Construction of High Powered Rocket Igniters and Electric Matches

NOTE: The chemicals used and the end products are not toys and require serious care in handling, cleanup, and storage. If you don’t treat them with respect, you may learn about natural selection the hard way.

Dark Flash is a true explosive and if you don’t know what you are doing, you shouldn’t mess with it. Ditto about H3. Formulae for both are readily found on the Web.

If you aren’t willing to work carefully and take your time to get things right, just save yourself some grief and buy commercially.

Electric Matches

The electric match, or e-match, is used un-augmented to trigger ejection and separation charges. By itself, it is not sufficient to reliably light a composite propellant motor. Commercial e-matches are available from several sources (QuickBurst

 & MJG among others), and usually cost upwards of $1.50 apiece. The commercial matches are very reliable, but are costly and the purchaser is requires to have a Low Explosives User Permit from the BATF.

Making your own e-matches is very easy, and if you are meticulous in your process, the results are very reliable. He simplest way uses pre-made “chips” which are small bits of circuit board with a thin (34 to 50 gauge) Nichrome bridge wire on the end and solder on both sides.

Leads are soldered to either side of the chip and then the chip is dipped to form a flammable coating that is ignited by the bridge wire when a current is applied. Matches made in this way will fire off of a very small battery – and should have no trouble firing off of any commercial altimeter. 

Needed:

-          Nitrocellulose lacquer, Dark Flash, H3, & Second Fire Pyrogen  (see section on chemistry)

-          E-match chips – available from many sources

-          22 – 24 gauge solid core wire, preferably two conductor. Standard is “shooter” wire which looks like thin lamp cord. I also have had success with the twisted pair wires in CAT-5 network cable. It’s not as easy to work with, but it is VERY available.

-          Solder

-          Soldering Iron (25 Watt is plenty)

-          Third hand, or other clip to hold everything while soldering

-          Multimeter

Procedure:

I will describe the process for doing a single match, but it is worth the effort to rig up a holding jig to allow you to do multiple matches in a batch.

1)      Cut a piece of shooter wire (or twisted pair) to the length you require

2)      Strip the insulation 3/16” off of one end – you should have something that looks like this:

3)      Using the third hand (or other setup) hold the chip with the wires on either side.

4)      Touch the solder and iron to the wire where it touches the chip just until the solder turns liquid and remove the heat. Repeat for the other side.

 

5)      Test the resistance from the loose ends of the wire with the multimeter. There should be continuity and the resistance should be around 1.4 – 2 Ohms depending on the bridge wire gauge. If you don’t have continuity, or if the resistance is wildly off from this, double check the bridge wire is intact and that your solder joints are good.

Optional: This is a good time to twist the ends of the wire together to prevent accidental firing of the match. If you want to make the joint more robust, this would be a good time to put a small piece of heat shrink tubing over the wire/chip junction.

6)      Mix a thin solution of Dark Flash in 5% nitrocellulose lacquer to use as a primer. Dip the tip of the match head into the mixture so that approximately 1/8” of the tip is coated. Don’t use too thick a mixture or it will tend to pop instead of burn. Too thin and it won’t light. My rule of thumb is that the tip should be black with no metal showing through, with only a very slight visible thickness to the coating. Let this dry for 1 hour before proceeding.

 

7)      Mix a syrupy solution of H3 in 5% nitrocellulose lacquer. This will be your main fire source. Dip the match so that approximately ½ the length of the match head is coated. This can be a thicker layer – I usually dip twice in the H3 so that the match has a noticeable rounded shape. Let this dry for at least an hour.


8)      If you are just planning on using these matches for firing ejection charges, you can stop here – give the match a dip in the lacquer to seal it and let it dry.

9)      To increase the energy output, and to make the matches more useful in igniters, I do a final dip in Second Fire pyrogen. Mix a syrupy solution with 5% lacquer and dip once to coat the entire H3 area with the pyrogen. Let dry overnight.

 

10)   Finally, re-test for continuity. Check again when you are ready to use them. And if you want to be really secure, use two of them for any mission critical (ie: will cost a lot if it fails) situation.

 

Electric Matches prepared in this way will pop and then burn (much like a regular match) when sufficient current is applied. If you test and the match is popping too much – such that it blows off the pyrogen and does not then burn, you probably need to use a thinner primer coating. The idea is to use just enough primer to light the H3 which then lights the 2nd Fire. Pyrogen. If all you will use the matches for is ejection charges, you can skip the 2nd Fire pyrogen and just do a final dip in lacquer after the H3 dries.

