E-Match

First a warning:  DO NOT USE THESE E-MATCHES TO IGNITE ROCKET MOTORS! These are intended as ejection ignitors, to be fired from a 9-volt (or so) altimeter.  There are NOT intended to be primary ignitors for rocket motors, especially when fired from the kind of system used at most rocket clubs.  The problem?  The "continuity test" button on the junction box produces enough current to fire this ignitor.  When you test for continuity, the rocket motor is very likely to ignite!  With you and who knows who else out there at the pad. This is clearly very dangerous.  So DO NOT USE these devices for your motor without appropriate safeguards.  

Next, a disclaimer:  I feel a bit embarrassed by my own term "JimmyFire."  I did not originate this idea.  Perhaps the clever name should be applied to the fuse paper ignitor, which might be an original.  

No, I got this from the one of the rocketry discussion lists where someone had mentioned using Christmas-tree bulbs for ignition purposes.  I don't recall who recommended it but if you are reading this, please stand up and take a bow!

I didn't think about it much at the time.  Some really good ideas go "whoosh" right over my head.  I'm not ready for them.

But I started getting ready while poking around in a store after Christmas and noticed these little strings on sale for half price:  $.50 for a string of 50.   I bought a couple of strings for camping.  Teresa's research requires that we go camping quite a lot.   We have to go into the boonies and study some aspects of the wildlife therein.   It makes for a long day, and we don't quit until it is too dark to take pictures.   Sometimes not even then.

So by the time we reach camp, it is pretty well night.   We hang these strings of little lights over the tree branches and plug them into a power inverter running off the spare truck battery.   They provide a very pretty light for dinner, dancing, and stimulating conversation way out in the woods. These are much more pleasant than the light-blaster Coleman lantern, which I treasure but don't like to look at. And they don't attract insects either.  Entomologically correct.

When winding the string back up, I noticed something...   They are wired in SERIES!   There is one wire running from light to light to light, and another running from the far end back to the plug.   Series!  So let me count: 110volts divided by 50 lights...  well, that is a little over 2 volts per bulb, right?   I did that in my head!  Aren't you proud?  But that means these are long strings of penlight bulbs for a penny apiece!

And a 9 volt altimeter battery should light a single bulb up really well, right?

I tested one with my Co-Pilot altimeter and found that it does indeed light it up.  So well, in fact, that if you let it stay on for more than a few seconds, it burns the bulb out. That filament isn't very big, but it shines brilliant white.  So its temperature would be what, 5000 degrees?  Seems like that could be used to ignite something, if placed in close proximity to some readily ignitable substance. Voila!  The JimmyFire! Here is the stuff I'll be using.  Pistachio nuts are a recommended option.

Open the box, take out the string, choose one lucky bulb, and remove it from its socket.  They are press-fitted into their sockets, so two wiggles and a pull should do it.  Each glass bulb is in a plastic base with two wires sticking out, one on either side.  The bulb is held in just by the bent wires.  Straighten the wires, and pull the bulb out of its base.  The plastic bases make nice dog treats... Here boy!  Catch!  Hey, don't call PETA on me.  The cat made me say that.

   

The wires look pretty greebly now and could be a pain to solder.  So we are going to clean them.  One of my favorite tricks is to dip anything to be soldered in muriatic (hydrochloric) acid.  Cleans the heck off of it.  Wire goes from black to bright in about two seconds.  It'll clean the leads right off these bulbs given time, so I give it a dip a baking soda solution to neutralize the acid, then wipe it all dry.

   

Yeah, I could clean them with fine sandpaper just about as easily.   Or I could leave them dirty and count on the flux to do the job.  But I like to solder clean wire.  I like to see the redheads smiling at me.  It has a good feeling about it.  Feels like a reliable connection. I have split a foot-long piece of shooting wire an inch or two, and stripped a little insulation off each end.  These wires are clean, but if they had not been, there is that little vial of acid just waiting...

Sometimes I use twisted-pair network cable instead.  It works fine, it's just a little more trouble to get the pairs out of the cable covering.  I will use a twisted pair in one of the examples here.

Now you will think I'm totally weird.  All that trouble to clean the wires, and I'm dipping them in flux?  Please bear with me, there is a reason.

The soldering iron has been heating quietly on the side.  Sometimes I think it should have a whistle like my tea kettle.   There is already a drop of solder on the tip, so all I have to do is touch it to the cleaned, fluxed copper and the solder flows like magic.  Aaah! This means that I don't have to juggle three things with two hands.  By doing a bit of prep work, I have enough hands to go around.

That was normal.  Here is where I earn my pay.  I think I should get some kinda reward for this solution.  The problem?  To break that little bulb open without smashing the filament, getting cut, or throwing glass shards into sensitive areas.

How?  Well what would any red-blooded pyromaniac use... fire, of course!  
Have you ever put glass marbles in a frying pan?  Heat them really hot and drop them in cold water? 
I suggest you try it some time when the kids aren't looking.  They rarely shatter, but craze like crazy.  

