Straw igniter
- Cut 80-100mm of a Mc Donald straw.
- Wrap about 10mm of aluminium tape with a single layer around one end. This will prevent the straw from melting when adding the hot glue.
- Insert a fuse head aproximately 20mm into the straw add the taped end. When using a x-mas bulb make sure the filament is about 20mm into the straw. The fuse head of filament will be fully surounded by the pyro charge.
- Add hot glue and roll the end through your finger such that the glue will distribute evenly and to ensure good bonding with the inner surface of the straw. Let it harden and make sure the wires leave the straw straight.
- Add 1gr (or any other ammount) of 3F or 4F black powder. Firstly add halve of the ammount and tap it lightly, secondly one can add the remainder and tap lightly again.
- Add some wadding, cotton wool or a small ball of toilet paper and use a cylindrical piece to tamp and compress it. Be carefull not to damage the filament of the x-mas bulb.
- Check the igniter for continuity if possible.
- Cut the straw that it will extend 5mm out of the wadding and warp a small aluminium tape around this end as well.
- Add the hot glue, roll it through your fingers and let it harden.
- Check igniter for continuity again and you are ready to go.
Link: SMD resistors as bridgewire elements of pyrotechnical initiators
I do however, have a formula that may fill your bill. A fellow club member sent me his recipie, it may help you.
All you do is get some powdered graphite (the kind you bulb action spray into locks to lubricate them) and mix it about four or five to one with black powder (guess you could use Pyrodex here but I have not tested this) and use acetone to make a loose paste. You dip the bare ends of the igniter wire in this mixture to make a “bridge” between the two wires. This is the conductive part of the igniter.
Now for the pyrogen that actually lights the engine. I used common old everyday sparklers that the kiddies twirl around at Christmas and the Fourth of July to make a wonderful (to them) light show. I broke the “pyrogen” off of the wire that holds it and crushed it into a powder which was added to clear Pactra Dope. The mixture might or might not need to be thinned with acetone to get it to a consistency that is good for dipping.
After two dippings, you might come up with a *large* igniter head. This can be overcome by “squashing the pyrogen before it becomes dry/hard. I have thought about putting a drop of CA on the head of the igniters to keep them from disintegrating under rough storage, but have not really found it necessary yet.
A bit of chat recently about igniters, so I thought I’d share the mix we’ve
been using for ages.
It’s conductive so you just dip two wires into it. I can’t take credit for
this mix, but I can definitely vouch for it!
Some of these quantities are below the resolution of our scales so you can
either double the quantities or just improvise which should be okay (that’s
what I do):
Lamp Black 0.25g
AP 1.9g
Ti 1.7g
Copper Oxide 0.05g (in other words: SFA)
+ enough NC lacquer to wet to pancake batter.
We use wire pairs from network cable (cat-5 solid core) for most igniters.
Strip 10mm of insulation from the ends and twist the end a bit tighter to
hold the copper bits about 1-2mm apart, then dip in the mix. It pays to get
your wires all sorted before you start as the lacquer dries out fast, the
quanitities above will make at least 50 igniters. A good tip is to use a
plastic film canister with a small (5-10mm) hole in the top. This reduces
evaporation and gives you more time to dip wires. You can get away with
diluting the mix with more NC maybe twice..
We make smaller igniters (fit up a 4mm diam nozzle) out of computer ribbon
cable, and really tiny ones (up a 2mm nozzle) from the solid core version of
the fin pitch SCSI cable.. Leave em to dry overnight, the resistance should
drop to a few tens of ohms..
Another option is to use SMD resistors for bridgewires. I wrote a paper on
this once, if anyone wants to host it I’ll make it available.
PK