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To the original post -
I'll use anything available as the initial heat source. For small motors, I tape a strip of blue propellant to the ignition source. Once the motor is up to an N, I use a single pyrodex pellet instead. At O, I use two pyrodex pellets. In these cases, ignition is typically about 3/4 second and it is a soft start. It is very reliable so far.
One can also use thermite but many use formulations which for all practical purposes detonate. I don't like the concussion inside the motor.
One wants to dump enough heat into the core so that a notable portion of the propellant surface is raised to above its ignition temperature. IMHO one does not want to dump enough heat in to raise the whole surface above ignition temperature - I want the igniter to burn out before the motor comes up to working pressure so that it doesn't add to the initial chamber pressure.
This is for ground level ignition of course. I presume at sufficient altitude it may be necessary to use an igniter design which will also aid in the initial pressurization of the motor in some cases. Besides, at lower pressures and temperatures one will need to dump a little extra heat in to assure ignition.
NASA published a paper on igniters years ago. One can find .pdf scans of it with a bit of search.
Gerald
I'll use anything available as the initial heat source. For small motors, I tape a strip of blue propellant to the ignition source. Once the motor is up to an N, I use a single pyrodex pellet instead. At O, I use two pyrodex pellets. In these cases, ignition is typically about 3/4 second and it is a soft start. It is very reliable so far.
One can also use thermite but many use formulations which for all practical purposes detonate. I don't like the concussion inside the motor.
One wants to dump enough heat into the core so that a notable portion of the propellant surface is raised to above its ignition temperature. IMHO one does not want to dump enough heat in to raise the whole surface above ignition temperature - I want the igniter to burn out before the motor comes up to working pressure so that it doesn't add to the initial chamber pressure.
This is for ground level ignition of course. I presume at sufficient altitude it may be necessary to use an igniter design which will also aid in the initial pressurization of the motor in some cases. Besides, at lower pressures and temperatures one will need to dump a little extra heat in to assure ignition.
NASA published a paper on igniters years ago. One can find .pdf scans of it with a bit of search.
Gerald