4.5 inch (114mm) diameter solid rocket motor with a D-slot grain fails in flight. The grain was recovered and the cause of the failure was over pressurization of the motor case due to the D-slot grain not being completely inhibited by the spiral wound phenolic casting tube. The spiral grooves in the casting tube allowed the flame front to burn on the backside of the grain, increasing the burning surface area (and chamber pressure) beyond the design limits of the motor. Rocket was a full scale model of an ARCAS HV. Balls Experimental Rocket Launch Black Rock Desert Nevada Sept. 19, 2005