IIRC the first version of the Guppy – transporting Saturn rocket segments – was not pressurized – later they built one with a pressurized cockpit.
However the case could be that a MEL (Minimum Equipment List) problem required unpressurized flight, so FL100 was observed.
And when they take along a crew of mechanics / engineers on the main deck (only cockpit pressurized) they have to stay at or below FL100. The most probable case here.
…perhaps the fuselage is not pressurised?
The cargo bay of the Beluga is not pressurised. So whenever they carry delicate stuff (rocket stages, racehorses…) they must fly low.
Why would rocket stages be sensitive to depressurisation? Is it that they have internal cavities that are filled with some gas that is at sea level pressure and which cannot simply be opened to the environment?
No, it sometimes go higher.
But other than the newfangled self-landing ones, they are not making it back in one piece. And even those and the splashy gunpowder jobs used for the space shuttles need some work done before they get to fly again…
Why would rocket stages be sensitive to depressurisation? Is it that they have internal cavities that are filled with some gas that is at sea level pressure and which cannot simply be opened to the environment?
During ascent this would be possible. But on descent it needs to be re-pressurised, otherwise it will get crushed by the rising ambient pressure. Moist ambient air is not the first choice for that, so some pressurisation system with an inert gas would be required. Too big, too heavy. But anyway, that rocket stage was just an example. Usually they get shipped on barges in pressurised containers directly from the factory to the launch site.
