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Overall view of the middle of the interior 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.

So, there are two "up" tubes for each "down" tube, and this is obvious here: The gaps between the sets of "up" tubes make room for the introduction of the "down" tubes. These "cat eyes" also 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.