Livermore Unit of the National Association of Rocketry              July/Aug 2002

Advice was requested on igniting a large BP cluster, for a rocket using 8 carefully selected Estes igniters. The idea of using 16 alligator clips was not appealing, and even if the igniter leads on adjacent motors are twisted together, that would still be 8 sets of leads to deal with.

The first suggestion was to build a custom ignition harness for this job, soldering 8 igniters together with only 2 leads for the alligator clips, giving fewer connections and fewer spots to get shorts.

The use of alternative igniters was suggested, with options including rolling your own, buying a bunch of nichrome igniters, or useing a FireStar igniter. The latter was acknowledged as being an expensive solution, however. For rolled nichrome, FireStar Pyrogen, or the Igniter Man stuff, was suggested; just dip up a bunch of home brews, make a "clip whip" setup and push the button.

Estes igniters are ok for single motors, but for clusters, especially big clusters, a hot, reliable igniter is a must. Sounds like homemade ignitors are the way to go then. More reliable and longer leads. Small ignitors can be made using #28 wirewrap wire and Rob's secret sauce (Igniterman kit).

When building a custom ignition harness, parallel was suggested as being better than serial ignition, but perhaps a network configuration would be the best. For 8 igniters, maybe a 2x4 network. But if you're using a simple clip whip, it's all parallel pretty much. It kind of depends upon how much space there is between motors as well. If you can't get a simple Estes igniter lead to reach over to the next one, then you suddenly have options to consider. If you're making your own, use 28 gauge wire wrap wire, or 24 gauge red/white bell wire with nichrome ends, and make them long enough to reach. Twist all the white ones together, and all the red ones together. It's a parallel hook-up. Or, to make the network, twist two whites together and two reds together, and daisy chain them. You'll have four sets of parallels in series. This will help keep the overall resistance lower than having all eight in series and possibly not firing. One thing to note, however, is that they should all be really close to the same resistance value or you run the risk of popping one first (the one with the lowest resistance value) and leaving the rest high and dry. That's why I think that plain ol' parallel is best. If the launcher can push the juice, it will work the best.

As for metallic vs. plain BP. No real need to use a metallic pyrogen, but it always burns hotter, and if there's a little clay blocking the nozzle, it will probably not be a problem. You get a lot of "burn for your buck", and with a big cluster, why chance it? Pyrogen is cheap (if you have it already - or if you can bum some!) compared to the possible failure and embarrassment!

The reference to "pyrogen" is talking about the compound from the Igniterman kit, which is normally used as a second coat over the black "conductive primer" when making igniters for composite motors. For BP motors, the Igniterman instructions say that you can make igniters using just the black primer, but a light coat of the gray pyrogen (evidently some sort of powdered metal/oxidizer/lacquer mixture) will get things going all the more vigorously .

As for firing several Igniterman dips in parallel, the LUNAR high power system should handle it... when you make them, check the resistance of each igniter after it dries and pick out matched sets.

I'm sure the LCO at any LUNAR launch would be willing to pop a "test fire" for you if requested. (For safety, it might be best to do this when there were no rockets hooked up on the other pads of that set, avoiding any risk that might arise from the fact that you would be out at the pad area watching the igniters when the system was armed to fire them...)

A couple of other suggestions... use 2 different colors for the wire, to save confusion when connecting the igniters together - you can then connect all the red (for example) wires together, and all of the blue wires together, knowing that there's no way you could accidentally connect both wires of any of the igniters to the same terminal. Also, it's probably a good idea to leave a long-ish tail on each igniter (perhaps a couple of feet) so that if first motion does occur before the last igniter fires, it has a moment or two to get going before the wires pull out.

Lee Teicheira’s check list looks like:

• Igniter man igniters are the only way to go.
• made with wire wrap wire Per Rob's directions, the will go into an Estes D or E with ease. I taped them in as per Rob's recommendations. I'll do it that way again.
• do use a spark plug gauge to set the gap when making the igniter. Try to keep it constant. I can constantly build igniters at 1-3 ohms by keeping the gap constant at .010 and the length of the bare copper constant as recommended in the directions. Both effect the resistance. Lower resistance means more current which means better ignition. A gap of .010 aint very big.
• David has it right when he said to check the resistance and use a closely matched set. One low resistance igniter in a cluster will steal the current and ignite before the others.
• DEFINITELY wire them in parallel. In series, current stops flowing when the first igniter fires. Not a good thing.
• Make each igniter with 2 different colors of wire. Makes connecting all these igniters in parallel a no brainer. Just twist all the same color wires together. That's a good thing.
• Even for BP, dip the igniter in pyrogen, at least once. More, hotter igniter fire (good grammar huh) is a good thing in clusters. If your pyrogen is to thick and makes a big lump of an igniter, thin the pyrogen with a little acetone. The on-line directions for IgniterMan igniters talks about this. Do a search. I think I found them at Pratt Hobbies. Don't try this with other brands unless recommended by the manufacturer. IgniterMan is the best. (How am I doing Rob?..:)
• I also use the sealer on all of my igniters. I had some igniters melt together once that were not sealed and stored together in a baggie.
• Take a BIG rag to wipe that grin off your face when this thing goes. BP clusters are way cool. Especially at night. Mix a few Es into that 7 motor cluster, and you get a BP mild *H*.