Build Potato Cannons, Paper Match Rockets, Cincinnati Fire Kites, Tennis Ball Mortars, and More Dynamic Devices
By William Gurstelle, Todd Petersen, Rob Nance
Chicago Review Press Incorporated Copyright © 2012 William Gurstelle
All rights reserved.
Keeping Safety in Mind
GENERAL SAFETY RULES
When you were a child, people told you not to play with matches for a good reason — they can be dangerous! If you don't follow the directions closely, any of the experiments in Backyard Ballistics could cause harm to you and your possessions. Remember to always follow the instructions closely. Do not make changes to the materials or construction techniques. It can lead to unexpected and unintended results.
A Very Important Message
The projects described in the following pages have been designed with safety foremost in mind. However, as you try them, there is still a possibility that something unexpected may occur. It is important that you understand neither the author, the publisher, nor the bookseller can or will guarantee your safety. When you try the projects described here, you do so at your own risk.
Some of these projects have been popular for many years, while others are new. Unfortunately, in rare instances, damage to both property and people occurred when something went wrong. The likelihood of such an occurrence is remote, as long as the directions are followed, but remember this — things can go wrong. Always use common sense and remember that all experiments and projects are carried out at your own risk.
Be aware that each city, town, or municipality has its own rules and regulations, some of which may apply to the projects described in Backyard Ballistics. Further, local authorities have wide latitude to interpret the law. Therefore, you should take the time to learn the rules, regulations, and laws of the area in which you plan to carry out these projects. A check with local law enforcement will tell you whether the project is suitable for your area. If not, there are other places where experiments can be undertaken safely and legally. If in doubt, be sure to check first!
These are your general safety rules. Each chapter also provides specific safety instructions.
1. The experiments described here run the gamut from simple to complex. All are designed for adults or, at a minimum, to be supervised by adults. Take note: Some experiments involve the use of matches, volatile materials, and projectiles. Adult supervision is mandatory for all such experiments.
2. Read the entire project description carefully before beginning the experiment. Make sure you understand what the experiment is about, and what it is that you are trying to accomplish. If something is unclear, reread the directions until you fully comprehend the entire experiment.
3. Don't make substitutions for the specific liquids and aerosols indicated for use in each experiment. Stay away — far away — from gasoline, starting ether, alcohol, and other powerful inflammables. There are few things as dangerous as flammable liquids or aerosols. They can and do explode, and the consequences can be disastrous.
4. Use only the quantities of fluid listed in the project instructions. Don't use more propellant than specified.
5. Don't make substitutions in materials or alterations in construction techniques. If the directions say to cure a joint overnight, then cure it overnight. Don't take shortcuts.
6. Read and obey all label directions when they call for materials such as PVC cement, primer, and other chemicals.
7. Remove and safely store all cans or bottles containing naphtha, hairspray, or any other flammable substance prior to performing the experiment. A good rule of thumb is to maintain a hazard-free radius of at least 50 feet around the area in which you plan to work.
8. The area in which the projects are undertaken must be cleared of all items that can be damaged by projectiles, flying objects, and so forth.
9. Keep people away from the firing zone in front of all rockets, mortars, cannons, etc. Use care when transporting, aiming, and firing, and always be aware of where the device is pointing.
10. Wear protective eyewear when indicated in the directions. Similarly, some experiments call for hearing protection, blast shields, gloves, and so forth. Always use them.
* The instructions and information are provided here for your use without any guarantee of safety. Each project has been extensively tested in a variety of conditions. But variations, mistakes, and unforeseen circumstances can and do occur. Therefore, all projects and experiments are performed at your own risk! If you don't take this seriously, then put this book down; it is not for you.
* Finally, there is no substitute for your own common sense. If something doesn't seem right, stop and review what you're doing. You must take responsibility for your personal safety and the safety of others around you.
WORKING WITH PVC PIPE
Several of the projects contained here involve cutting and joining PVC pipe. This section tells you what you need to know in order to make safe and secure joints.
First, you should be aware that there are at least four types of plastic pipe and plastic pipe joints available: PVC, CPVC, ABS, and PB. The letters are abbreviations for the type of plastic material that composes the pipe. Pressure rated schedule-40 PVC pipe and pipe fittings are made of white polyvinyl chloride. This is the type of pipe and pipe joints recommended for these projects.
You can purchase the PVC pipe and pipe fittings at your local hardware or home store. Note that even among PVC pipe there are different kinds, and some, such as cellular core PVC (recognizable by the designation ASTM 02241 printed on the pipe), are not suitable for use in these projects. Be sure to use only pressure rated schedule-40 PVC pipe with the pressure rating printed on the outside of the pipe by the manufacturer. If there is no pressure rating or schedule number printed on the pipe, don't use it. Such pipes are likely made from cellular core materials, which are not strong enough. If you read the words "cell core" or "cellular core" on the pipe, don't use it!
