Detail of the exterior 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 photos of the bifurcation point on the
interior of the thrust chamber.