E-Match Construction

Electric matches are ubiquitous in experimental rocketry.  They form the core of an ignitor.  The following ematches can be fired with a 12 volt car battery, which gives adequate ignition.  But I usually use 110volt AC house current through long drop cords, lending accuracy to the term "exploding bridge wire."  They burst with enthusiasm.

These are for ground-based ignition only, where lots of current is available.
They will NOT fire from a 9-volt battery, so do not use them with your altimiter or staging device!

Materials and equipment:

Well, I don't really need all of this stuff but I am a junk freak, and most of it will be used before this project is finised.

The bridge wire is the interior of a lamp cord.  Stripping the insulation from 6 inches or so to reveals 40 or 50 very thin strands.  These measure .006 inch on my caliper.  Separate one strand from the bundle and draw it through the superfine sandpaper until it gleams.  This will make soldering much easier.  Sand gently, especially if using the "coarser" 320 grit sandpaper, as these strands break very easily.

The computer geeks at my workplace are always throwing out sections of network cable.  I lurk around the corner, sneak up to the trash pile, grab a cable, wrestle it to the ground and subdue it with a knot.  Whew!

A sharp knife is used to slit the covering two or three inches down, revealing four pairs of wires, neatly twisted together.  Hence the name "twisted pair" cable.  They are nicely color-coded, and are ideal for making ematches.

Then I pull and pull to get more of the covering off.  This is a lot of work, partly because there isn't much to grip, and the covering tends to break if you go at it too hard.

Once a foot or two is stripped, I tie the covering to something sturdy and use both hands.  This makes it easier to apply just the right pull.

Here I have cut 2-foot sections for 8 potential matches.  I like to start with long wires.  They can be re-used by cutting off any damaged sections after use.  Long, virgin ignitors are good for launches.  Short, experienced ones are fine for static tests and ignitor experiments.  This saves a bit of work.

A knot is tied 4 to 6 inches from one end.  The wires are untwisted up to the knot, knot keeps them from separating any further.  About 1 inch of insulation is stripped from each wire.  Sometimes I wonder if I should wait on this, for safety purposes to prevent accidental ignition, and to keep the wire clean until launch time.  Then I get paranoid about coming up to the launch stand without a wire-stripper, so I go ahead an strip them.

Now to the other end, where the bridge wire will go.  The wires are untwisted about 2 inches, and one of them is cut 1/2 inch shorter than the other.  About 1/8 inch of insulation is stripped from the end of each wire.  If a stripped end is not shiny, sand it or scrape it clean!

The bridge wire is wrapped 3 or 4 times around the stripped end of the short wire.  Apologies for the last picture, the wire had slipped partly off.  I corrected this error before soldering, but forgot to get a photo.

These thin wires are pretty squirrely to handle, so I find it better to clamp down the soldering iron and bring the wires to it.

Tug gently on the bridge wire after soldering.  If it comes off, do it again.

Now twist the wires back together, at least as tightly as they were before.  This is to reduce the strain on the thin bridge wire.  Wrap the bridge wire strand around the longer wire 4 or 5 times on the insulation, then 3 or 4 times on the stripped end.  Solder the end.

Now I like to trim the ends, usually cutting off a bit of the network wire at the far end, and getting rid of any frills at the shorter end.  Isn't it pretty!

Not a bad idea to test these with the voltmeter for continuity.  But I usually don't.  They invariably work at this point.  More important is to test one just before an important firing, to be sure the slings and arrows of time and travel have not broken the bridge or compromised a solder-joint.

Longer matches are wrapped around three fingers.  Shortish ones around two.  This makes it easy to find the right length on the first pick.

So here is today's production.  Took me about an hour to do this.  Some folks prefer to buy ignitors.  That is fine with me.  I prefer to make everything I can.

To be an effective ignitor, these will need some pyrogen.  That web page is coming, but the short of it is:  wrap the bridge-wire end in masking tape with 1/4 gram of black powder.  Add a few magnesium turnings if you have them.  These work very well with my candy motors.