Sometimes you'll see the letters DWV printed on the pipe. They only signify its applicability for use in certain plumbing situations and does not mean that a pipe is usable or not useable for the projects that follow. Just make sure there is a pressure rating printed on the pipe.
The pressure rating of the pipe changes with temperature. As the air temperature rises, the plastic gets weaker. At 90ºF, the pipe's pressure rating is only three-quarters of what it is at 70ºF, and at 100ºF, the pipe is only two-thirds as strong. While projects in Backyard Ballistics are designed so pressures are always well below ratings, keep in mind that on hot days your margin of safety is much less.
Unlike pipes, PVC fittings (elbow joints, tee joints, and so forth) usually do not come with pressure ratings on them. Typically they are rated for 150 psi, although some are rated for more, and a few for less. If you are in doubt of the pressure rating of the fitting, ask the person you are buying it from. Be sure to inspect the pipe end and fittings for cracks, dirt, and abrasions. Don't use damaged PVC pipe or fittings.
Cutting and Fitting PVC Pipe
PVC pipe is easily cut with a regular crosscut saw or a fine-bladed handsaw. It is important that all the cuts be made as close to 90 degrees to the centerline of the pipe as possible. That way, you won't leave any interior gaps, which will weaken the joint. Be sure to remove any burrs.
You may want to "dry fit" the pipe into the joints before you apply any cement to see how things fit. Sometimes the dry-fitted pipes and joint fittings stick together so tightly it is hard to get them apart. If that happens, you can carefully whack the fitting loose with a wooden block.
Joining and Cementing PVC Pipe
The process of joining and cementing PVC pipe is technically called "solvent welding." The solvent melts the plastic so when you push the pipe and the pipe fitting together, the two parts fuse as the solvent evaporates. Each type of plastic pipe has its own special solvent. Some solvents are advertised to work on several types of plastic, but it is strongly recommended that you use the solvent that is meant solely for the type of plastic you're working with. At the hardware store the solvent you need is called "PVC cement."
The solvent works only on clean surfaces — surfaces with no dirt, no grease, and no moisture. Wipe the inside of the fitting and the outside of the pipe with a clean cloth. Then, apply PVC primer (called "purple primer") to the ends. Allow the primer to dry before applying cement.
Next, coat the surfaces that you want to join with a thick coat of PVC cement. (PVC cement, which is a solvent, should only be used in well-ventilated areas.) Immediately join the pipe and fitting full depth with a slight twist to bring it into correct alignment. A continuous ooze of cement around the fitting indicates that you used enough solvent cement to ensure a leak-free joint. Let the joint dry for several hours before using. Be sure to observe the cure times shown on the PVC cement can's directions or see the table on the next page for average joint cure times.
GENERAL PVC SOLVENT SAFETY INSTRUCTIONS
1. Avoid breathing the PVC solvent cement and primer vapors. Work in a well-ventilated area.
2. Keep all chemicals away from open flames.
3. Read and follow the precautions that appear on the labels.
4. Joint Cure Time is the time required before pressure testing. In damp or humid weather, allow 50 percent additional cure time. How long should PVC cure? Use the table here.
The Potato Cannon
The potato cannon, or spud gun as it is sometimes called, is nearly legendary in amateur science circles. You'll be amazed at how easy it is to make a working ballistic device out of materials readily available at your local hardware store. Don't worry, the potato cannon doesn't use dangerous gunpowder or rocket fuel to blast the potato in the air. Instead, the cannon takes advantage of the considerable energy contained within the aerosol propellant of common hairspray.
Thousands of people, from adolescent boys and girls to serious experimenters at Ivy League universities, enjoy shooting homemade ballistic devices like this. It's appealing for several reasons. First, the cannon is simple to build. Second, it really does work well. And finally, it's easy to understand. Unlike the complicated configuration of a computer's silicon chips, the average person can figure out (with the help of this book) the physics of the cannon.
People love making the potato cannon. Don't be too surprised if the hardware store clerk takes a quick look at your materials and says, "Making a spud gun, eh?" It happens to me all the time.
* * *
Building the Classic Potato Cannon
WORKING WITH PVC PIPE
PVC pipe is the greatest home plumbing invention of the twentieth century. Unlike heavy steel pipe, the average person can quickly cut, join, and fasten PVC pipe with a minimum of materials and a small amount of practice. This makes it the perfect spud gun raw material.
PVC pipe is made of a polyvinyl chloride plastic. Manufacturers make these pipes in various thicknesses. You specify the thickness by referring to its "schedule." For our experiment, we need schedule-40 PVC pipe. It also comes in a variety of diameters: 1-inch, 2-inch, and so on. Buy it in 8-foot lengths and cut it to the size you need with a hacksaw.
PVC pipe manufacturers make a variety of connectors to join pipes in the way plumbers (and spud gunners) need. Couplings join pipes of similar sizes. Threaded couplings have female pipe threads cut into at least one end. Reducing bushings join a pipe of one size to a pipe of a smaller size. End caps simply cap the end of the pipe.
