As can be seen in close-up footage of SSME ignition, such as the video below,

there are small jets of an unknown gas/vapor emanating from specific points around the rim of each engine bell. I've been unable to dig up a schematic detailed enough to figure this out. What are these jets?


1 Answer 1


The lines that exited at the end of the nozzle were drain lines carrying leakage from seals, output of hydraulic actuator drain lines, etc. The following schematic shows the various systems attached to these drain lines. Source: Rockwell SSME Pocket Data Book, R/RD87-142.

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This graphic differentiates between the transfer ducts (which carried the hydrogen which cooled the nozzle) and the drain lines.

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Finally, this graphic shows a "looking down the length of the nozzle" view with all the drain lines annotated.

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Note: this answer represents a 1989 version of the SSME and may not be totally accurate for the versions flying at the end of the program; the important take-away is that what you see is only leakage, drainage, purge gas, etc, and not part of the propellant loops.

  • 2
    $\begingroup$ Very detailed, and answers every possible follow-up question I might have had as well. Thanks very much! $\endgroup$
    – dgw
    Commented May 30, 2016 at 17:18
  • 4
    $\begingroup$ Follow-up: why the kinks in the fuel lines? $\endgroup$ Commented May 30, 2016 at 20:27
  • 3
    $\begingroup$ I could speculate (allow for thermal expansion?), but I don't really know. $\endgroup$ Commented May 30, 2016 at 20:30
  • 9
    $\begingroup$ Thermal expansion is the usual engineering reason, and quite probable in environments that get as hot as an engine. The basic problem is that fuel lines are tubes, and straight tubes are quite stiff structures (think bamboo). A straight fuel line that expands as it heats can exert very high forces at both ends. And it it fails, it buckles.However, a corner in a tube can typically bend by a few degrees without significantly affecting the inner diameter of the tube. So that U-shaped bend will allow a few percent expansion in the fuel line it's part of. $\endgroup$
    – MSalters
    Commented May 31, 2016 at 8:41

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