I've fired 1 inch diameter rockets with B sized motors from a Sig 1/3 Spacewalker and also a 1/4 scale Cub. The Cub had two rockets, one under each wing, and the Spacewalker had three rockets on each wing. I used a shorted 1/4 inch phono jack for a safety interlock to arm the firing system just before takeoff, and had two microservos closing spring loaded microswitches to arm and fire the rockets while in flight. I tested both Estes igniters and flashbulbs with thermalite to ignite the motors. The flashbulb system was the best for the Spacewalker since different speeds of thermalite could be used to stagger the firing of the rockets. The launch rails mounted on the wings were simple 4" lengths of 3/16" dowel that were attached to the underside of the wing with a pylon to stand the rocket off from the wing by enough distance so that the delta shaped fins would have a small amount of tension on the bottom of the wing to stop the rocket from sliding forward if the plane was in a dive. The plane would be pulled into a climb, then the arming and firing servos closed to ignite the rockets. It was way cool. Is it AMA legal? Hell, no.
Charlie