Most useful shape for a mobile shipyard?

[MasterGwydion]

As you observe, very few Traveller deck plans are more than 5 decks tall. If you made your construction deck 18m tall, it would be sufficient for many vessels. The exact specifics depend on choices made at a particular table -- 'Small Ship' vs 'Large Ship' universes, and so on. I would tend to make the deck longer than it is wide; maybe 2x or 3x.

A 'Dispersed Structure' hull is a sensible choice, but I believe the 'Construction Deck' is a shirt-sleeve environment. I am not sure how a StarFleet-style Fleet Repair Dock similar to the first ST movie would be modeled in Traveller. You might start with an 'Interplanetary Jump Net' (pg 53; which is capable of moving cargo under several G of acceleration) and 'Collapsible Fuel Tanks' (pg 48; which can hold high pressure and regulate temperature in vacuum). Both occupy only 1% of their 'full' volume when not in use. 10 dTons of each would create 1000 dTons of temporary 'working space', especially if the work crews were okay with minor inconveniences like breather masks or vacc-suits.

You are, sadly, correct that, as described, Construction Decks are internal.

I am actually not aware of any ways to build ships externally to the enclosed structure. Is anyone else?

 

[rinku]

I also think a Dispersed Structure hull makes the most sense, but if For Reasons you need it to be an enclosed space, most Traveller ships seem to roughly be 2-3 times the length of their width and wider than they are tall. Spherical hulls are rare among the published commercial craft - which makes sense; per the revised HG they are more expensive to build than boxes, but are cheaper to armour up.

So I'd go with your second box.

You MIGHT be able to build it as a giant external fabricator to make the hull, supplemented with fabrication chambers large enough to produce engines and other components. That would impose TL limits on the hull... but not unreasonable ones. A TL15 external fabricator can produce TL12 objects.