This is a magic trick I learned about by watching the TV series "How I Met Your Mother." In it, the character Barney Stinson (Neil Patrick Harris) performs a magic trick where he shoots a fireball from his (apparently) empty hands. Neil Patrick Harris is actually an accomplished magician; he is the current president of the Magic Castle, an organization for magicians. To recreate the trick, I researched how the tricks work that you can buy off-the-shelf. The basic theory is fairly simple: a short (3-4") metal tube is sealed on one end. The sealed end has a glow plug screwed into it. A glow plug is a component of gas engines designed for RC vehicles. It contains a coil that heats up when 1.5V is applied across it. To load the launcher, you lightly tamp down some crumpled flash paper into the open end. To fire the launcher, you hit a button applying voltage to the glow plug, which then lights the flash paper. The burning of the flash paper propels it out of the tube, where it completely burns in the open air. Flash paper is tissue paper that has been specially treated to burn cleanly and rapidly, making it a material perfectly suited to this project.

Step 1: Materials Required

Needed for Fireball Launcher:
Flash paper
Aluminum Tubing 1/2" ID (OnlineMetals.com)
Aluminum Rod 1/2" (OnlineMetals.com)
Epoxy or other strong adhesive
28 gauge stranded wire
2 glow plugs (eBay)
1/4-32 Tap (eBay)
Tap wrench
Copper pipe strap
Slide switch (DigiKey)
Tactile switches (DigiKey)
AAA battery holder (DigiKey)
Calipers

Needed for Coloring Flash Paper:
Colorant chemical powder (UnitedNuclear.com):
Copper Chloride for blue/green
Potassium Nitrate for Violet
Sodium Nitrate for Yellow
Strontium Nitrate for Red
Strontium Chloride for Crimson
There are many other chemicals United Nuclear sells for pyrotechnics that can be used here, as long as it's water soluble it can probably be used.
Glass jars
Distilled water
Plastic spoons
Wax paper

Step 2: Firing Tube Construction

To make the firing tubes, cut two lengths of the 1/2" ID aluminum tubing that are each 2 3/4" long. Cut two pieces of the aluminum rod that are .230" long. They can be a bit shorter, but if they are longer than this they may block the glow plug from igniting the flash paper. Glue a section of aluminum rod into one end of the tube, trying to keep it level with the aluminum tube. Two of these firing tubes must be made. I used a file to clean up the glued end of each firing tube to give it a cleaner look, but this is not strictly necessary.

Step 3: Firing Tube Drilling and Tapping

The sealed end of each firing tube needs to have a threaded hole made through it to screw the glow plug into. First, mark the center of the cap. It is not particularly important the the threaded hole be at the exact center of the tube, but I tried to get it as close as possible, for a symmetrical look. Once the center is marked, drill a 7/32" hole through the cap using a drill press to ensure that the hole is straight. There are many techniques for tapping a hole. The technique I employed involves holding the 1/4-32 tap in the drill press and turning it a few times into the aluminum to start tapping straight into the hole. I then loosened the drill press and used a tap wrench to tap the hole. As always when tapping a hole, use a lubricant (I used 3-in-1) and back up the tap regularly to break the chips being formed. Once the hole has been tapped, use a drill bit or deburring tool to remove the burrs on either side of the tapped hole. It is important that the external burrs are removed so the glow plug can be tightened down completely and it is important that the burrs inside the tube be removed so that flash paper can be tamped down completely to contact the glow plug. Once the tapping operation is complete, clean the inside and outside of the tube with isopropyl alcohol or another degreasing agent (brake cleaner, soap, etc.).

Step 4: Firing Tube Assembly

To assemble the two firing tubes, rough up the area where the tubes will contact with a rough grit sandpaper (around 100-300 grit), apply your chosen glue (I used LocTite 5 minute epoxy) to the roughened area, and clamp them together. Ensure that the tubes are aligned when clamping. I used two v-blocks in my drill press vice to align them. Even with the v-blocks, however, their alignment had to be adjusted manually a bit when clamping them.

