(USE AT YOUR OWN RISK!) Making Your Own Igniters (USE AT YOUR OWN RISK!) Warren Massey 2/14/95 For igniting composite motors, Aerotech Copperhead igniters are the device most of us usually use, but they are less reliable than is desirable and are not well suited to ignition of clusters of motors or ignition of upper- stage motors in multistage rockets. More suitable commercial replacements for the Copperheads are available but are difficult to purchase if you don't have a B.A.T.F. explosives license and an approved magazine to store them in. The options available to the rocketeer are more limited than they were two years ago but there are still some good ones available. The Copperhead igniters are made from a thin insulating film with thin sheets of copper laminated onto each side. Long narrow strips are sheared off of the sandwich and one end of the strip is dipped into a conductive pyrogen that electrically bridges the insulating sheet between the two foil sides. When a voltage is applied to the two copper foil surfaces, a current flows through the conductive pyrogen and if the current is great enough, causes it to heat up to its ignition temperature. INSERT FIGURE #1 HERE The failure mode is often introduced into this system when the long narrow strips are sheared off of the sandwich. If the shear that is used to do the cutting is not perfectly sharp along its full length, some of the copper, rather than being cut cleanly will be stretched, pulled and torn down toward the other copper surface causing small short circuits along the edge of the igniter. A good Copperhead requires a one-ampere current to fire it. Depending on their number and size, these short circuits can easily make a bad igniter require 10 or more amps. INSERT FIGURE #2 HERE The time-honored way of making igniters for composite engines usually involved a length of thermalite with one end taped to a either an electric match or an AG1-B flashbulb. The mid-section of the thermalite was sheathed in a piece of teflon tubing which was long enough to extend from the flashbulb/electric-match outside the motor's nozzle up to the top of the motor's propellant grain. The free end of the thermalite extended about 1" to 2" from the teflon tubing and was peeled free of its wrapping and wires and folded over to form a double thickness of pyrogen. INSERT FIGURE #3 HERE Flashbulbs work for igniting thermalite (or back-powder ejection charges) because the exterior of the bulb gets quite hot enough to ignite things in contact with it when the bulb is fired. You have to be careful when using flashbulbs because the do not require very much current to fire them. The continuity test circuit for many of the common rocket launchers such as Estes, Quest or Aerotech are not flashbulb safe because the test passes more than enough current through the bulb to cause it to fire. The same is also true for most of the inexpensive Volt-Ohm Meters (VOM). Electric matches share this sensitivity to current. Twenty-five milliamps is usually enough to cause a flashbulb or electric match to fire so to test continuity, you need to use something that operates in the 1-to-5 milliamp range such as a good-quality Digital Volt Meter (DVM) or as an inexpensive alternative, a piezo-electric buzzer. This sensitivity to current has an upside however. It makes these devices ideal for air starting motors because only small onboard batteries are required to fire them. Flashbulbs and electric matches are both getting difficult to find. The sale of electric matches is being restricted because of a change in the regulatory atmosphere governing such things (they have a small dot of a sensitive high explosive as a primer for the pyrogen). Flashbulbs are getting difficult to find because even disposable cameras are coming with electronic flashes these days. Robby's Rockets is a source for pre-wired flashbulbs or you can resort to buying flashbars or flashcubes and disassembling them to get the bulbs. Beware though, not all flashbulbs are made the same way. "Magicube" flashcubes use bulbs that are mechanically fired and therefore have no wires which you can connect to. "FlipFlash" flash bars have wires but require much more voltage than is available in a rocket launcher. Thermalite is also getting difficult to obtain. Only Brazilian thermalite is still being manufactured and the current regulatory environment make even that difficult to purchase without an explosives license. Representatives of the high power rocket community are working with the regulators to try and bring about an easing of the regulations for us. Note that Visco fuse (cannon fuse) is not the same as thermalite and will not provide satisfactory performance if substituted for thermalite. If you can get (or already have) some thermalite there is an easy way to make reliable first-stage igniters from it without using nearly as much as the flashbulb method uses. You start with a 1-inch (2-inches for BIG, I/J/K/... motors) piece of thermalite from which the fiber sheath and all but one of the spirally-wound wires has been taken off. INSERT FIGURE #4 HERE Prepare two 12-inch pieces of 30-gauge wirewrap wire by stripping off 1 inch of insulation from each end of each wire. Take one uninsulated end of one of the wirewrap wires and starting about 3/8-inch in from one end of the thermalite neatly and tightly wrap the thermalite (and it remaining single spirally-wound wire) with the wirewrap wire. Then with the remaining piece of wirewrap wire, starting about 3/32-inch further in on the piece of thermalite from where the previous wrap ended, wrap this piece neatly and tightly around the thermalite. INSERT FIGURE #5 HERE If you started with a 2-inch piece of thermalite, fold it in half. Finally, you may choose to twist the two pieces of wire together to make a single "cable". This igniter works because the two pieces of wirewrap wire conduct the electricity from the launch controller up to and through the single strand of wire that is spirally wound on the thermalite. This stand of wire has a higher resistance than the copper wire leading up to it and so the small section of it between the two wraps of wirewrap wire will get hot and light the thermalite. While this method will work with wire heavier than 30-gauge, the finer wire is better because it will clear the nozzle better once the motor lights. Making the wraps neatly can be made significantly easier if you make yourself