No details. We don't know why. However, they had a problem scaling up the hybrid rocket form SpaceShipOne. It was a great innovation: solid rocket, but no oxidizer. Instead they blew nitrous oxide onto solid fuel.
SpaceShipOne won the X-Prize in 2004. It used hydroxy-terminated polybutadiene (tire rubber) and nitrous oxide (laughing gas). Solid rocket motors, such as the ones used by the Shuttle, use the same rubber, but they add powdered aluminum for more heat, and ammonium perchlorate to generate oxygen. SRBs also have a tiny bit of iron oxide (rust) as a catalyst to ensure the the aluminum burns quickly and smoothly. But burning rubber generates gas that produces thrust. SpaceShipOne used the same rubber, but no additives. Instead nitrous oxide was the oxidzer. That self-pressurizes at room temperature, so no need for any fuel pumps. It was a realy simple system.
For SpaceShipTwo they tried to use the same system. Scaled Composites designed and built both SpaceShipOne and SpaceShipTwo, but SpaceDev built the engine for SpaceShipOne. Sierra Nevada built the initial engine for SpaceShipTwo. They had a problem: ground tests showed the larger engine had stability problem at 20 seconds. Between 2005 and 2009, Scaled Composites themselves did numerous small scale tests of the engine. But between April 2009 and December 2012, ground tests were successful and they got FAA approval. In April 2013, they conducted the first test flight; but it didn't fly into space. However, further ground tests showed serious stability issues after 20 seconds. Investors and customers were getting ansy that no further test flights since April 2013. Virgin Galactic took over development of the engine in May 2014.
They replaced rubber with nylon. Ground tests showed it was stable for full duration burn. And the plastic fuel was projected to have better performance (by several unspecified measures), so should propel the craft higher. They conducted ground tests between May and October of this year. Then this test flight. No details what happened. No air tunnel tests, no unmanned tests, just this test flight.
Sources:
SS2 SS1
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