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The tail unit's hydrogen peroxide overflow and vent port.

One hose from the hydrogen peroxide servicing truck would attach to the hydrogen peroxide fill and drain valve to fill the 76-gallon hydrogen peroxide tank on the engine. Another hose would attach to the hydrogen peroxide overflow and vent port on the aft face of the tail unit and connect to the H₂O₂ overflow tank (a piece of ground support equipment, basically an enclosed bucket). Overflow hydrogen peroxide was diluted and discarded, due to the possibility of contamination.

The hydrogen peroxide overflow and vent port is directly adjacent to the steam vent.