 

And test everything. And again. And again. Just because it was good when you made it doesn’t mean it will still be good a year later after bouncing around in your tool box for months.

 

Standard Dipped Nitrocellulose igniters

These are the stock in trade of most folks who make their own igniters. They are easy to make, reliable, and cheap. They are easy to scale up for larger motors. They generally require a 12V ground support system to fire – don’t use for air starts or staging.

Needed:

-          Nitrocellulose lacquer, Dark Flash & Black Sparkle Pyrogen  (see section on chemistry)

-          34 gauge nichrome wire

-          22 – 24 gauge solid core wire, preferably two conductor. Standard is “shooter” wire which looks like thin lamp cord.

-          Solder and flux

-          Soldering Iron (25 Watt is plenty)

-          Third hand, or other clip to hold everything while soldering

-          Multimeter

 

1)      Cut a length of shooter wire (I usually use 3’ leads for smaller motors and go up to 8’ as needed)

2)      Separate the two conductors at one end and cut off one approximately 1.25” shorter than the other.

3)      Strip the insulation from 3/16” of each conductor. You should end up with something like this.

4)      Cut a piece of nichrome wire a couple inches long and wrap it tightly five times around the shorter lead’s exposed end.

5)      Wrap the nichrome around the longer lead going toward the end such that 5 loops wrap the insulated portion.

6)      Finish off by tightly wrapping the nichrome around the exposed copper end of the longer conductor 5 times. The end result looks like this.

7)      Wrapping by itself works well, but for extra reliability, you should solder the nichrome to the copper. Nichrome wire is stainless steel and hard to solder. To make it work, you should use a tinning flux (small containers of this are available anywhere plumbing supplies are sold). Get your soldering iron good and hot, apply a small amount of flux to both points where the nichrome is wrapped around the copper. Apply the iron and solder. The flux will help the solder bind the joint for a good connection.

8)      You now have a small heating element not unlike what is inside your toaster. Use the multimeter to verify continuity. Now all we need to do is add the combustibles. Just like with the electric match, we want a cascade effect where the hot wire will ignite a primer which will ignite the pyrogen which in turn lights the propellant.

9)      Mix a thin solution of Dark Flash in the NC lacquer. Dip (or roll) the prepared wires in the dark flash – making sure to coat the entire length of the nichrome wire. You want a thin coat, but should not be able to see metal through the coating. It should be black.

10)   Next, mix Black Sparkle pyrogen with NC lacquer to a thick syrupy consistency. Dip/roll the coated wire in the mixture to form a nice even layer. Let dry 15 minutes.

11)   Repeat this process to form a nice thick coating.

12)   Let dry at least an hour, then dip in NC lacquer as a sealer and let dry overnight.

13)   To modify for larger motors, simply fold the wire over to double- or triple-up the thickness and dip as before.

 

14)   One additional trick for those hard to light motors is to add some slivers of propellant to the igniter. Two or three thin slivers of Blue Thunder can add a lot of ‘oomph’.

 

Air -start/Staging Igniters

There are two main considerations for an air start or stage igniter: that it fire reliably off of an altimeter or timer, and that it can reliably light the propellant. The usual method of ensuring that the first criterion is met is to use a low-current electric match as the initiator for the igniter. To meet the second criterion, it is necessary that some sort of augmentation is used – i.e. that the match be dipped in a pyrogen of some sort. It is possible to make an air-start igniter without an e-match, but e-matches make it a lot less hassle.

Cesaroni motors are a special case here in that they already have a plug of pyrogen in place in the end grain of their motors. The igniter supplied with a CTI reload is actually just an electric match which is used to ignite the pyrogen and thus the motor.  You should be able to get away with using the CTI igniter as-is with your altimeter since the nominal fire current required is 1.2 amps with a resistance of between 1.2 and 1.8Ω. However, given the nature of air starts and 2nd stage ignition, it is not unlikely that the e-match could slip down away from the pyrogen. If that happens, the match will not be able to light the propellant (unless you are REALLY lucky). In which case, it makes sense to have the insurance of an igniter that can light the propellant whether or not it is properly seated at the factory installed pyrogen plug.

It is my belief that the best bet for this sort of igniter is to have a system that has a high probability of lighting, and which will burn for a long period of time to give the propellant a better chance of lighting. This can be accomplished through careful choice of pyrogen and through the use of the proper geometry of pyrogen to initiator (the e-match).

