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 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.
The gaps between the sets of "up" tubes both make room for the introduction of
the "down" tubes and serve as slots through which the turbine exhaust gas (from
the exhaust manifold, just forward of the fuel manifold) enters the combustion
chamber. Dumping the turbine exhaust into the thrust chamber in this fashion
enhanced the engine's specific impulse.