Batch 2017-batch-05

Another try at making our own igniters, using the "epoxy fire" formula from Scott Berfield. Moving from the pre-made ematches that "pop" to home-made wire-wound bridge wires with 6 turns of 30ga nichrome wire, and we'll try the "slow" West Systems hardener. We'll also try using slightly larger magnesium particles, sifted from the "saw chips" bag, not the powder.

Made up a 15g batch with the following formula:

  • 68% KNO3, 10.20g, hand ground with mortar and pestle to fine grain size
  • 7% magnesium powder, 1.05g
  • 1% air float charcoal, 0.15g
  • 24% West Systems 105+206 epoxy, 3.60g
    • 3.03g 105 resin
    • 0.57g 206 hardener

Two folded papers were used to weigh out the KNO3 in one, and a mixture of the magnesium powder and charcoal in another. The epoxy was mixed in a small disposable plastic cup with a popsicle stick, then the magnesium and charcoal were poured in and mixed until complete wetted. Then the KNO3 was added and mixed until everything became a homogenous mix. It was a dry-ish, crumbly consistency, but packed into lumps easily.

Two sizes of plastic drinking straws used. One is about 3/16-7/32" diameter, the other about 1/4" in diameter. Both were cut into 1" lengths. The procedure was to slide the ematch through the straw, pack material into the straw from the "back" end until it was mostly full, then pull the ematch lead until the match head was just at the forward end of the straw, then pack more material in the front end of the straw until the match head was just covered. Using nitrile-gloved fingers, it was then easy to mash on both ends simultaneously to make sure we had good packing with minimal voids, leaving both ends flush with the straw.

The matches were left to cure overnight, and then the straws were carefully sliced lengthwise with a razor blade and peeled off, revealing what looked like very solid cylindrical igniters. They look a lot like the Quickburst ones.

These igniters were made on 2 June 2017.

Results

We got 6 of the larger and 3 of the smaller igniters out of this batch. It was very difficult to pack the smaller straws, probably due to the larger Mg particle size. It could be that we need to go back to using the finer Mg powder when making smaller igniters, and reserve the "chunky" stuff for large ones?

At NCR's Mile High Mayhem 2017, we used a 12V battery to test one of the larger igniters, and the bridge wire burned and burned a small amount of the pyrogen, but then it fizzled out without doing anything really useful. That made us skittish about trying them on the rail, and we used up the rest of our commercial igniters and bought a couple more that weekend instead.

Once home, we tested the rest of the batch. About 2/3 ignited and burned ok, but fairly slowly and not very evenly. About 1/3 did what the one we tested at the launch did, burning slightly then fizzling out.

The burn time was at least 5 seconds on each igniter, with about 1" length of pyrogen. That really seems too slow. 1-2 seconds would be better?

Observations

We think the larger magnesium particles, which make nice "spitty" igniters when the igniter actually ignites, would probably help start big sugar motor grains, but we really can't call this batch successful. Our theory is that the failed igniters hit a big chunk of magnesium that they didn't have enough energy to ignite before enough of the epoxy-based pyrogen was burning to sustain combustion.

Robert noted that large magnesium particles also seem to raise the risk of starting a grass fire around the launch pad if the motor does light and the rocket takes off before the igniter completely combusts.

Next, we'll try going back to the powdered magnesium with bridge wires.