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Detail of the thrust chamber at the point at which the additional 180 tubes are added to the 360 full-length tubes comprising the thrust chamber at the fuel manifold. The addition of these tubes is best seen on the interior of the thrust chamber.

Picture 2 of 4.

The J-2 thrust chamber is constructed of 360 full-length "up" tubes running from the 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.