Krushnic Effect... But What About The F-1's Nozzle Extensions

Discussion in 'Propulsion' started by lakeroadster, Mar 14, 2019 at 8:45 PM.

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  1. Mar 14, 2019 at 8:45 PM #1

    lakeroadster

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    I'm No Rocket Scientist... so can somebody explain: Was the Rocketdyne F1 engine's thrust adversely affected by the Nozzle Extensions?



    f-1-cut-away-thrust-chamber-sm.jpg
     
  2. Mar 14, 2019 at 9:18 PM #2

    dhbarr

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    IIRC at sea level yes, in vacuum no. Krushnic is just shorthand for grossly overexpanded.
     
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  3. Mar 14, 2019 at 9:31 PM #3

    Nytrunner

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    The nozzle extension serves to expand the exhaust gas further before exit to atmosphere. Not having worked on this particular engine, I can't say for sure, but I'd believe the 16:1 expansion at the bottom of the bell extension was optimized as best they could for the portion of the flight where the 1st stage was active (relatively low altitude compared to 2nd and 3rd stage operation). When you watch the slow-mo liftoff video from the pad of a Sat5, you can see the exhaust products streaming out pretty dang straight, so I'd say that if there was an effect, it was negligible.

    The upper chamber bell was actively cooled with fuel running along those bifurcated tubes (down and return), but the lower extension bell had gas generator exhaust products pumped out the interior to help protect it from the much hotter combustion exhaust. (you can see the darker gg exhaust initially in the slow-mo)

    Back to the Krushnic effect: The pressures and power of the F1 exhaust are orders of magnitude greater than our little motors (and by that I mean A-O :cool:). That whole structure is essentially just the final clay or phenolic or graphite nozzle of our rockets. I'm imagining the ratio of a C6 nozzle exit (not throat) diameter to a BT-60 and what that would look like scaled to an F1 engine.
     
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  4. Mar 14, 2019 at 9:35 PM #4

    lakeroadster

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    The CM had a huge nozzle extension too..

    CM.jpg
     
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  5. Mar 14, 2019 at 9:47 PM #5

    Nytrunner

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    Man that's a great picture

    In the SM's case, that is a much smaller motor with lower pressures, but it's operating in the vacuum of space where ambient pressure is essentially 0
    If that engine were to fire at sea level, the resultant low pressure inside the bell, and the 14.7 psi outside would give you something closer to Krushnic, but would probably Krush the nozzle in! It's an extreme case of (a) in the image below)

    Think of the Falcon 9's Merlin engines. there are 9 on the 1st stage and their bells are ~3.5 feet across (that number may be off, it's been awhile). They operate within the atmosphere and are optimized for lower altitude, and as you watch the video, you can see the radiating exhaust plumes as atmos pressure lowers and the exhaust gas expands (case (c) below)
    On the 2nd stage, there is a single Merlin-Vacuum engine which is basically the same combustion chamber, with mostly similar plumbing, same fuel, but a big 9' nozzle extension on it! It operates in the very high atmosphere and vacuum of space and requires the much larger expansion ratio. (if you fired the M-vac at sea level with the skirt on......you would have a very bad day)

    upload_2019-3-14_16-38-33.png
     
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  6. Mar 16, 2019 at 4:40 PM #6

    lakeroadster

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    Thanks for the responses... I always learn something every time I log on here.

    Have I mentioned lately that I love this forum? ;)
     
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  7. Mar 16, 2019 at 6:29 PM #7

    aerostadt

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    In some ways looking at the picture in the first post of this thread, I can see why someone might ask this question because it looks like there is hardly any converging section before the throat. I am wondering now if the F-1 propellant flow rate is so high and the combustion so fast that the mass addition acts like an area contraction. In the idealized world of 1-D compressible (inviscid) fluid flow theory a large mass addition acts the same as an area contraction. That is after the propellants have combusted and turned into high pressure, high temperature gas, they are at a high subsonic Mach number and the area contraction needed to get to Mach=1 at the throat does not need to be too much.
     

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