On 11/1/2014 10:19 AM, David Weinshenker wrote:
Henry Vanderbilt wrote:The white cloud was almost certainly the nitrous supply venting. Ken Brown's photos can be seen at http://i.imgur.com/bfGB0da.png. All what follows caveat "as best I can tell." The first shot shows what looks like a clean burn just after dropoff and start. (Hard to say for sure from a still; rocket motors can look just fine in stills when a vid might show significant pressure variations.)Hmmm... comparing the photos at this link with the nydailynews.com link below... that has what looks like a post-drop photo a little earlier in the burn sequence, with SS2 slightly behind the carrier plane - vs. the first image in the link you posted which shows it having pulled about even (as seen from the camera angle): the exhaust plume appears very different in the first and second images (presumably from the same camera, closely spaced in time, so the photographic transfer function should be consistent...): the first one has a very luminous plume, to the point of apparent probable saturation of the color rendition. The second image shows a much paler plume with visible "shock diamonds", almost transparent. I don't know how they light that thing - it could be that the first image represents part of the ignition sequence, with the output of a solid-propellant igniter charge contributing to the visible plume - but if both images represent "mainstage" hybrid combustion, it looks like there might have been a -major- combustion mixture ratio shift...
From watching previous flight test vids, they seem to flight-start their SS2 hybrid with some sort of igniter charge that vaporizes then lights the fuel grain. (The sequence in the past has looked as if it almost went out before the main burn stabilizes - this may be an artifact of partially-burnt fuel smoke hiding the flame late in the igniter burn and early in the main burn.)
The sharp square end to the whiter portion of the plume in that post-drop first photo at your dailynews URL http://www.nydailynews.com/news/national/virgin-galactic-spaceship-crashes-test-flight-article-1.1994672 is very likely where the igniter first lit.
Note the thinner vapor plume extending farther aft beyond that, at a slight angle to the WK2 engine plume. They started injecting something else before they lit the igniter. I wouldn't think they'd start nitrous flow before lighting the igniter; that would seem begging for a hard start, but perhaps our hybrid experts might clarify this point. (Methane injection, maybe? See below.)
The bright orange exhaust flame is likely a combination of the igniter plume plus the main burn just starting up and possibly still somewhat fuel-rich. (Methane? I've seen somewhere that this latest RM2 version injects both helium and methane into the motor at various points, and methane burned fuel-rich gives a reddish flame. That's speculative though, as I haven't seen their actual motor operating sequence described anywhere.)
Henry
