Back in Why Build it? I alluded to the logical problems of what would drive a civilization to construct what would be essentially a project on the level of the pyramids. Today, over breakfast, I think I finally have a satisfying answer.
The answer is a complicated mess. But I think it's a satisfying mess. The quick telling: Giant ships of project Gilgamesh and Iliad are being built to skirt naval arms limitation treaties. The goal of both lines of ships are not so much to create finished vessels as to maintain a class of craftsmen who are capable of constructing such vessels.
Now as to why that is important, we can look to a similar program that exists in the United States. The reason to construct Nuclear Aircraft carriers is to be able to retain the ability to build future carriers. Their schedule of construction is designed to keep everyone in the industry comfortably employed while avoiding fits and starts that lead to booms and busts in the industry.
However, in this world we have a similar "peace" as existed after the armistice from WWI. The various world powers agreed to avoid an arms race by limiting the size of their military fleets. So, instead of building massive fleet carriers, ISTO is building improbably sized commercial vessels that could be easily adapted into a fleet carrier if war should ever break out.
Project Gilgamesh was an excuse to build capital ship sized hulls into vessels suitable for continued operations around the Solar system.
By the late 21st century, Krasnovia started to have ambitions of expanding outside the Solar system. At the same time, there were quite a number of Project Gilgamesh vessels that were darting around the solar system. Justifying further construction for them on economic grounds was getting downright silly. Enter Project Iliad.
Project Iliad was intended to be a semi-enternal make-work program for both the Psyche and Ceres colonies. The vessels produced would be blasting off on what would essentially be a one-way trip. There are countless stars that could be reached by one of these vessels. This also solved the problem of integrating populations fleeing the collapsing ecosystem on Earth. You didn't have to. You could just direct that torrent of refugees onto these ships, and then set them loose on the cosmos to start their own world.
Thus, the design process for the Gilgamesh and Iliad project never really start with a function for the vessel. They are all about coming up with a justification for why a high-tech propulsion system needs to be married with a small planetoid's worth of metal, and occupy millions of man hours of shipbuilder's time.
Another interesting point is how many ships in the ISTO fleet are leased from the "PSYCO" (Psyche ShipYard COrporation). Of course, only know do I realize I am setting up for an ultimate showdown between that Krazis and the Psychos.
On another related note, I realize that I now have two major settlements where it would seem there should be a natural monopoly. I am going to justify it thus: Psyche's shipyards eventually evolve to deal with initial construction of vessels. Ceres shipyards evolve to focus on maintenance and refurbishment.
Psyche is the exposed core of a defunct planet. It is a source of cheap metal, and while it has some silicates, it is mainly devoid of water. The colony there has to basically import all non-metallic building materials, soil, etc.
Ceres is a miniature planet. Thus it has a balanced composition that permits the cheap minining of soil, water, and silicates. While there is metal present in Ceres soils, it isn't in the raw elemental form found on Psyche.
The two planetoids travel in close enough orbits to permit ready communications between them. The resources on both worlds are concentrated enough and massive enough to sustain a civilization. At the same time, the weak gravity of both worlds demands that those settlements exist in sophisticated vessels with their own gravity and life support systems.
I have been playing with a few designs, but I think this one is my favorite:
The idea is that civilization is broken up into a number of cities such as these which dot the planetiod's surface. Tubes run through the planet connect the cities together with a tube system:
I am imagining the very low gravity of Ceres and Psyche would dictate some sort of system that used opposing tires to maintain traction on the inside of a tube. The trains would have to maintain their own atmosphere an life support, thus making them more akin in design to a jet aircraft than a conventional train. While the tires look pathetically small for the amount of mass the vehicles are carrying, remember that we are dealing with a fraction of the force of gravity. Their role is to maintain traction, with the pulling force distributed over a large number of tires. Given the low escape velocity of the planetoids, travelling on the surface would be more dangerous than it is worth. The tube shape is dictated by the drilling equipment, which I imagine to be a Tunnel Boring Machine.