Detail of some of the "cat eyes" in the middle of the thrust chamber wall.
The J-2 thrust chamber is constructed of 360 full-length "up" tubes running
from reverse-flow manifold to the the
injector, plus an additional 180 "down" tubes running from here at the fuel
manifold to the reverse-flow manifold. This "pass-and-a-half" construction
increases the number of tubes comprising the thrust chamber as the diameter of
the nozzle increases, allowing for a high expansion ratio.
So, there are two "up" tubes for each "down" tube, and this is obvious here:
Gaps between sets of two "up" tubes allow the introduction of the "down" tubes
which originate in
the fuel manifold.
The gaps between the sets of "up" tubes also serve as a slot through which the turbine exhaust gas
(from the exhaust manifold, just forward of
the fuel manifold) enters the combustion chamber.