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The sign accompanying the thrust chamber section. It reads

RL10 Thrust Chamber Section

This is a section of an RL10 thrust chamber wall. The chamber is composed of 360 tubes that circulate liquid hydrogen. While combustion gases hurtle from the thrust chamber at 6,000 degrees F, super-cold LH₂ enters the tubes at -420 degrees F. As the liquid hydrogen cools the thrust chamber wall, the heat transforms it into a gas. That gas, called "gaseous hydrogen" (GH₂), is routed through the engine to drive the fuel turbopump.