User:Geek.not.nerd

The book Backyard Ballistics is a popular how-to book by William Gurstelle that was published in 2001. It is full of fun experiments that can be done relatively inexpensively and that are easily executed. It also includes the history and mechanical principles of some of the inventions and projects. From catapults to rockets, this book describes accessible ways to create these at home or in the classroom.

Safety In Mind

The safety considerations of these projects are included, but none of the projects is fool-proof. When playing with fire, it is generally a good idea to have at least one adult present. Safety glasses, gloves, and hearing protection (if needed) would be proper safety precautions. Hair could get in the way, so securing long hair is always a good idea. Smoking near flammable materials could start a fire and is strongly warned against. All flammable materials should be in separate containers and kept away from heat or sparks. A misstep could cause an unintended explosion. Keep a fire extinguisher near in case of a fire.^[1] Local regulations regarding the launching of model rockets must be followed. For example, some cities temporarily ban model rocket launches when conditions are exremely dry and fire danger is higher than usual.

With any projectile-launcher it is important to aim only in a safe direction and to refrain from releasing the projectile until the path and what is beyond it is clear.

Cannons

A potato cannon, or spud gun, is a common introduction to projectile motion. It gives people a chance to learn important safety as well as physics principals in a low-cost, fun way. PVC piping, PVC cement, and a spark cap are the basic building materials. One can cut the PVC with a hacksaw to the specified lengths, and attach the coupling pieces to join the larger combustion chamber and the longer narrow barrel. PVC cement is then applied to ensure secure holding. The potato is pressed onto the end of the “cannon” and the cut-out is shoved down to the end of the narrow section with a broomstick. The spark cap is taken off of the large end, aersol hairspray is sprayed for a couple of seconds, and the cap is quickly placed back on the pipe. The spark button is flicked, and the spark ignites the hairspray. The pressure from the combusting hairspray propels the potato out of the cannon.

The appeal of the carbide cannon is the bright flash and loud bang it produces. Various fittings and lengths of PVC pipe are required. A hacksaw can be used to saw the piping. Before using PVC cement to secure the pipes and fittings together, a dry fit of the piping is suggested so that the person assembling it can see how all of the parts fit together. The calcuim carbide is placed in a water reservoir that is included in the assembly. Acetylene gas is formed, which is very explosive. The gun is very loud, so standing back and hearing protection are recommended.

Pnewtons Petard is a pressurized cannon to fire potatoes, however it is without the combustion factor. It is much stealthier in the way of being almost soundless. It is made of PVC pipe: the PVC is used to create a pressure reservoir, a pressurization valve, a barrel, and a trigger valve. One must look at the pressure limit of the PVC pipe and make sure not to go over it. The pressure limit is usually around 100 PSI. After assembling the Petard according to instructions, it is time to test the performance. Ram a potato to about 5 inches from the end of the pipe with a broomstick. Pressurize the reservoir to no more than 30 psi using a foot stablized air pump. Be very careful where the cannon is aimed, as it is now pressurized and capable of shooting the spud for over 200 yards. Snapping the valve open quickly will release a burst of air that propels the potato.

Rockets

The paper match rocket is a very simple rocket that uses matches, aluminum foil, and pins. The foil is wrapped carefully around the match head and pins. After the pins are removed, exhaust ports are made. The foil must be molded perfectly to have to rocket work correctly. The launcher can be made the easy way or in a more complicated version. The easier version is to bend a paper clip at the angle desired to launch the rocket. For the more complicated launcher, a copper tube is attached to a hinge, which is attached to the block by a screw. Both launchers will work, however the difference in performance is noticeable.^[2]

The hydro pump rocket is built with a plastic bottle and a straw for a nozzle. The straw is taped securly to the mouth of the bottle. The bottle is partially filled with water, and a bicycle pump is used to pressurize the water. A stopper is in place beforehand, but when the pressure peaks the stopper will fly out and the rocket will fly to heights of 60 or 70 feet high.

The pneumatic missile is a plastic tube that is fitted over a wooden dowel. The tube has a stopper with a small opening, to which a tube is attached. The tube leads to a rubber or plastic ladder. When stomped on, the bladder sends a big puff of air down the tubing into the missle. The missile is forced off of the dowel and flys surprisingly high. Several commercial toys are based on this design.

Flingers and Catapults

The Greek fire catapult was a giant catapult that hurled barrels of fire at their opponents. This is a more scaled down version. A saw, various wooden beams (specified in the directions), a hammer, cord, vaious washers and bolts, nails, and glue will be needed. The catapult uses a torsion spring (thus the cord) to propel the cup holding the projectile in question. To fling the catapult, carefully pull the arm back and latch it with a hook and eye and place the projectile in the cup-shaped washer. Attach a thin string to the hook. The more tension in the torsion spring, the farther the projectile will go when the string is pulled.

The ballistic pendulum is a device that can help one learn about muzzle velocity and energy of a projectile. It is built with a cardboard box, weighs exactly 64 ounces, and uses various eye hooks and pieces of plywood. The box is suspended from the string on either side, and it is placed in front of the weapon. A pen is secured to the bottom, with the tip touching a mounted piece of paper. When the weapon is fired, the pen makes a line on the paper. The line is used to see how much the pendulum rises. The projectile must be weighed beforehand. With the information of how heavy the projectile is, the weight of the pendulum, and how high the pendulum goes, the muzzle velocity can be accurately calculated.^[3]

About the Author

William Gurstelle mostly works in the realms of ballistics and technology. Popular Mechanics magazine dedicated 4 pages to his work in 2010. He is currently the co producer of an Emmy nominated television show called MAKE:TV. William Gurstelle has also written The Art of the Catapult and Absinthe and Flamethrowers. ^[4]