for the head, and some leather or cardboard for cutting the fins. You also need fiber-reinforced strapping tape, duct tape, and some razor blades. Refer to the drawing below.     It is my experience that the tennis ball should not be glued to the tube. The shot last longer if the ball can actually move a little on the end of the tube; that helps dissipate impact shock. If the ball is glued, it holds so strongly to the tube that too much impact is passed through the tape, which rips.     The tape should be attached to the tube a few inches behind the ball; this helps streamline the head a little.     Use a leather punch to make small holes at the ends of the slits cut for the fins. This keeps the tube from tearing farther.     When you tape the fins to the tube, try to keep it as smooth as you can. Clean lines will help the bolt fly farther and straighter.     Capping the butt end of the tube is not necessary if you are using some kind of wide-area contact surface between the tube and the ballista string. If the string contacts the tube itself, you will need to cap the butt of the tube with a PVC cap or a piece of leather or something to keep the string from gouging the end of the tube. Since energy is wasted in gouging the tube, this robs the shot of range as well as damaging the tube.     The bolt bodies can be made a little stiffer by stuffing them with foam. They bend less and stand up to being stepped on a little better. They also weigh a little more so that affects their flight; try shots using both stuffed and unstuffed projectiles and see which works best for you.     The round pipe insulation found at hardware stores is great for stuffing bolts. Whatever you use, cut it in small chunks 4 to 6 inches long, and use a broomstick to shove it down the tube. Pack the foam LOOSELY; a tight pack makes the bolt much stiffer but also adds so much weight the result isn't worth the foam.     No, you can't use the same expanding insulating foam used to make arrowheads. It won't cure properly because it can't outgas through the plastic tube. And even if it did, the cured foam is too brittle for bolts; if stepped on it will crumble to powder.     It's a good idea to cap the nocks of the bolts; they will stand the stress of launch better. I think the best way to make the nocks is to simply glue in a disk of styrofoam. Nothing stronger is needed unless you're using a dreadfully strong weapon, and most other materials are heavy enough to make the bolts tail-heavy, which would impair their flight. To avoid problems with inspecting marshals, don't use wood or other hard substances for the nocks.     Golf tube bolts must be kept shaded as much as possible. If left in direct sunlight they will bend toward the sun, and you can't expect accuracy from a bent shaft. It would be a good idea to rig a cloth shade over your ammunition carrier, and load the bolts only just before they are fired.     Between fights, inspect your ammunition and try to put it back in good shape. Clean off any big clumps of mud or other debris, and do what you can to straighten the tubes and fins. At home, you can actually straighten your bolts with a heat gun or hair dryer. Use the heat gun to warm the tube until it softens slightly, and then roll the tube back and forth on a hard, flat surface until the body straightens out. Swivels    You can make a uni-directional swivel for a ballista mount by using the mechanism from a disassembled office swivel chair. With that connecting the weapon's draw works to the mount, you can move the weapon in any direction. Cheap swivel chairs can be bought from a used office equipment store for very low prices. Rail/Arm Length    In theory, the longer the longer the launch rails, the better. (I'll apologize in advance to the physicists if I state this badly.) Basically, every milisecond that the string is applying force to the projectile, the projectile's kinetic energy increases. The more kinetic energy it has the longer it will fly. So, the longer the projectile is under accelaration, the greater its range. That means that if all other factors are equal, the longer you make the launch rails, the longer your weapon's range will be.     Of course, any time you change one factor, you may have to change others to accomodate things. If you increase the length of the rails, your string must cover more distance from its cocked position to its rest position. That means you may have to change the way your ballista's arms are built.     If you are using torsion power, lengthening the rails will require other changes. It may be possible to accomodate the change by simply pulling the string farther, bringing the arm tips closer together when the weapon is ready to fire. Whether that is safe or not depends on the structure of the arms and the amount of torsion; you may need to slightly slacken the skeins if you do this.     If you do not change the tensioning, you must use a longer string, and that will require longer arms to hold it in the proper attitude. If you lengthen the arms, the distance from the string's rest position to the front end of the rails will now be shorter. Because of that, you may have to set the arms so they are are closer to parallel to the prod box when at rest, or lengthen the front ends of the launch rails as well.     If you use elastic cord, longer launch rails also change your string length factors. You could simply stretch the cord more to reach the cocked position, but that will be more difficult, and may be hazardous because it will put more stress on the structure. You could lengthen the arms to accomodate a longer string. If you do that, you also have to face the considerations mentioned in the paragraph above.     For any ballista, you may want to start with arms that are a bit longer than you think you need at first. Then, if you lengthen the rails later, you can accomodate a longer string by just moving the string anchor points farther out the arms. On a torshion ballista, having that extra mass on the arms beyond the string anchor points may affect performance by changing the leverage in the arms. But, you can always cut off the excess once you are satisfied with the performance, and if you do decide to lengthen the rails later, it's much easier to move the string anchor points than to rebuild the arms.