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On 2002-07-08 15:30, JayUtah wrote:
Mysteriously, the F-1 exhaust plumes are dark for the first 8’ after the end of the nozzle, then ignition of a very fuel-rich exhaust plume occurs in the atmosphere.
You probably know the clip in question. It starts with a dark, grainy image looking up the skirts of a Saturn V. You see cryogenic oxidizer cascading from the nozzle in slow motion followed by the ignition. Then the view shifts to pad level and you watch the rocket rize slowly. As you see the nozzles rise above the MLP, you can see a portion of laminar flow, and underneath it a brighter turbulent flow.
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Apparently Mr. Wood doesn't realize his "dark plume" argument is based on footage taken from a high-speed armored DAC and thus is trememdously underexposed. Though taken in daylight, most of the rest of the frame is dark. It's only when the F-1 plume becomes visible that the frame is sufficiently lit. Not only does this plume completely saturate film at normal exposure, we even get optical scattering in some lenses, centered right at the nozzle where the plume would be brightest.
The plume is not "dark" coming out of the nozzle, it's merely very slightly darker than the more turbulent flow beneath it. In terms of human visual perception both would be too bright to look at with the naked eye.
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Jay, here for (I think) the first time, I am going to challenge you.
There does indeed seem to be a section of the exhaust plume from the F-1 engine that is objectively dark. It's visible in at least two Saturn V launch photos I have in my possession, as well as the famous footage you described above. It extends from the bottom of the engine bell for about 2/3 the size of the engine bell below it -- maybe 4-5 feet. It is very dark indeed -- much darker than the sunlit sides of the S-V, and far, far darker than the incandescent (and greatly overexposed) exhaust plume below that. It's a sort of golden brown color.
The effect is so clear and surprising that for a long time I assumed that the F-1 engine had a long "skirt" of some relatively thin metal, through which the brilliant exhaust plume was visible. However, I have now seen pictures of the F-1 that show very clearly that the engine bell ends above this dark region. One unambiguous marker is the plumbing for the LOX coolant that wraps around the bell about halfway down. The dark plume is clearly not part of the engine bell.
I have a couple ideas about what might be causing this effect, but they would be pure speculation at this point. However, one thing I would argue strongly: the dark portion of the exhaust plume is really dark, and would be no harder to look at directly than any other sunlit scene, if the incandescent plume were not present.
If anyone has definitive information about the causes of this phenomenon, I'd be very happy to learn more.
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Truth be told, they never actually got this fixed. The Saturn V was known for its "pogo" effect -- longitudinal vibrations caused by combustion instabilities. They just figured out how to work around it and limit its effects.
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But wasn't pogo a problem mainly with the second stage? My recollection is that the S-IC was a very smooth-burning launcher, but the S-II had the pogo problem -- so bad that the second unmanned launch would probably have been aborted if there had been a crew aboard.
Incidentally, Rocketdyne tried a lot of things to fix the F-1 problem, including changing the position and angles of the LOX and fuel sprays, moving the impingement positions somewhat lower in the combustion chamber (at a cost of some engine efficiency), and adding baffles to the injector plate (which had to be cooled by circulating LOX). None of these was a "magic bullet", but in combination, eventually the F-1 became so stable that they could detonate a small bomb in the combustion chamber, and the resulting oscillations would damp out within 100mS.