Judging from the dark plume of smoke, my source believes is it was a rubber/nitrous oxide that Virgin Galactic’s subsidiary, The Spaceship Company, is reportedly working on.
The source reports the engine ran pretty roughly toward the end, as tends to happen with rubber hybrid motors.
As you might recall, the rubber hybrid appears to be an alternative to the alternative nylon/nitrous oxide engine that Virgin Galactic switched to when Sierra Nevada’s rubber/nitrous oxide engine failed to perform as needed.
A second source says work on the rubber engine is continuing because Virgin Galactic has a great deal of money tied up in tooling for producing that type of engine. It would be expensive to switch over for another type of hybrid.
There are still plans to use the nylon/nitrous oxide engine for SpaceShipTwo flight tests, which are believed to restart soon. On Oct. 4, Virgin Galactic Vice President Mike Moses said powered flight were “going to start imminently, literally very imminently.” That was three weeks ago.