Looking more into their design turns up all sorts of issues with nitrous oxide as a propellant. It's used all the time in dentist offices, so it's easy to misunderstand as an innocuous gas. At higher pressures and relatively normal temperatures it can go supercritical and become very, very volatile. At that point it's an even better solvent than supercritical CO2, and still a very strong oxidizer:

http://www.knightsarrow.com/rockets/scaled-composites-accide...

http://www.dtic.mil/dtic/tr/fulltext/u2/a489459.pdf

(In that Air Force research lab report there's mention of supercritical N20 causing a spontaneous explosion with coffee grounds.)

Well, you want it to be a strong oxidizer, that's its purpose in the rocket.

Strong oxidizers are almost universally hard to handle. About the nicest ones around are Lox, Nox and hydrogen peroxide.

Agreed.

Don't forget that this is not the first fatality associated with this engine. In 2007, 3 people were killed in an explosion during a static test at Mohave Air & Space port.

So they traded sometimes dangerous moving parts that could lead to a mission fail to a rarely dangerous pressurized vessel that would lead to casualties?

Is this a case of not seeing safety's big picture? Cutting accidents in half (or by whatever) is great, but not if that means the remaining accidents are fatal. A more frequent controlled failure is a much better scenario, but I imagine hurts the bottom line too much.

I'm wary that for-profit companies aren't going to invest enough in private manned spaceflight's safety when there's a profit incentive at work dictating otherwise. At least without a fair bit of regulation.

I think it's very hard to make that determination. In spaceflight, all accidents are likely to be fatal. Also, I have no problem with a profit-motive; killing your passengers is very bad for business.

Yes in this market, the invisible hand will tend to ensure the success of companies that don't blow up their passengers.