E9-4 motor nozzle blew out on ignition in an Estes MegaMosquito. Rocket hopped about 1" or less on rod then sat there while grain burned out with no significant thrust. Ejection charge fired as expected and enough to pop nose cone but not rest of recovery system. No damage to exterior of the motor case. I flew another motor from this pack earlier in the day with excellent results and have one motor remaining.