Joining the PVC
The insides of the connectors are either smooth or cut with screw threads. Sometimes we'll want to join two smooth pieces, which can be "solvent welded" together using special PVC cement. (Note: Always use special-purpose PVC cement on PVC pipes and connectors. Regular glue won't work.) Other times, we'll want to join two threaded pieces that can simply be screwed together.
Go to the local hardware store's plumbing section and ask the clerk to help you find the items on page 14. Yes, the big commercial hardware stores usually have all of these items (except the lantern sparker and hairspray). However, I recommend going to your local hardware store because the clerks are usually much more helpful. Sometimes, they will even cut the pipe to size for you and not charge you for a full 8-foot piece of pipe.
* Shaping file
* (1) 36-inch length of 2-inch diameter schedule-40 PVC pipe
* (1) 3- to 2-inch diameter reducing bushing
* (1) 14-inch length of 3-inch diameter schedule-40 PVC pipe
* (1) can PVC primer
* (1) can PVC cement
* (1) 3-inch coupling, one side smooth, one side threaded
* Electric drill with 1/8-inch drill bit, 5/16-inch drill bit
* (1) flint and steel lantern sparker. (This small device is widely available where camping goods are sold. If you are purchasing online, just search for "lantern sparker." It is designed to ignite the mantles of lanterns. It consists of a steel wheel that is rotated against a flint by means of a knurled brass handle. It generally retails for less than five dollars. Common brands and models include the Coghlan 503A and the Coleman 829B705T.)
* Large adjustable wrench
* Duct tape
* (1) 3-inch diameter threaded PVC end cap
* (1) 4-foot length of 1-inch diameter wooden dowel or broom handle
* Hairspray in a large aerosol can (Be sure it's an aerosol can and not a pump spray. Spud gunners typically buy the most inexpensive brand of hairspray. Our interest is in its hydrocarbon propellant, not its holding power or scent. Check the label to make sure it does include volatile ingredients such as alcohol, propane, or butane.)
* Protective gear including safety glasses, earplugs, and gloves
* Bag of large russet potatoes
* Sharp knife
Place all of your materials and tools in front of you. Crafting a spud gun from raw materials takes at least two hours of filing, cutting, and drilling. You may need an extra pair of hands to hold things in place while you are gluing. After the pieces are put together, you'll need to let the PVC cement cure overnight.
1. Use the hacksaw to cut the PVC pipes to the desired lengths.
2. Use the file to taper one end of the long, 2-inch diameter pipe section so it forms a sharp edge. The edge will cut the potato as it is rammed into the muzzle of the gun.
3. Use PVC primer before cementing. Attach the 3-inch side of the 3- to 2-inch reducing bushing to one end of the 3-inch pipe using the PVC cement. Be sure the joints are clean and that you apply the cement according to the directions on the can. Don't forget to observe the directions for curing times. You must let all the connections cure overnight.
4. Carefully cement the smooth, unthreaded side of the 3-inch, one-sided threaded coupling to the 3-inch PVC pipe. Do not get any cement on the exposed pipe threads. If you do, you won't be able to screw the end cap into place.
5. The 36-inch long, 2-inch diameter pipe is the muzzle of the potato gun. Cement the untapered side to the 2-inch side of the reducing bushing.
6. Carefully drill a hole large enough for the sparker (usually ¼ inch or 5/16 inch, but match the twist drill you use to the diameter of the sparker's hollow bolt) to snugly fit through the middle of the 3-inch threaded end cap.
7. Take the hollow, threaded bolt assembly from the sparker and insert it through the hole made in step 6. Depending on the type of sparker you have, you may have to drill (counterbore) a 3/8-inch diameter depression on the outside surface of the end cap. Make it no more than 1/16-inch deep in the PVC. You just need to reach and engage the hollow bolt's screw threads with the nut. (See diagram 2.5.)
8. Mount the sparker by unscrewing the knurled end cap from the shaft. Be aware there is a spare flint inside the end cap, so watch for it. Unscrew the nut and remove the metal angle piece. (We don't need the metal angle piece, so just throw it away.) Insert the sparker shaft through the hole and tighten the nut until the sparker is firmly in place. The shaft will slide in and out, but it won't come out. Replace the end cap and tighten the lock screw.
9. Allow the entire assembly to cure overnight. Do not "test fire" the lantern sparker inside the gun until the cement has cured and all solvent fumes have dissipated. Solvent fumes are volatile and can ignite.
10. For extra safety, wrap the barrel and joints with multiple layers of duct tape (excluding the threaded end cap). (Continues...)
Excerpted from Backyard Ballistics by William Gurstelle, Todd Petersen, Rob Nance. Copyright © 2012 William Gurstelle. Excerpted by permission of Chicago Review Press Incorporated.
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