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Detail of the interior of the fuel manifold inlet.

Picture 2 of 5.

Liquid hydrogen from the turbopump is routed to the fuel manifold, where it enters the ends of these half-length regenerative cooling tubes. The fuel then travels through these tubes, down the thrust chamber, to the exit plane. There the reverse-flow manifold (variously called a fuel return manifold or turnaround manifold on other rocket engines) routes the hydrogen up the full-length regenerative cooling tubes to the injector.

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.