Simple E-match-Initiated with Bottom-up Burn
 

This is a reliable igniter that lights at the bottom and burns upward like a backwards sparkler. The 2nd fire pyrogen is tough and burns slowly to give more time to ensure that the propellant lights. The use of a single e-match means that most altimeters and timers can fire multiples of these at one time.

Needed:

-          Electric Match, prepared per previous instructions

-          2nd Fire Pyrogen

Procedure:

1)      Test and verify that the e-match has continuity and a resistance in the 1.4-1.8Ω range.

2)      Fold the head of the match over approximately 1” below the solder joint. Try to crease it so that there is no gap between the wires. For larger motors, you can fold a longer length of wire to add burn time, and/or do a double or even triple fold to allow the igniter to hold more pyrogen.

3)      Dip the entire folded portion into a syrupy mix of 2nd Fire pyrogen. Make sure that the match head is covered. Let dry for a couple of minutes until no longer dripping.

4)      Dip a second time, again making sure the head gets covered.

5)      Let dry over night and give a final dip in the lacquer and let dry.

6)      Test to verify no change in continuity or resistance.  Re-test just before use.

Double-ended E-match-Initiated with Dual-End Burn
This igniter lights at the top and bottom and burns toward the middle. It has built-in redundancy so that if one match head fails, the other will probably light and get your motor started.  The 2nd fire pyrogen is tough and burns slowly to give more time to ensure that the propellant lights. Verify your altimeter or timer can fire two matches at one time.

Needed:

-          2 Electric Matches, prepared per previous instructions

-          2nd Fire Pyrogen

Procedure:

1)      Test and verify that both the e-matches have continuity and resistance in the 1.4-1.8Ω range.

2)      Fold the head of one e-match over just below the solder joint. Put the leads of the two matches together so that the bent over match head is approximately 1 to 1.5” down from the tip of the other match. You can secure this by twisting the leads together, with a bit of duct tape, some wire, heat shrink tubing, or whatever. Trim the leads to all come together neatly and twist the matching leads together. You want both matches to fire at the same time off of the same output from your altimeter or timer.

3)      Dip the entire folded portion into a syrupy mix of 2nd Fire pyrogen. Make sure that the match heads are both covered. Let dry for a couple of minutes until no longer dripping.

4)      Dip a second time, again making sure the head gets covered.

5)      Let dry over night and give a final dip in the lacquer and let dry.

6)      Test to verify no change in continuity or resistance.  Re-test just before use. Be sure that your altimeter can fire two matches at once.

An alternate way to do this is to mount both match heads to a single piece of shooter wire. Strip 3/16” off the ends of the wires and solder a chip as described above. Strip the insulation off of both wires 1 to 1.5” down from the end. Bend the wire 180 degrees and solder a chip to the exposed leads. Dip both heads as is making regular e-matches. Use tape, wire, or heat shrink tubing to hold the bent wires together. Dip the whole thing in 2nd Fire. This is a little harder to make, but is thinner and easier to handle once built.

 

Cast Epoxy Igniters

 

For some time, I have wanted to come up with an igniter that is durable, moisture-proof, and which burns really hot. Several sites on the Web talk about making igniters with epoxy as a fuel and binder, but I could never get them to work right – the epoxy would shatter instead of burn, was really hard to light, etc… So I went back to the first principle of lighting a motor – the cascade. These instructions cover a way to use an electric match as the initiator for a cast epoxy igniter. These will light easily off of a 9V battery and so could be used for air starts and staging, as well as ground-based igniters. This design lights at the end and burns downward like a very nasty sparkler.

Needed:

-          Epoxy Fire Pyrogen

-          One of your enhanced electric matches

-          A plastic drinking straw

-          Multimeter

 

Procedure:

1)      Use the multimeter to verify that your e-match has continuity and as resistance around 1.6Ω.

2)      Cut a 1” long piece of the drinking straw

3)      Mix a batch of Epoxy Fire Pyrogen – make sure it is very thoroughly mixed.

4)      Now you need to pack the epoxy pyrogen into the straw such as to embed the e-match with the match-head just at the end of the straw. This part is a little messy/tricky and I am sure there are better ways to do it, but here is my current method.

5)      Insert the head of the e-match through the straw so that it is just on the other end of the straw.

6)      Use a small wooden stir stick to pack the epoxy mixture into the straw from the back (the end away from the match head. Pack it in loosely until the straw is about 2/3 full.

7)      Now hold the back end of the straw (wear gloves!) so the epoxy stays in the straw as you pull the wire so that the match-head is pulled into the straw. Stop when the tip of the match is just even with the end of the straw.

