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
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.