Ejection E-matches and Motor Igniters

E-matches as a general category are widely used in rocketry. The classics were the Daveyfire 27B and the 27F. The B for Burst and the F for Flame. Key attributes are power and purpose and they're generally inversely proportional.

When homebrewing igniters it is imporant to keep that distinction (application - burst or flame, current requirements) in mind. It is also important to keep the concept of ignition chain in mind. As with most pyrotechnics the first fire is rarely the final fire and that is  because the device rarely takes fire itself. So an ignition chain that the flame passes through is designed to take fire from whatever ignition source through intermediate composition(s) to the actual device itself

You will see this is particularly important when finessing the brief burst of heat from a short 50 gauge bridgewire into something that can reliably fire black powder 100% of the time. Indeed, it's critical for this application. Note also that a scaled up quantity of the same composition(s) used for a motor igniter may fail. Consider Primes Burn - Primers Expolode!  Indeed, I've over-primed motor igniters only to have them POP and not pass fire to anything.

Past the ignition chain also is applicability. A large APCP motor will do better with flaming bits of hot metal or slag that are totally inappropriate or unnecessary for an ejection charge (and may burn holes in your chute too).

E-Matches

Acquiring e-matches for ejection charges is more problematic now. The DaveyFires are OOP and while I generally went commercial for this critical recovery component I now make my own if not for direct use also as spares if they're unavailable at a launch. And a larger project with redundant recovery electronics will consume four e-matches per flight.

I've played around with the "christmas tree bulb" igniters and while they work there's really no point (to me) past the novelty given the possible reliability issues of them as well as the general availability of commercial e-match heads.

Commercial e-matches generally seem to use a very short bridgewire of 50 gage nichrome. You can homebrew these by winding nichrome around strips of PCB material, soldering, and then cutting out the chips. I would suggest this as preferable over christmas tree bulbs. Alternatively quality chips are available by the hundred or thousand from ODA or you can buy smaller quanities of the same from Skylighter (Part GN5030 Electric Match Heads) and I believe FireFox.

Constructing Ejection "B Type" E-Matches

We will use 50 gage nichrome igniter chips. we will prime them with a very sensitive prime called dark prime and finish with a less sensitive coating such as H3 or black powder. It's almost as easy to make 50 as it is to make 5 so plan ahead.

Gather Ingredients

Prepare Leads

Cut the shooting wire into your target lengths. I always found Davey fire and others much too long (and re-used the trimmings). Just make them long enough to comfortably wire your altimeter bay. I generally cut mine to about 24" long. Cut three or four dozen.

Strip the ends. Highly recommended are the self adjusting wire strippers. They're a godsend as they don't cut the wire, require no adjusting, and will strip both leads of twin-lead at the same time.

The ignitor chip end needs only about 1/8 to 3/16" stripped. Any more than that and the length may interfere with the chip end.

The altimeter end only needs  3/16" to 1/4". I like to start this end at about 1" and stop the stripper after 1/2 or so. leave this insulation on.  Then give this end a half to three-quarter twist to short the leads. When ready to use give a twist and pull to expose the ends.

Solder the heads

Solder the e-match heads on. Use the short stripped ends of the leads and solder to the bridge end opposite to the nichrome bridge wire. The key is to keep the bridgewire end clear and unencumbered for proper and consistent priming.

Prime matches

The prime selected is often called dark prime. It is a very sensitive compound so only a small amount is is needed or appropriate.This compound is also impact and shock sensitive. Another reason to not over use.

Mix 5 grams of Potassium Chlorate with NC lacquer to make a syrupy slurry. Mix thoroughly. Add 5 grams of Antimony Trisulfite. Mix thoroughly adding NC lacquer as to make a syrup. Add a little acetone to thin.

Dip the head of the pre-prepared igniters into the slurry. Dip only about 1/8" to just coat the end of the chip. Do not dip deeper. Do not dip again. You need just enough for a first fire to transfer fire. Any additional of this compound risks a pop and no fire transfer or ignition.

The slurry will thicken as you work through your leads; add acetone to thin as necessary.

Allow these primed leads to dry for a couple hours or so before proceeding. Properly discard the remaining composition.

Second Fire

Mix 5 grams of your choice of H3 or black powder with NC Lacquer to make a syrupy slurry. The H3 will be more sensitive but good quality black powder is acceptable.

Dip each igniter head completely and onto the wire insulation. Re-coat if necessary but note that it's easier and better to build up by multiple dips than trying to get one or two single thick dips. After final dip allow dry for several hours or overnight.

Seal

Dip each igniter into 5% NC lacquer and allow to dry. Dip past the composition to ensure a good seal. It's not strictly necessary to dip but it is highly advisable and will increase reliability of the stored product. Only one dip is necessary.

Test

3 or 4 percent of the igniters should be tested to ensure they don't pop and that they take fire reliably. Indeed, it's best to test a few at the prime dip stage and adjust that compound (in particular to not pop) before proceeding.

Note that these e-matches are very sensitive and may fire with some higher current continuity testers (esp including multimeters)

Motor Igniters

Motor igniters are almost as problematic as e-matches for ejection with the slight benefit as being less sensitive to contstruction - so one may therefore trust more aftermarket (esp hobby) suppliers. Indeed, a ignition failure, unless in a cluster or staging, requires only a recycle to the next relay with a new igniter.

Constructing Ignition "F type" Igniters

It's important to choose a wire gauge suitable for your motors and in particular your nozzles. The shooting wire based igniters such as the OOP Daveyfire 28F will ignite a wide range of motors. But they themselves won't fit in the smaller motors and require augmentation for the larger motors.

Prepare Leads

Cut the shooting wire into your target lengths. This length based on the length of the motors as well as suggested excess to secure the leads around or to the launch pad (motor length + 2 feet or so).

Strip the ends. Highly recommended are the self adjusting wire strippers. They're a godsend as they don't cut the wire, require no adjusting, and will strip both leads of twin-lead at the same time.

Strip the alligator clip ends. Start an inch to inch and a half to leave plenty to wrap around the alligator clip. A full squeeze of the strippers will leave about half an inch or so exposed. Leave the insulation on so they're not pokey but give it a 3/4 to full turn to short the ends for safety.

The business end should have one lead stripped about 1/4" and the other lead cut back 3/4" and then stripped back about 1/4". That results in a 1/2" insulated span between the copper wire.

You now have a choice of resistor or nichrome elements.

Resistor Elements

Resistor elements are easy though perhaps not as robust  nor quite as fast as nichrome elements. You want 1/2 watt, 10 ohm, carbon resistors. They're very inexpensive at about 3 dollars for a hundred pack at any mail order supplier.

Do not substitue any other wattage, resistance value, or type of resistor. This {value; rating; type}is proven to work.  In particular you're intentionally over powering the resistor, concentrating the heat in a small area, and having it also survive long enough to ignite your composition. A 1/2 watt resistor would, for example,

Position a 1/8watt 10 ohm carbon resistor between the exposed wire leads. Twist to secure and solder. Personally I find it easiest to twist the cut back end, re-position the resistor against the wire lead, twist off the lead to the end, trim, and solder.

Nichrome Elements

Nichrome elements are a bit more work but are very robust. Generally a 30 or 40 gauge nichrome wire is selected. Buy a hundred feet or so so you can maintain consistency (or remember and replentish with the same size). Nichrome is available from Omega as well as many online suppliers.

For wrapping the nichrome, some people use a wire wrapping tool, but I find it as easy to hold and wrap by hand. Note that if you plan on not soldering (I strongly recommend soldering) then you really should use the tool to ensure as tight a wrap as possible.

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