Low pressure ignition
As someone with experience in non-professional rocketry, I can tell you that at the sort of release altitudes you are planning ignition will be a small problem.
The low pressure will be your biggest factor you will need to keep the igniter in the motor until you get sufficient heat to ignite the propellant (550 degrees F is required). You must achieve this without blocking the nozzle as once ignited the pressure will continue to build.
A note of caution APCP (that’s the propellant in use in your selected motor) and from memory the Aerotech propellant formula is their Black Jack variety, a Smokey propellant :-) will continue to increase the pressure until Lohan has a CATO so use caution in how strong you seal the chamber.
Once the propellant reaches 550f then the reaction will be self-sustaining as the APCP will provide the needed oxygen and heat to continue the process until burn-out of your main engine. However should your motor chamber be exposed to the near vacuum before the engine is up to full pressure it may chuff and not burn successfully.
Unfortunately due to the small motor selected my favourite igniter technology copper/magnesium-thermite is unavailable to you.
So the lesson will be, keep the igniter sealed in the motor but not so tightly that engine and Lohan are recovered in flaming pieces.
Onward and Upward, want more information visit http://www.tripoli.org (A Tripoli members recent project.. http://www.youtube.com/watch?v=rvDqoxMUroA and a little traffic Lohan needs to watchout for)
Bubba Von Braun