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Detail of the interior of the sustainer engine thrust chamber, where the tubes bifurcate, or split in two.

The regenerative cooling tubes of which the thrust chamber is constructed are formed to allow a high expansion ratio (ratio of the thrust chamber's diameter at the throat vs. the exit plane). However, given the size and material of the tubes, they can be shaped only so much, so additional tubes are necessary to support the increasing diameter. Thus, two tubes are spliced onto each tube at this point.

See my photo of the bifurcation point on the exterior of the thrust chamber.

Picture 1 of 2.