a wirewrapping tool. The easiest way I've found to do this is with three 2-inch pieces of straight brass tubing. One piece is very small and has an ID just large enough to accommodate the stripped end of the wirewrap wire. The second piece has an ID just large enough to accommodate the stripped piece of thermalite. The third piece has an ID just large enough to accommodate the other two pieces of tubing. It should look like this when finished. INSERT FIGURE #6 HERE To use it slide the stripped end of the wirewrap wire up into the small tube and slide the larger tube down over the thermalite to the point where you want to make the wrap. Hold the free end of the wire and the thermalite in one hand and spin the tube between the fingers of the other hand. A nearly perfect wrap should result. If you do not have access to commercial thermalite and are a bit adventurous, Firefox sells a "recipe" and all the necessary chemicals to make your own. Recipes are also available to make the pyrogen from scratch that is used on electrical igniters. For those who are less ambitious, Firefox also sells pre-mixed bottles of "Electric Squib & Igniter Pyrogen" which makes it very easy to make your own ignitors. There are several ways to fashion igniters assuming you have this pyrogen. The method you choose will depend on the size of the nozzle you will be trying to pass the finished igniter through. With the use of zinc chloride soldering flux the nichrome igniters can be assembled by soldering, however I have found that a wirewrapping technique similar to that used for the thermalite is easier. Again you will start with 30-gauge wirewrap wire but this time only strip 1/2-inch of insulation off of each end of each wire. You will need a wirewrapping tool similar to the one used for the thermalite igniters but instead of a big and little tube inside a bigger tube, this one will have two little tubes (each has an ID just large enough to accommodate the stripped end of the wirewrap wire) inside a bigger tube which has an ID just large enough to accommodate the two smaller tubes. INSERT FIGURE #7 HERE Next, assuming you have some 30-gauge nichrome wire (Firefox sells this too), cut off a 3/8-inch piece. Wrap the wirewrap wires on the nichrome wire approximately like this: INSERT FIGURE #8 HERE For the large-throated motors, form the wires like this: INSERT FIGURE #9 HERE and for the small-throated motors, form the wires like this: INSERT FIGURE #10 HERE If you have a good DVM which can measure low resistances, check the continuity through the igniters. The resistance should be approximately 0.5 to 1.0 ohms. If the resistance checks out O.K. then the wrapped portions of the igniters can be dipped in the pyrogen and then left to dry. If your nozzle throat will allow it, multiple dips may be made to build up the pyrogen on the igniter heads. Note that a large deposit of pyrogen in the area of the nichrome wire will be more difficult for the hot nichrome wire to ignite because it will have a higher thermal mass. For quicker ignition using less current, use a relatively thin layer of pyrogen over the nichrome wire and build up to larger thicknesses in the area of the wire wraps and even down over the twisted 30-gauge wirewrap wires leading away from the head of the igniter. These igniters can be fired from a fresh D-cell and they work very well off a 12-volt battery. They require too much current to work off of a typical on-board battery or capacitor staging system made for use with flashbulbs or electric matches but will work with systems capable of delivering one or more amps of current for a short period of time. Rather than working with the 30-gauge nichrome wire, it is also possible to use Estes Solar Igniters (ESI) to make igniters for large-throated motors. Cut off the ESI's leads just below the pyrogen (make sure you are also below any oxidation that may be on the wires at this point). INSERT FIGURE #11 HERE Wrap the wirewrap wires onto the stubs of the ESI. Bend/arrange the wires as shown making sure that the bare wires do not touch. Check for good continuity and no shorts and then proceed to dip it in the pyrogen like the other igniters. INSERT FIGURE #12 HERE Firefox also sells small printed cicuit chips that consist of a copper "M" trace printed on a very thin fiberglass backing. To use them you are supposed to solder a wire to each leg of the "M" and then dip in pyrogen. Based on tests I have done, these do not work well with the "Electric Squib & Igniter Pyrogen" that I have tried so far. With a small battery they are slow to heat up but will eventually ignite the pyrogen. With a large battery they will blow the pyrogen off without igniting it. The bottled pyrogen from Firefox comes with its own instruction sheet that explains how to use it. It is supposed to mixed to a batter-like consistency by adding acetone to the mixture. Since acetone evaporates so quickly, a few drops of toluene or MEK will help to improve the shelf life of the sealed bottle. This mixture is extremely flammable! Make sure you take all reasonable precautions to protect yourself and to prevent ignition of the mixture. Use only non-sparking materials (copper, brass or wood) to stir the mixture. Sources of Materials Robby's Rockets [Flashbulbs, teflon tubing] P.O. Box 171 Elkhart, IN 46515 (219) 679-4143 Magnum Rockets Hobbies & More Inc. [Teflon tubing & other rocketry supplies] P.O. Box 124 Mechanicsburg, Ohio 43044 (513) 834-3306 Firefox Enterprises Inc [Chemicals, Thermolite, Pyrogen, Nichrome wire, Zinc Chloride] P.O. Box 5366 Pocatello, Idaho 83202 (208) 237-1976 $2 for catalog & mandatory order form. Must be 21 or older. I recommend group orders to save on HazMat shipping charges. (The following are {Northern California, Bay Area} potential sources for wirewrap wire, teflon tubing, fiberglass for fins, batteries, switches, lights, piezo-electric buzzers, DVMs and much more) HSC Electronic Supply (Halted Specialties Co.) [Previously owned electronics] 3500 Ryder Street 4837 Amber Lane 6819 Redwood Dr Santa Clara, CA 95051 Sacramento, CA 95841 Cotati, CA 94931 (408) 732-1573 (916) 338-2545 (707) 792-2277 Haltek Electronics [Previously owned electronics] 1062 Linda Vista Ave. Mountain View, CA 94043 (415) 969-0510 Excess Solutions [Previously owned electronics] 568 S. Milpitas Blvd Milpitas, CA (408) 263-9232 Fry's Electronics [Retail electronics] 440 Mission Ct. Fremont, CA (510) 770-3797