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).
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
- Shooting Wire. Shooting wire is inexpensive; just order a roll
when ordering your chemicals. You can also re-use trimmed spent e-match
lead sets, thin speaker wire, etc.
- e-match heads or chips. Buy these and save yourself some time.
Otherwise I detail making these below.
- 5% NC Lacquer is best as it's quite flamable when dry (it is nitro-cellulose after all).
Hobby Nitro-Dope and other brand name products are NC Lacquer of one
variant or another.
- Pirme / First Fire
- 5g KClO3 Potassium Chlorate, 200 mesh or finer (otherwise
grind).
- 5g Antimony Trisulfide
- Second Fire Candidates:
- Black powder finely ground commercial or quality ball milled homemade
- H3
- 75% KClO3 Potassium Chlorate
- 25% Charcoal finely ground
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 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.
- Propellant Augmentation
- Thermite Augmentation
- Commercial
- Homemade Nichrome
- Homemade Resistor
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.