I've decided to use copper thermite to ignite my motor(s). How should I measure and mix it, and how should I construct my "motor ignition devices" using it?
Here's a quick list of the supplies and equipment you ought to have on-hand if you want to work with copper thermite:
Once you mix your thermite, you are going to be using it to ignite one or more motors. To do this properly, ensuring near-instantaneous ignition, you want the copper droplet cloud to expand in all directions within the core of the motor, coating (and igniting) it everywhere at once. One or more electric matches (depending on the size of the motor and your need for redundancy) will provide the initial flame, but we need to construct something to hold the powder in contact with the match(es) until it's lit, which will also not impede the dispersal of the copper cloud once it has been lit.
For smaller motors and nozzle throats, I like to use the paper wrapper from a drinking straw to contain the thermite. It's fragile and easily combustible, so it won't impede the spread of the copper droplets. And, it's narrow, so it will easily fir through the nozzle throat of most J or K motors. Plus, you can use the straw to transfer the thermite to the wrapper -- it's like it comes with a ready-made perfect-size tool.
For larger motors, plastic wrap works well, as does tissue paper. In a pinch, a tube rolled from a single layer of wide masking tape, or notebook paper, can work quite well. Just don't make the "container" too strong: you probably want the burning thermite to spread molten copper in all directions evenly, and not to just spray out in one direction.
It's probably instructive to relate the story of igniting a fairly large motor, to give an idea how it all comes together:
I made the ignition package for a Q motor launched at Black Rock a few years back. It used three electric matches, with their heads spaced about two inches apart from each other, both for redundancy and to ignite the thermite charge at multiple locations along its length. A sheet of notebook paper was rolled around a wooden dowel as a form, and cut and taped into a cylinder one-layer thick. The match heads were positioned in the center of the tube, lengthwise, and the bottom end of the tube was securely taped to the wires, sealing that end off. The wires were taped along their lengths to a 1/8" wooden dowel, to provide support. Once the tube was filled with thermite, the top end was taped shut, and we were ready.
At the pad, once the rocket was vertical, the pad was cleared of all non-essential people, and all on-board electronics were armed. Then, a spare e-match was connected -- far away from the motor -- to the clip leads to test for accidentally-energized launch equipment. It didn't light, and that gave us the confidence to test again, this time with the thermite package laying on the ground at the far end of its dowel and wires. After that was also confirmed NOT to light, even when the continuity test button was pressed, it was disconnected from the clips again, and installed in the motor.
The ignition package was inserted about three-quarters of the way up the motor core, and the dowel was taped to the launch pad, to hold it in place. This position was selected because it seemed like it would give a good spread of copper droplets throughout the core. After hooking up the leads, with NO body parts under the rocket, or in a position to get hit by exhaust, the last of us retreated to the minimum safe distance (1500', IIRC) to watch the countdown and launch.
At the end of the countdown, the LCO pushed the button, and the rocket ignited instantly, with no chuffing or hesitation, and leapt off the pad for a perfect flight.
A long discussion had preceeded the construction of this igniter, with different people arguing for more or less thermite to be used to ensure instant and complete ignition of the 8" diameter Q motor. The general agreement, such as it was, seemed to be that somewhere between 40 and 120 grams of thermite ought to be the "right amount" to do the job. However, even after all of the discussions, there were still folks arguing for more or less to be used, and no real consensus had ever emerged.
I mixed up 108.4 grams of thermite (4.42:1 ratio, with 20 grams of aluminum and 88.4 grams of CuO), intending to err towards the upper end of the range discussed, but ended up only using 55 grams, as that was all that fit in the tube I had made. Ignition was instant, so it appears that 55 grams was definitely sufficient.