In Drag Racing it is common to "purge" (of gaseous N2O) the Nitrous line prior to actuation of the valve at launch. The purpose is to get pure liquid Nitrous at the point of injection.
I think you've nailed it Brett.
Shoot, the Germans had it figured out by 1945.
Not really, and the Germans never did anything much like this, although they probably had enough information to figure it out if they had experimented. I think these guys are using hybrids because they are seen to be "simple*.
The problem in the 2007 case was nailed (and predicted) in 2005 by someone else. But I now hear that they are in fact considering *combustion instability* as I speculated above, caused by the new fuel grain. This is a *very tricky* problem to solve, and in reality, no one has come up with an acceptable analytical means of predicting it and dealing with it. The Germans did find this, but in 1963, not 1945, and the found it out the hard way. There's a lot of cut and try, and interesting testing you can do. For the F1 engine on the saturn 5, for instance, they would tape bombs to the inside of the combustion chamber. When the bomb goes off, it perturbs the combustion, and then you can watch the pressure, etc, and see whether it recovers and how long it takes, which is interpreted as a measure of the stability
The problem with this on a NO hybrid is that nitrous oxide can decompose spontaneously due to pressure spikes in the liquid system. This was the cause of the 2007 accident as well. If combustion goes unstable, say, from the natural frequency of the (very large) combustion chamber happens to line up with the natural frequency of the fuel grain - which is rubbery/plastic substance, not rigid - can cause the normal operation of the engine to cause it to go into resonance and become unstable. The effect can be something like runs, stops, runs, stops, runs chuff-chuff-chuff. But at very high frequency, hundreds or thousands of hertz, with the pressure spike at each start getting larger and larger. This will send pressure spikes up the oxidizer line, and cause high or low local pressures, and in the right conditions, the oxidizer explosively decomposes and blows up the entire system. Alternately, the combustion chamber itself can fail from pressure and fatigue, same result, or burn through from high local temperatures. Thats what we think happened when one of our bi-propellant engines was starved of oxidizer.
These are all fairly-well-recognized failure modes, and hybrids are particularly prone to it because the combustion chamber is very large since it contains the entire fuel grain, so the natural frequencies are low which means it takes less energy to get going. That's why they suddenly ran into it when they tried to scale up small engines to the F1, and why the model-sized engines are casually tossed together with fairly reliable results. You can test and test and not get it, then the next test, it blows.
This is all still speculation, of course. One person died yesterday, and another is in the hospital with serious injuries, and no one wishes to pile on. But it is extremely frustrating to be able to see and predict the problems, have people point them out and then have some of the principals go around bragging about how they never waste any time on "endless analysis" and how all their predecessors are stupid or hidebound, and how "we will just fly it and see" because we are way smarter and in a big hurry. It doesn't cost much time at all to download a report and read it.
People have been warning about this sort of approach for years, now there have been three separate dangerous problems, two of which resulted in deaths that appear to have been completely unnecessary. That's what is frustrating.
Brett
Topic: Virgin Galactic Spaceship One Crashed (Read 5349 times)