Making your own Igniters

by Warren Massey

Before Estes supplied the plastic plugs with the motors, Estes recommended and everyone used a small wadded-up ball of recovery wadding pressed into place with a pencil. The ball of wadding should be fairly small and pressed deep behind the igniter with a pencil or other pointed object.

Also, it is much easier to keep the two igniter wires from touching each other and shorting out if the paper tape is removed from igniter before you install it.

 

End View --- Proper Wadding ball Insertion
WRONG! Igniter leads shorted together	Correct - Paper tape retained Igniter leads come out on opposite sides of the wadding ball.	Correct - Paper tape removed Igniter leads come out on opposite sides of the wadding ball.

It is ecologically beneficial too! Removing the paper tape and not using the plastic plugs has an additional benefit of reducing the amount of trash left out at the pads.

For igniting composite motors, Aerotech Copperhead® igniters are the device many people usually use, but they are less reliable than is desirable and are totally unsuitable for 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 several 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.

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.

There are several companies that sell the necessary supplies so that you can make your own igniters.

Firefox
Firefox sells a "recipe" and all the necessary chemicals to make the pyrogen. They also sell pre-mixed bottles of "Electric Squib & Igniter Pyrogen". Since this pyrogen comes premixed, a HazMat shipping fee makes its purchase more costly. The bottled pyrogen comes with its own instruction sheet that explains how to use it. It is supposed to be 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. After dipping the igniter elements into the pyrogen they are left to dry until hard. This pyrogen dries to a hard consistency that does not tolerate bending without flaking off.

FireStar Electric Match
Jerry Dyben sells kits that can be used to put together good quality igniters. The pyrogen is shipped in two parts to avoid HazMat fees and possible BATF regulation. You add a powder to a liquid when you receive it and then mix to form the pyrogen. Jerry supplies everything you need to get started and even insists first-time buyers buy a starter kit that includes a video tape showing you how to use the material. The video tape is very obviously an amateur production and can be quite tedious to watch in places but it will show a beginner igniter maker how to properly make the Firestar igniters. The Firestar pyrogen shares the solvent evaporation problem with the Firefox pyrogen but Jerry tells you how to deal with it and includes a bottle of solvent (lacquer thinner) with the starter kit. One area where the Firestar pyrogen is better than the Firefox pyrogen is that the Firestar stuff remains somewhat flexible after it dries, making it easier to handle without crumbling.

Jerry also sells igniter wire assemblies in several sizes consisting of two lead wires with a fine COPEL (copper/nickel) bridge wire element soldered in place. These wires are a bit pricey and Jerry makes several poor excuses for why they are so, but if you don't want to be bothered making your own then these at least appear to be well made. I have also seen one unconfirmed report that FireStar sells COPEL wire in 35/ 38/ 40 gauges if you want to make your own wire assemblies.

Rocketflite
Greg Dyben (Jerry's son?) is selling an igniter kit that sounds very much like the Firestar kit but at a lower price for the wire assemblies and without a stated requirement for first-timers to buy a starter kit.

Unrocketry
Robert Briody is starting to sell a "strip-and-dip" construction system. There are no bridge wires to wind or soldering required. The Aerotech copperhead uses a similar nonmetallic resistance element in place of the bridge wire. You just have to strip a small amount of insulation off the ends of two parallel wires and then dip the exposed wire ends into first, a conductive slurry that forms a bridge element when it dries, and then a pyrogen. There may also be a protective lacquer overcoating.

Trailing Edge Technologies Inc.
These people sell pre-manufactured igniters similar to the ones you can make yourself for not much more money. One notable thing about some of these igniters is that they can be ordered with tungsten bridgewires (like in light bulb filaments) which means they probably work with less electrical power than the other types.

There are several ways to fashion bridgewire igniters assuming you have the pyrogen. The method you choose will depend on the size of the nozzle or propellant-grain slot you will be trying to pass the finished igniter through.

Nichrome wire can be used for the bridge wire element but if you are going to solder it you will need to use zinc chloride (Rubyfluid is one brand that works) soldering fluid. The COPEL wire requires a special 2% silver solder (Stay-Brite) if it is to be soldered.

Craft stores carry 32 gauge iron "beading" wire that works well with normal 12-volt launch systems and the iron wire is easy to solder using rosin-core electronic solder.

It is possible to use Estes igniters as a source of a fine bridgewire for making igniters for larger motors.

The tiny Christmas tree lights that come in strings of 60 to 100 are a good source of a bridgewire that works with low-power ignition sources. (These should work great for ejection charges)

Almost all igniter manufacturing starts with twisting wires together. I have found the following wirewrapping technique to be a good means of accomplishing this. The wirewrapping process will go much faster if you make (or possibly buy from RadioShack) a wire wrapping tool. If you make your own it 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. Hobby stores sell brass tubing that works well for this.

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. Optionally solder and check for good continuity and no shorts and then proceed to dip it in the pyrogen like the other igniters.

Firefox also sells small printed circuit 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 large battery (12-volt gel-cell battery) they will blow the pyrogen off without igniting it.

All of these pyrogen mixtures are extremely flammable! The dry pyrogen can be sensitive to shock and friction! 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. Be Careful!

Historical Information - Using Thermalite

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 they do not require very much current to fire them. Static electricity can cause them to fire! Unlike LUNAR's flashbulb-safe launch system, 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 the resistance check circuit of most inexpensive Volt-Ohm Meters. 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 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 try to buy 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 and the current regulatory environment makes any you do find 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.

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.

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.