Did I miss an update somewhere? Populations housed in orbital habitats are supposed to count towards planetary populations and they can run planetary installations like factories and the like. If you think it's too unrealistic to commute to and from orbit every day, you could load up the OH with cargo holds, allowing it to haul its own installations and simulating the increased size. If you think that OH are too expensive for their return, you're in good company. Steve will be increasing their capacity in the upcoming version.
From what I've read, OH population was separated from planetary pop. At the same time, you couldn't transfer them from planet to planet if you moved the OH. Unless I'm working off of outdated info or it's just plain wrong, this is what I've played by.
The reason that PDCs get a special designation is that they operate under special rules: no engines, some free armor, no maintenance, can be prefabricated. What would the rules be for space stations that differentiates them from ships?
As soon as I made that last post, I realized that I've spoken out ideas without giving anything concrete, anything to be critiqued. I did plan on editing, but then this came up, so I'll just do it here.
Hull Type
Space StationNo engines, minimum of ten years worth of supplies, crew number based on infrastructure rather than crew quarters, free maintenance if over colony, stronger shields, cannot be prefabricated, but can expand after construction.
Engines and supplies is self-explanatory, but the infrastructure is to basically condense all the needs of the species under one name. If that's too simplified for you, then it's more than possible to break it down and have different modules for different life support.
The maintenance is free over colonies cause then it's assumed that the colony is responsible for the station. Since the station orbits a colonized planet, it wouldn't be that much of a stretch to say that those planet-side would make sure the huge sink of resources just outside the atmosphere doesn't end up fireballing into their cities thanks to a power source or weapon malfunctioning.
Shields is there for defense, once everything's said. Shield sizes can't be changed as far as I know, so in making shields there'd be an option where you decide whether it'll be on a ship or station. The station one is larger and has a better fuel-shields ratio, though it'll still drink more fuel than its smaller counterpart.
The lack of prefabrication is basically because OH can't be prefabbed. Since this is similar to OHs, I'm not gonna try and force a difference between the two where it isn't needed.
Now this here would be the unique aspect. There'll be a module called simply the construction module, where each module comes with a workshop and the space needed for a single construction ship. It'll have the same BP as a factory, it can build on a planet it orbits, and it'll even speed up shipbuilding if a shipyard is present, whether inside the station or just on the same planet. This means that you can tow this station over to, i don't know, say the edge of your empire near the frontline of a war between you and some aliens. Navy getting battered? Repair them at the station, and build them reinforcements at the same time without having to worry about risking your standalone shipyards and worry about trying to ferry minerals to said shipyard. In another scenario, you have a terraforming station that's just finished its job, and now the population is beginning to migrate to the surface because you didn't plan on placing down any infrastructure in anticipation of a zero cost colony. The station can build the beginning factories, mines, mass drivers, and whatnot thanks to the minerals it has in its hold and the construction modules on the station itself.
Another add-on to the last scenario, now the colony needs defense. So what do you do? Train some troops to temporarily pacify the locals, but a PDC is needed to stop the messages from popping up in the event log. Or, you could just retrofit the station by building some weapons on it, since it counts as being part of the planet when in orbit. Because the construction modules comes with its own ship, you can add installations onto the station after it's built, without needing to go back home for an overhaul.
While typing up the construction module one, I remembered about armies. Well then, another module. For planets there's the training installation that lets you train troops from the population. Since a station has its own population, all you need is a place to train them in... and obviously house them. So, one module to train them and another to house them, though the latter technically already exists. The station can only train infantry however; there are limitations to structural integrity that shouldn't be tested by battalions of tanks rolling around firing everywhere.
I've mentioned biodomes a few times in this thread, so I'll expand here. Biodomes reduce the supplies needed for a station to keep its population alive. If you can grow your own food and use your own poop as fertilizer and pee as water after filtration, then at bare minimum you've become self-sufficient. I'm not sure if a station should be allowed to get to a point where it doesn't need supplies, but chances are it'll need some other resource so it won't be able to survive in deep space by itself forever anyways.
The more I go, the more I come up with, like space elevators that affect the station's efficiency percentage like political stability does on planets in case you have more modules than your station's population can use. I think at this point the issue of coding this all in should pop up, so someone tell me whether any of this is possible or not before I lose me head in the stars.
The only substantive request you've made is for installations to be able to be supported or replicated in empty space, without a planet or a token asteroid to anchor then to. We are already getting a boost in that regard for deep space maintenance bases and civilian hangars.
Though I think you summarized my desire down pretty well, what came after's still nothing more than a pit stop really, which isn't that close to what I'm suggesting at all. DS maintenance and civie hangers just make our current options easier to implement, and maybe open up a few new ones.
At the very end let me say this. Roleplay can benefit from stations too. Just imagine the stories about stations that orbit a planet lost to abominations the scientists cook up in labs that end up massacring the population.
I'm such a dreamer...
