The three core stages of the Delta IV Heavy are venting oxygen as the propellant boils off from its cryogenic liquid state at minus-297 degrees F. The gaseous oxygen is dumped to prevent a pressure buildup inside the tank. The liquid hydrogen also vents as its temperature increases above minus-423 degrees F, but the gaseous fuel is carried away from the rocket before being cast aside. Once the tanks are filled, pumps continue to trickle the propellants into the stages to replace the small amount that boils off.
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Thanks for the update sir.