The launch of space shuttle Atlantis (STS-135) was delayed approximately 2 minutes because of the Gaseous Oxygen Vent Arm, also called the Beanie Cap. You may recall seeing it looking almost like part of the launch tower that holds the shuttle down until it lifts off.
Here is a picture I snapped as the Rotating Service Structure (RSS) was moved back the day before launch. The vent arm is raised about 2 1/2 minutes before launch, a move that takes about 25 seconds. At 1 minute and 45 seconds to launch, it is retracted against the fixed service structure. If you check the previous post, you’ll note that at 31 seconds to launch, the onboard computers take over launch. At that moment, the computer registered that the arm had not fully retracted and put the countdown into hold.
Mission control then used remote cameras to verify with eyeballs that the arm had retracted enough. It had, and countdown restarted.
Now you know.
What IS the arm for, though?
Vapors are created as the liquid oxygen in the External Tank (the big central orange tank) boil off. The hood vents the gaseous oxygen vapors away from the shuttle.
The arm is attached between the 207-foot and 227-foot levels. The arm is about 65 feet from the tower to the hood. The vent is 13 feet wide.