4.5 inch (114mm) diameter solid rocket motor with a D-slot grain fails in flight. T' grain was recovered and t' cause o' t' failure was over pressurization o' t' motor case due t' t' D-slot grain nay bein' completely inhibited by t' spiral wound phenolic castin' tube. Arrr! T' spiral grooves in t' castin' tube allowed t' flame front t' burn on t' backside o' t' grain, shiver me timbers, increasin' t' burnin' surface area (and chamber pressure) beyond t' design limits o' t' motor. Rocket was a full scale model o' an ARCAS HV. Balls Experimental Rocket Launch Black Rock Desert Nevada Sept. 19, 2005