During the HTV-6 launch I saw a large, enclosed fire pit next to the launch pad. (On the left of the picture below). What is its purpose?
Video of the fire pit (@ 24:54):
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They're burning off hydrogen in what is called a "flare stack". Probably the hydrogen boiled off from chilling down the engine and/or the hydrogen boiling off from the tank.
These dim orange flames are typical hydrogen diffusion flames.
The water is there to prevent anything from catching fire. Hydrogen burns pretty hot.
Here's a video of the flare stack at KSC launch pad 39A: