dsc93267.jpg

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 4 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.

Note the fuel manifold brackets brazed onto the pairs of down tubes.

I speculate that the gold color is due to an increased amount of brazing compound used at this joint in the thrust chamber during manufacture, coupled with the fact that this is a relatively protected area of the thrust chamber, resulting in less erosion of the compound over the years.