8)      Pack epoxy mixture in from the match-head end until the straw is full and the match full embedded. Most straws are translucent enough that you can see if there are voids. Pack it tightly.

9)      Make sure the tip of the match is at the end of the straw with just a tiny bit of the epoxy mix over it.

10)   Set aside and let cure for several hours.

11)   Use a razor blade to split the straw and peel it off of the igniter and let cure for a few more hours.

 

Chemistry

Nitrocellulose (NC) Lacquer  – The easiest way to get this is to buy it, but if you are feeling cheap and/or like to make your own stuff there is a simple way to make it at home.

-          6 ping pong balls

-          16 oz acetone

Cut ping pong balls into small pieces and add to 16 oz of acetone. Seal in an air-tight container and wait. In about 3-4 days, the ping pong balls will have completely dissolved into the acetone. Give it a good shake. It should pour with the consistency of thin syrup. Dissolving six balls in 16 ounces of acetone will produce a 25% solution. You can then thin this with acetone as needed to get to the recommended 5% solution.

Dark Flash and H3 -- Dark Flash and H3 are true explosives and if you don’t know what you are doing, you shouldn’t mess with them. Dark Flash in particular will go off violently from friction, impact, and cross looks.

Formulae for and procedures for mixing both are readily found on the Web but will not be included here.

NOTE: Both Dark Flash and H3 use Chlorates as oxidizers. DO NOT coat over either of these with anything containing Sulfur, Copper, or Ammonia compounds.

Second Fire Pyrogen – This is a slow burning, durable pyrogen which uses Red Gum as a binder and fuel. This is a variant of the original formula was found on Alan Yate’s site.

-          Potassium Perchlorate (KClO4) : 67%

-          Red Gum: 19%

-          Air Float Charcoal: 9.5%

-          Magnesium:  3%

-          Titanium Flake: 1.5%

Use a mortar and pestle to get rid of any clumps in the Potassium Perchlorate. Diaper the other ingredients together thoroughly and add to the 5% NC lacquer. Stir completely.  Gradually add the potassium perchlorate to the mix to make a paste. Add acetone as needed to get to the consistency of thick syrup. Note that this material must cure overnight.

Black Sparkle Pyrogen – A hot, sparky pyrogen.

-          Potassium Perchlorate (KClO4): 70%

-          Aluminum (400 mesh): 10%

-          Magnesium Powder (doesn’t need to be super fine): 10%

-          Air Float Charcoal: 5%

-          Titanium Flake: 5%

Use a mortar and pestle to get rid of any clumps in the Potassium Perchlorate. Diaper together the other ingredients thoroughly and mix with 5% NC Lacquer. Gradually add the potassium perchlorate to make a paste. Add acetone as needed to get the syrupy solution needed.

Epoxy Fire Pyrogen – This is a castable, water-proof, very tough pyrogen that uses epoxy as its primary fuel. It produces a lot of smoke and ash and doesn’t smell great – so test well away from the house.

-          Potassium Nitrate (KNO3): 68%

-          Magnesium Powder: 7%

-          Charcoal: 1%

-          Epoxy (West Systems): 24%

Use a mortar and pestle to thoroughly break up the potassium nitrate – you don’t want clumps. Carefully measure and mix the epoxy. Diaper the magnesium and charcoal together and stir into the epoxy. Gradually add the potassium nitrate and stir until mixture is a uniform paste. It will be crumbly, but should be wetted through. This stuff has a very definite pot life so you need to use it as soon as it is mixed so it doesn’t harden before you can use it. Don’t mix too much at a time.

Sources

Most of the chemicals required are available in small quantities from various Web suppliers. Some of the materials are readily available locally as well – for example stump remover is typically good purity potassium nitrate.  If you poke around on the Web, you can find information on how to make/acquire a lot of this stuff. But the easy way is to purchase commercial grade chemicals from suppliers.

I have ordered from the following at various times.

Skylighter: http://www.skylighter.com/
Chemicals, electric match chips, shooter wire

Firefox: http://www.firefox-fx.com/
Chemicals, electric matches, electric match chips, shooter wire, nichrome wire

United Nuclear: http://www.unitednuclear.com/
Chemicals

Aerocon Systems: http://www.aeroconsystems.com
Electric Match chips, shooter wire, nichrome wire

Other:

Alan Yates’ Site -- An excellent source for various pyrotechnic formulae: http://www.vk2zay.net/composition/

Richard Nakka’s Site – tons of useful info of all sorts: http://www.nakka-rocketry.net/igniter.html