Huge fan of space flight development since a kid in the 60's. Thru some little reading and internet chasing, it appears ......... Who knows how accurate....
2014 change of engine
In May 2014, Virgin Galactic announced a change to the hybrid engine to be used in SpaceShipTwo, and took the development effort in-house, terminating the contract with Sierra Nevada and all development on the first-generation rocket engine.
Rather than use rubber-based HTPB in the solid portion of the hybrid rocket motor—which had experienced serious engine stability issues on firings longer than approximately 20 seconds—the Virgin Galactic-developed SS2 hybrid rocket engine would now use thermoplastic polyamide (i.e., nylon) as the solid fuel component of the propellant. The plastic fuel was projected to have better performance (by several unspecified measures) and was expected to allow SpaceShipTwo to make flights to a higher altitude.
As of May 2014, the new engine formulation had already completed full-duration burns of over 60 seconds in ground tests on an engine test stand. However, four additional ground tests of the polyamide-fueled engine are anticipated before the SpaceShipTwo flight test could resume with the new-fuel rocket motor.
PF04 malfunction
On 31 October 2014, the new polyamide engine fuel formulation was used in flight for the first time in the PF04 powered test flight of SpaceShipTwo. At 10:12am PDT, VSS Enterprise suffered an engine malfunction, and subsequently broke up in mid-flight. The inflight mishap resulted in the death of one test pilot and severe injuries to the other test pilot, and a total loss of the vehicle.
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Appears it may have been the first inflight ignition of the new propellant combination in the motor