> Was ut to sustain altitude, pressure, G and speed?

Heat expansion. That's also why it would leak fuel when on the ground (fuel lines would expand at speed so they needed expansion gaps when on the ground).

Outside emergency situation, the crew had to wait inside the plane after landing, waiting for the surfaces to cool down enough that they could safely get out without melting their gear.

If I recall correctly the plane was built to stretch at high speeds, so the panels were designed to be loose, and the plane would leak a lot of fuel until it reached optimal speeds.

Q: How do you know there's fuel in the Blackbird?

A: It's leaking.

Same goes for oil in old Land Rovers!