US crash investigators began combing through the debris sites from last week’s fatal failure of the Virgin Galactic SpaceShipTwo for what could be a year long probe.  But they’ll likely also look into questions about whether Virgin ignored warnings about the unconventional fuel propulsion system used to blast the craft up to space.

SpaceShipTwo appeared to explode and breakup at about 45,000 feet during Friday’s ill-fated test flight.  One pilot, 43-year old Peter Siebold, managed to parachute to earth and was seriously injured.  He’s reportedly conscious again, and talking with investigators.  The other pilot, 39-year old Michael Alsbury, was killed.

The ship’s engine was powered by a relatively new hybrid propulsion system of plastic fuel and nitrous oxide – or, “laughing” gas, like your parents’ dentists used to use.  For decades, rockets soared into space on carbon-based fuels. 

It turns out engineers have been talking about this for at least a year.  Nitrous was described as “benign and stable” on Virgin Galactic’s website as recently as six months ago.  But rocket engineers like Carolynne Campbell of the International Association for the Advancement of Space Safety (IAASS) say that in a rocket engine, nitrous is “extremely unpredictable – it has a bit of a devil in it”.

What’s more, it’s not the first time Virgin had a problem with it.  In 2007, Nitrous Oxide was a factor in an explosion at the Virgin Galactic spaceport in which three workers died.  “They knew that three people were killed by this stuff, and yet they persisted in presenting it as safe, stable and benign,” Carolynne Campbell said.