Here I am using a small torch to heat the tip of the bulb. I got distracted taking the picture and overheated this bulb, creating a problem. It only takes a couple of seconds - when the orange flame appears, it is hot! The tip of the bulb is dipped in shallow water, causing it to fracture.  As soon as it hisses, take it out.  It's done!

I have used a candle flame and a butane lighter for this purpose, and they both work quite well.  It just takes a little longer.

Here are our three bulbs.  They have all fallen for the latest craze.  Click on the picture to bring up a larger version, and notice the one in the middle.  This is the one I overheated - see the melted spot just before the tip?  That's a problem, because when it hits the water, it doesn't just craze, it fractures and draws up water.  See the meniscus of water just to the left of the melt-hole?  Note the water at the internal blob-of-glass insulator?   All is not lost.  Just break the wet bulb open and let it dry.  Or if you are in a hurry, heat it until the water boils off.  The other two don't have any water, despite having lost their tips.  They were properly heated, and barely dipped. But do check them all for water intrusion because that little bit of water could render the pyrogen non-flammable.

Guess you could see this coming.  I take the rusty pliers and pinch off the crazed glass. It's very easy, and the risk of getting cut is low.  But if you want to wear gloves and goggles, I would certainly not make fun of you.  Not to your face anyway.  Here are our three little bulbs again.  The middle one still has some water in it, but it tested OK, as we shall see. The bulbs are ready to seal up with a little pyrogen.

A reservation:  These bulbs are designed to short out when they fail.  This is so the rest of a string will continue to shine even if a bulb or two burns out.  See that little ring of wire just above the glass glob insulator?  I am guessing that is the shunt - when the filament burns out, the wires spring apart and the wire ring makes contact, preserving continuity. 

Thus it is conceivable that the filament could be damaged but the unit test out OK with a voltmeter, and give a false positive in the altimeter.  This is a good reason to be suspicious of these e-matches, and to use redundant ejection on any project where their failure could pose serious danger. 

On the other hand, I understand that commercial e-matches are also designed to fail to a short, so this caution could apply to them as well.

In my tests here, two of these bulbs did not short out when fired, they tested negative for continuity.  I'm assuming the pyrogen residue got in the way. One of these used Red Dot (my standard now) and the other used black powder.  I goofed and did not make a note which was which.  Guess is that the Red Dot match would be the one that tested continuous after the burn, as it would have had less residue.  Maybe I should do some more tests...

In any event, it's not a bad idea to put on the granny glasses or a magnifier of some sort, and look at the bulb after breaking the glass away, check for water and make sure the filament is intact.

Pyrogen!  Let's start with this pretty one.  It is laid on a four-inch strip of masking tape, base near the middle.  The strip is then folded up and over but not completely closed - we still need to add pyrogen.  It is pinched at the edges to make a little cup to receive the powder.

   

Now we can dribble in the pyrogen.  Here I am adding 0.2 grams of Red Dot, which I find to be excellent for this purpose.   Black powder also works, and I'm sure there are other things.  This is NOT the full ejection charge, it is just enough to set fire to the ejection charge, plus a little more just to be sure... It's a bit of a chore to get the powder past the sticky tape, but be sure some goes in the little glass thingy.  I use the point of the loading wing to pack in gently in.  Gently, so that the filament is not broken - at this point you can't see it.

So let's test this match!  No - more is better!  Let's test all three...

Each one of these tests is fired using a single 1.5 volt AA alkaline penlight battery.  It's not even a new one.  It's been hanging out in a flashlight since the hurricanes last  year, when it got used quite a lot.  

Test 1:  Red Dot  You could hardly see that one fire.  Smokeless powder is also pretty much flameless powder.  But you will note that it had enough force to bend the wire and jostle the little plastic vise.  These have plenty of hot air to ignite an ejection charge - I have used them on about two dozen flights so far (as of 12/05) and none of them have failed to ignite the charge.  Test 2:  Black Powder  At least this one has a bit of smoke and a bad smell.  Red Dot smells like baby powder.  But black powder offers good bit of visible flame to convince the public that this is indeed an ignition source. Test 3:  Black Powder with Ti flakes  Finally, some drama.  It is hard to imagine a pyrotechnic compound resisting ignition with those little balls-o-fire dancing around.  Sometimes I use magnesium flakes, which are similar to Ti in their effect.  If Mg flakes are hard to obtain, one can get an "emergency fire starter" from the camping section of Wal-Mart - it's a bar of magnesium with a sparky striker on one side.  Just use a knife to scrape off some flakes. But the metal is superfluous.  Plain BP or Red Dot is perfectly adequate for this purpose.  Metal would be more appropriate for motor ignition, and that is not what these ignitors are for.  See warning above.

Note that I am not the least opposed to purchasing e-matches.  I have admired the Daveyfires, and look forward to trying other commercial matches. 

But the light-bulb solution is simple, reliable, and cost-effective.  I will keep this technique, and use redundant ejection when the stakes are high.

Jimmy Yawn
4/12/05
rev 12/11/05

Recrystallized Rocketry