How did the shuttle designers make those decisions? What made it a better decision to point the exhaust of the igniter initiator toward the main igniter's grain at an angle? What made it a better decision to point the exhaust of the main igniter straight down the bore of the SRB? Was it just because the main igniter was much shorter and "boxier" than the SRB?
My question is inspired by this answer. This image (from there) shows the main igniter's nozzle pointing straight down the axis of the motor and the igniter initiator's (labeled "initiator chamber assembly") nozzles pointing at an angle against the grain