@radix
They had abandoned the engine mixture related to the 2007 accident. That engine had a rubber-based fuel (HTPB) and was being developed by a sub-contractor (Sierra Nevada Corp -- SNC).
That 2007 accident actually had nothing to do with the engine all. They were testing the flow of nitrous oxide between two tanks. No cause was ever released, but there is speculation that one of the nitrous oxide tanks was overheated in the desert sun, and the gas inside reached critical temperature.
The HTPB engine SNC was supplying had serious vibration issues with burns longer than 20 seconds.
So earlier this year, Scaled decided to abandon HTPB and take the engine design in-house. They switched to a fuel based on plastic (polyamide) and they subsequently completed several successful ground test firings with the new polyamide fuel.
Today was the first attempt to use the new polyamide fueled engine in a powered flight test.