The low pressure propellant ducts in SSME are routed around the engine in a somewhat complicated way. Both fuel and oxidizer lines leave the low-pressure turbopump, go around, down and then turn back to the high-presure turbopump. What is the reason for such contrived routing?

I can think of 2 possible reasons behind that:

  • Length contraction due to temperature changes (both fuel and oxidizer are cryogenic).
  • Flexibility: since the low-pressure turbopumps are fixed to the orbiter, but the rest of the engine is gimballing, the line must be flexible. This is solved by articulating joints on the tube.

Are there other reasons? Could these technical issues be resolved differently - for example using a wire braided flexible hose directly connecting both turbopumps? Although I am not sure if there is a material that is flexible at the temperature of liquid hydrogen.

SSME tubing

  • $\begingroup$ I think the major reason was, as you state, the gimbaling, but I'm not sure I've ever seen this written down. I'll take a quick browse through my notes. $\endgroup$ Commented May 31, 2018 at 12:59
  • $\begingroup$ I’m not sure long, large-diameter flex hoses would be viable here due to acceleration and vibration loads. In satellites, much thinner ones are subject to pretty strict support requirements. $\endgroup$ Commented May 31, 2018 at 15:25

2 Answers 2


I found a reference that backs up your reason #2 -

enter image description here

The flexible joints welded into the fluid interface lines, allow movement of the engine for vehicle steering, while maintaining the internal pressure and temperature environment of the lines. Since the fluid interface lines connect either between the engine (which gimbals) and the vehicle, or between the engine and a nongimballing component, they must be flexible.

This document (p. 373) gives rules for the design of these ducts which the SSME ducts follow...however, infuriatingly, the book doesn't explain why some of these rules should be followed!

In ducts connecting two components involving large relative movements, a minimum of three flexible sections is required. The longitudinal axis of at least two of the bellows sections should be positioned at a right angle to one another.


The three flexible sections are kept in the plane of the engine gimbal point. This results in minimum displacements of the sections for a given engine movement.

  • $\begingroup$ Thank you, this confirms that the tubes must be flexible. Is there a reason why they are so long and wrapping around the engine, instead of going directly from the low-pressure to the high-pressure turbopump? $\endgroup$
    – mpv
    Commented Apr 30, 2019 at 13:33
  • $\begingroup$ @mpv still haven't found that in writing :( $\endgroup$ Commented Apr 30, 2019 at 13:53
  • $\begingroup$ A related question could be, why the articulated joints had to be on this duct and not upstream of the low-pressure turbopumps. Not sure if it would be part of this question or should be asked separately. $\endgroup$
    – mpv
    Commented May 3, 2019 at 5:16
  • $\begingroup$ The low pressure pumps were mounted on the vehicle structure and didn't gimbal but as to why...yeah, good question. $\endgroup$ Commented May 3, 2019 at 11:48
  • 1
    $\begingroup$ In steam piping (vibration & thermal issues, but a different regime) the “right angle rule” is to convert length changes into small angular changes that the next joint could then handle. I.e vibration in one pipe is small back-and-forth of a joint, which moves the large-angle elbow, which is then an angular motion at the next joint “around the corner”. $\endgroup$ Commented May 13, 2019 at 5:46

The principle reason is for the gimbaling capability. The engine had a 13 degree gimbal requirement and you needed the feed line length to make that possible.

  • 2
    $\begingroup$ Thank you for the answer. Would you know about some resource describing the details? $\endgroup$
    – mpv
    Commented Mar 27, 2019 at 19:45

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