Apparently CalOSHA has issued their report, and it remains unclear what caused the explosion at Scaled last summer. Charles Lurio notes (as I've been saying for, well, forever, or at least since I heard about the proposal to go with a nitrous hybrid):
...largely because of its ability to self-detonate - nitrous oxide has every now and then created unhappy surprises whose causes are difficult or impossible to explain. This may turn out to have been the case at Mojave. If in the end no cause for that incident is identifiable, Scaled should perhaps consider an alternative oxidizer for its hybrid; liquid oxygen (LOX) may be less convenient to transport and manage but doesn't have nitrous' particular unpredictabilities.
It also performs much better, whether with hybrids or liquids. This is very bad news. If you don't know what caused an accident, it's very difficult to know how to prevent it from recurring. Even if it causes a delay in the schedule, I think that they will have to go to some other design, and I also think (as I've always thought) that they should subcontract it out to an established propulsion house, such as HMX or XCOR, who are right there on the field.
Maybe when Burt has recovered from his recent health problems, he'll be in better shape to grasp that nettle than he has been.
You know to me this is a load of hooey. It is well known that nitrous oxide has a phase transition at 97.5 degrees. I was in Mojave two days before the explosion and here are the conditions that I observed.
At our solar trailer that was about 500 feet above their altitude it was 98 degrees and near zero humidity as my measurements taken there. It was also very windy (I was at a wind farm where my solar trailer is). In conditions of high temperature, high wind, and near zero humidity, what happens? You get static electricity, a LOT of static electricity. I got shocked every time I got out of my seat and touched a metal surface.
So what do we have?
We have a temperature at which is very close or above the liquid/gas phase transition of nitrous.
We have extremely low humidity which fosters static electricity, which is your ignition source.
We have wind, which disperses the gas, a lot of gas if it has gone from liquid to gas phase.
We also have a composite combustion chamber, which unless extremely well grounded or with at metal matrix, retains and gathers electric charge.
Now we have all of the elements necessary for an explosion.
What happened? Did one of the guys get out of a seat and walk over and touch the engine? That is all that it would have taken.
That is my opinion on this subject.
It is possible to test the conditions above.
It needs to happen on a summer day similar to the day this happened but just put a small tank of nitrous, monitor the temperature, when it gets high enough, open a valve, then operate a spark gap system to generate a spark and see what happens.
This is a simple test and would settle this particular issue.
I haven't read the report yet (and don't know any who have) so maybe they've thought about Dennis Wingo's possible explanation. If not it sure sounds like something one would want to test.
A half inch spark is 40,000 volts. The temperature of that spark is well above the autodetonation temperature of nitrous.