Which specific chemicals should I use to make copper thermite for motor ignition? Why?
The most commonly used (and therefore, the most well-understood and probably the safest) chemicals to make copper thermite for motor ignition are:
Generally speaking, the finer the chemicals, the more thoroughly and intimately they can be mixed, and the faster the resultant reaction will be. Faster doesn't always mean better! If the reaction goes too fast, the mist of copper droplets will be too fine, and they will transfer their heat to the air in the motor core rather than the actual propellant, possibly resulting in nothing more than a loud noise and an unlit motor. Also, finer mixtures are easier to ignite, and therefore potentially more sensitive or prone to accidental ignition due to static electricity.
We want the reaction to be fast enough to provide near-instant ignition, but slow enough to burn and throw larger copper droplets, rather than pop, when lit, and we want the chemicals to be relatively safe to handle and mix.
A number of rocketeers have experimented with various sizes of aluminum powder in copper thermite mixtures, and 325 mesh seems to provide the optimum balance between slow enough to guarantee motor ignition, and fast enough to make it "instant", without risking accidental premature ignition.
400 mesh Aluminum works in place of 325, although it does react quite a bit faster. If you can't find 325 mesh, 400 mesh is a marginally acceptable substitute. Try to find 325 mesh, however. It's much more trustworthy....
Experiments with smaller aluminum particles (625 mesh spherical) have been very unsatisfactory: they burn far too fast, resulting in a sharp "crack", rather than a "poof" when ignited. Because of this, the 625 mesh aluminum hasn't seen any practical use, and its sensitivity (to static, friction, impact, and other potential sources of premature ignition) is unknown, but presumed to be higher than desired.
Therefore, we recommend that you DO NOT EVER USE metal powders finer than 400 mesh in copper thermite mixtures for motor ignition under any circumstances. 325 mesh is optimal.
Coarser metal particles have been tried with some success, however. An aluminum flake powder intended for use in automotive paints, with approximately a 125 mesh size, was tried, and worked successfully, but motor ignition was slightly delayed. If your goal is "instant on" ignition of your APCP rocket motors, you should use 325 mesh.