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The sign accompanying the bricks. It reads

RL-10 Thrust Chamber Section

This is a section of an RL-10 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.