Step 5: Handle Construction

A big part of hiding this trick in your hand is holding it in such a way that it doesn't look like you're holding anything. One of the ways to do this is by using a handle that lets you hold the launcher with your middle and ring fingers only. My design of this handle can be slid along the launch tubes, enabling the grip to be adjusted for the particular way you want to hold the launcher. To make the handle, start off by flattening two copper pipe straps. These are sold in hardware stores for holding copper pipes to a wall. Once you have two flat pieces of copper, you are going to want to bend one of them into the part you hold onto and bend the other into the part that holds onto the launch tubes. I bent them using a combination of a bench vise and pliers. The part that holds onto the launch tubes should be bent so that it holds onto the launch tubes tightly but can be slid along them to adjust your grip. Once you have the two copper strips bent properly, they need to be joined together. To do this, I soldered them together with rosin-core electronic solder. Heat them using a small blowtorch, and apply solder to the joint. Let the handle cool, then flip it over to the other side, heat it again, and apply solder to the joint. Once the handle has cooled, I used a wire brush to clean off the black residue left over from the soldering process. If you want to use this trick for serious magic, you may want to trim down the part of the handle that protrudes above your hand and paint it a color that matches the color of your skin to make it as invisible as possible.

Step 6: Circuitry Construction and Attachment

The glow plug is active when 1.5V is applied across the protruding portion and the portion attached to the threads. Since the glow plug is a simple resistive element, the polarity is probably not important, but I powered it with the same polarity convention as spark plugs: 1.5V is applied to the protruding portion while the portion attached to the threads is grounded. Power comes from a AAA battery, which provides enough power to light the flash paper but is still fairly small. I experimented with using AAAA batteries and N cell batteries, but neither of these could supply enough power. If you find that the AAA battery is not lighting your flash paper as quickly as you'd like it to, you could use a AA battery instead, or connect a supercapacitor in parallel with the AAA. Once charged, the supercapacitor provides additional current to rapidly heat the glow plug.

The circuit itself is fairly simple: a small slide switch switches the connection between the glow plug's ground and the battery's ground, while two tactile switches connect the positive lead from the battery to each glow plug's positive connection. The tactile switches aren't rated for the level of current involved steady-state, but they seem to work fine controlling that amount of current for a few seconds. I've tested each switch on the order of 20-30 times without failure. The pictures below outline how to assemble the circuit.

Step 7: Glow Plug Insertion and Soldering

Once the circuitry has been completed, the next step is to insert the glow plugs and solder them to the circuitry. The glow plugs came with copper o-rings that can be seen in the first picture below. I tossed these so that I could make the plug that they screw into as long as possible, making construction and alignment of the plug as easy as possible. To attached the glow plugs, screw them into the tubes, and tighten them with a wrench. Don't really crank them down, but tighten them firmly with enough torque that they're not going to unscrew on their own. It is important that they seal well or some hot gases may leak out the back of the fireball launcher, reducing the effect of the trick. It is not necessary to follow the exact order shown in the pictures below; it's fine to screw in both plugs at once. To solder wires to them, strip about 1/4"-3/16" of the wires leading from each tactile switch. Wrap this stripped portion around the upper slot on the protruding portion of the glow plug, and solder it with rosin-core electronic solder. Be careful to minimize the application of heat with the soldering iron. Once the wires from the switches have been soldered, solder the ground wire coming from the slide switch to the left glow plug (viewing from the back with the handle up) and solder a short wire between the two glow plugs so they are both grounded by the slide switch.

Step 8: Tying Up the Wiring

To clean up the wiring, use a thin strip of electrical tape to hold all the wires together. This should help prevent the wires from snagging on anything and coming loose. Congratulations! You now possess one of the world's finest handheld fireball launchers. Insert a AAA battery, and you're ready to test it! Skip the colored flash paper section if you just want to go ahead and try it out with regular flash paper.

