A phenolic-like material, made using something in between sintering and casting-based on the appearance of the final product.
I'd like to stress that we don't necessarily know that the motor was at fault for this.
In other news, with some of the data timing issues resolved, I think i can fairly conclusively say that the nosecone came off of the rocket at about 3.446 seconds into the flight. Both z-accels show negative acceleration before that point (inward acceleration in a spin or corkscrew), and at that point, they both show a return to essentially zero acceleration-the nosecone coming loose and continuing straight away from the booster, no longer accelerating inward. Shortly after (~0.014s) there is an equally sharp negative acceleration, which conceivably could be the shock cord going taught. This is accompanied by a violent drop in internal pressure, which is pretty unmistakably the nosecone coming off, exposing the baro sensors to atmosphere.
I am suspicious but cannot prove at this time that the shock cord did not break instantly. Notice how the baro sensors show elevated pressure up until t~3.9s? I think what happened is the nosecone came off, the cord went taught, and the booster's momentum continued to drag the nosecone along for the ride, facing downward (open end up) for another .3s, between 3.6 and 3.9 seconds on the axis. This would explain how the heavily filtered baro sensor on the raven showed the pressure stop dropping, and the faster-response Aim Xtra actually showing pressure increasing as the open face of the nosecone was hauled forward into the air. As the shock cord slipped around on the metal airframe, it wore, until the terrific load of the nosecone caused it to tear apart over the edge of the aluminum. Then, with the shock cord broken, we see the nosecone make a sudden, although noisy, return to a decceleration we would expect to see for the nosecone flying through the air by itself.
