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Why does the YF-75D nozzle material have such a strong spiral pattern?

Is it a result/biproduct of a manufacturing process only, or does it have some specific function?

edit: The last, cropped images that have enhanced contrast and sharpening show a bead-like appearance around the bottom edge of the nozzle. This gives the impression that the nozzles is fabricated from fused rods or "ropes" of metal. This is consistent with them being nearly vertical at the top, and increasingly spiraled as the bell shape widens.

Could these have been made by some additive process?

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above: YF-75D engine cropped from here.

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above: YF-75D engine cropped from Chinaspaceflight. (In Chinese)

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above: YF-75D engine from Chinaspaceflight. (In Chinese)

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above: YF-75D engine nozzle detail, cropped from Chinaspaceflight. (In Chinese)

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    $\begingroup$ @OrganicMarble possibly, but the pitch seems to coarse to me. Look up at the top where it mounts to the engine - those lines are almost vertical. I'm thinking it's an additive process like making a clay pot from ropes of clay, but I still don't know why it would look like this. $\endgroup$ – uhoh Nov 4 '16 at 13:26
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    $\begingroup$ This engine uses the expander cycle, so propellant is circulated through the nozzle walls. Could be you're looking at tubes brazed together. $\endgroup$ – Hobbes Nov 4 '16 at 17:42
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    $\begingroup$ @Puffin agree, there appear to be none of the usual ducts to supply or return coolant from the nozzle extension. This looks like a radiation cooled nozzle extension to me. $\endgroup$ – Organic Marble Nov 6 '16 at 13:35
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    $\begingroup$ @uhoh Google translate suggests coolant going through 242 spiral tubes embedded in nozzle and then being dumped through 726 "micro-nozzles". Also from this asi.org/adb/04/03/09/01/sep-engines.html "Cooling Method: hydrogen is heated to 150K during cooling of combustion chamber wall along 128 longitudinal channels for injection in gaseous form. The nozzle cooling system employs 242 helical tubes; 0.15 kg/s of hydrogen is dumped overboard." $\endgroup$ – Ohsin Jun 18 '17 at 17:29
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    $\begingroup$ @uhoh More information from translated text here bernd-leitenberger.de/ariane4.shtml "The nozzle consists of 242 individual tubes from Incotel 600, which are helically welded together. The nozzle is cooled by 150 g of hydrogen / second. The temperatures reach a maximum of 1080 K at the nozzle neck. While the hydrogen of the combustion chamber cooling is combusted with the remaining hydrogen, the hydrogen that cools the nozzle is exhausted at its end through the open tubes. This creates a small extra thrust." $\endgroup$ – Ohsin Jun 18 '17 at 17:56
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According to the paper from the 64rd International Astronautical Congress, Beijing, China The Development of the LOX/LH2 Engine in China; IAC-13, C4.1, 1x18525, the nozzle was welded from spiral-shaped elements. So the spiral pattern is here by design.

The spirals are "dump-cooled", an old Rocketdyne idea described in this 1966 NASA Technical Note TN D-3532 Design and Cooling Performance of a Dump-Cooled Rocket; dump-cooling basically means that you use a low-temperature fluid (in the Rocketdyne paper that is LH2) which, once it has performed its cooling function, is "dumped" overboard without having participated in a combustion process.

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  • $\begingroup$ Anyone know what's dumped in this case? $\endgroup$ – drjpizzle yesterday
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    $\begingroup$ It would be surprising if it weren't H2. $\endgroup$ – Organic Marble yesterday
  • $\begingroup$ Thank you for the links. I understand that the spirals are by design, it would certainly be strange if the spirals were manufactured without being designed that way. But my question is Why... such a strong spiral pattern? If the explanation why is in the links, then this becomes a link-only answer unless you can include a brief explanation or quoted passage from one of your sources. Thanks! $\endgroup$ – uhoh yesterday
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    $\begingroup$ In the IAC paper it says The nozzle extension welded in spiral tubes bundle, adopts dump cooling. and also The nozzle extension is made of welding in spiral tubes bundle in order to reduce its mass. My guess is that LH2 is pumped through the spiral tubes for cooling, before being dumped at the lower end (@uhoh's last picture shows the shiny inside of the tubes?). The shape may be advantageous for three reasons: 1) vertical injection at the top 2) spiral realizes optimal path length 3) spiral helps with structure: vertical weld lines are not optimal to take the pressure of the exhaust $\endgroup$ – Everyday Astronaut 18 hours ago
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    $\begingroup$ (speculation only) $\endgroup$ – Everyday Astronaut 18 hours ago

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