Step 9: Colored Flash Paper

Let's say that you're not content to just replicate the fireball magic trick as it's been done before. Let's say that you want to go that next step, and perform the trick in such a way that Barney Stinson would be impressed. This is where we turn to science. Our goal here is to make regular flash paper burn in colors other than its ordinary yellow-orange. Be

fore we start, however, I'd like to say that this section is a work in progress. I haven't ironed out all the bugs in this process, particularly in making this process work with the fireball launcher. Still, I have gotten some very cool results, and have managed to launch fireballs in at least two different colors.

Making flash paper burn other colors is fairly simple, in theory. You take any one of various chemical powders and mix it with distilled water. You then immerse flash paper in this solution, dry it, and it should burn a different color depending on the chemical. Some of the common chemicals for this sort of thing are:

Strontium Nitrate - Red (makes a really cool fireball, but gunks up the launcher)
Strontium Chloride - Crimson (Doesn't burn well enough to launch a fireball)
Sodium Nitrate - Yellow (Also doesn't burn well enough)
Barium Nitrate - Pale Green (Toxic enough that I decided not to try it)
Copper Chloride - Deep Green (Actually burns blue-white, doesn't burn well enough to launch)
Potassium Nitrate - Violet (Burns a white-purple, works best out of the chemicals I tried)
Copper Chlorate - Blue (I didn't know about this chemical until I had made my order, so I haven't tried it yet)
Barium Chlorate - Green (I also didn't know about this one)

Bear in mind that these chemicals vary in toxicity, so you're going to want to follow proper safety procedures when working with them: wear safety glasses, a dust filter mask, and chemical-resistant gloves, use nonmetal containers and stirring tools when mixing, store and dispose of the chemicals properly.

I tried this process with five of these chemicals. They all burn in open air, but only two of them work when the flash paper sheet impregnated with them has been loaded into the launcher. I recorded a video of test burning flash paper impregnated with these chemicals and embedded it in this email.

The color variations are more impressive in person than they appear on the video, but they vary in how different they are from the original flash paper color. I also have pictures below of the different colors they burn.

There are a multitude of pyrotechnic chemicals UnitedNuclear.com sells that may work better than these. I plan to experiment more with some different chemicals to get a larger range of colors to work with the fireball launcher.

Step 10: Loading

There are lots of different ways to load flash paper into the fireball launcher, depending on the kind of effect you want to have. There are two basic kinds of effects you can get, with plenty of variations thereof: I call them a "poof" fireball and a "traveling" fireball. A poof fireball burns out close to the launcher (1-2 feet) while a traveling fireball goes considerably farther before burning out (6-8 feet). To get a poof fireball, loosely crumple up a 2x3" piece of flash paper and loosely tamp it into the barrel. I cut a 3 1/2" long piece of aluminum rod and chamfered the ends to act as a tamping rod. By tamping the flash paper loosely, there is more oxygen for the flash paper, letting it burn faster.To get a traveling fireball, crumple up a sheet of flash paper more tightly, and tamp it down tightly. Because the flash paper has less oxygen, it takes longer to burn, so it can travel farther. If you want a really impressive fireball, start by tamping down a crumpled sheet of flash paper to act as a propellant stage. Then, stack two sheets of flash paper together and crumple them up tightly, then tamp them down into the firing tube. When ignited, the propellant stage with launch the tightly crumpled ball, which will burn slowly enough that it can travel many feet. The slow burn rate doesn't detract significantly from the effect, the fireball is still plenty bright and impressive.

Step 11: Cleaning

Although flash paper burns fairly cleanly, some residue does build up in the fireball launcher. To clean it out, first pour some isopropyl alcohol into each firing tube. Next, take a quarter of a paper towel sheet and roll it up. Then clean out each firing tube by rotating this roll counterclockwise. It is important to rotate it counterclockwise instead of clockwise because the glow plug heating element is soldered to the glow plug in such a way that rotating it clockwise could snag the heating element and detach it. Pour out any isopropyl alcohol left in the firing tubes and wait for them to dry before using it.

Step 12: Safety

Before using this magic trick, make sure that you can use it safely. I'm not a pyrotechnician, so I make no claim that it is an inherently safe device. This magic trick uses highly flammable, pyrotechnic materials and toxic chemicals, so make sure to follow all reasonable safety practices.