As one shuttle is being loaded up for flight, another is taxiing up into place, ready to be lifted onto the track once the current flight has departed. A pressurised 'helmets-off' cabin environment is maintained up on the upper walkway, so as to maximise the air reserves carried in EVA suits as backup for the duration of the flight into orbit. Heavier cargoes than can be ported down the gangway can be lifted into position over the upper hatch by crane.
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