Re: APCP Burning Characteristics in REHAB
I'd agree, adding a large pressure (vacuum!) tank to the system will act as a buffer to suck the gasses given off by the igniter and therefore minimize the added pressure.
Where you would get a large-ish container capable of withstanding that amount of vacuum i am not sure. Perhaps bodge it from welded steel plate - it doesn't have to be complex, just a hollow box with an outlet pipe.
However I would go against the idea of changing the rocket motor nozzle characteristics. I think once lit, the internal pressure of the motor will not be dependent on the ambient low pressure outside.
Shrinking the nozzle diameter could lead to a rather nasty bang.
In terms of a burst disk with a hole just large enough for the igniter to poke through into the motor, I think that may be over-complicating it. The fuses are designed to be a rather tight fit in the nozzle already, so I doubt an additional diaphragm/disk would make much difference to be honest.