Quote Originally Posted by Yora View Post
I tried coming up with some kind of very rough map where the systems inhabited by the various species are roughly in relation to each other, but that turned out to be more complex than I thought. The Galactic Republic and the Federation are both federal republics in which each speciws has their own worlds, but they all can freely move between the system. The Federation only has borders with three significant neighbors, so Star Trek can easily handwave that away. .with 12 species thst that each have several states, knowing who is actually neighbors with who becomes a relevant factor, though. But I still want to also eat my cake and have all 12 species appear in the same places on the frontier, so some clever thinking will be required here.
One thing to keep in mind here is that unless you're operating at extremely large scales, space empires occupy a spherical map, and even at extremely large scales, the galaxy is a disc of ~1000-1500 light years in thickness (assuming using the Milky Way as a base, other galaxies have different parameters.

You also want to keep in mind how much total territory you're working with. A ~1000 light year radius sphere will contain something on the order of 8 million star systems - assuming stellar densities similar to those in our local neighborhood. Most of these stars will be red dwarfs, there will also be some number of brown dwarfs (estimates of abundance vary wildly). That's an awful lot of star systems to spread only a dozen species across.

As by the rules for Hyperspace travel as have been established, arrivial points in a system are quite random, and it takes several hours to go between the jump zone to the inner planets where most colonies and outposts are. Unless you're really unlucky, you will always be able to jump out of a system long before any ships present in the system can get to you. Or even missiles. All you need to make another jump is to have enough fuel on your ship. This means that it is impossible for any fleet to block a system for traffic passing through. So having to go through a system controlled by a different state to get to one of your new colonies would not be a big issue. Not much different than having to go through an uninhabited system. In a way, states would be limited to controlling only the inner system (and with giant stars not even that), while the outer syste, would effecrively remain "international waters", "off the coast". That could actually be interesting. Would be really annoying to have an enemy cruiser hanging out at the edge of the system, listening to all long-range communication (though with encryption of couse) and monitoring the coming and going of any ships. Even if you can send ships to threaten them, they can just jump to safety and be back in a day or two.
This means you need to think about bombardment very carefully. In a 'retro-future' situation bombardment ought to be a very powerful tool, since it was one of the principle functions of heavy gun warships from the time period - massive naval bombardments were a major feature of the timeframe from the American Civil War through WWII - and a ship in space has some rather severe gravity-based advantages in terms of bombarding any target deep within a gravity well and a huge targeting advantage on any target affixed to a small planetoid. If defenders cannot rapidly reinforce installations in the outer system basically any ship at all can summarily annihilate them by dropping rocks from high places. You've specified no energy shields, which eliminates the traditional anti-bombardment solution.

However, an important part in the previous part was "having enough fuel". I quite like the idea of super-preheating propellants with fusion power before running it through the sublight engines to create thrust with much better fuel efficienctpy than just burning the fuel normally. I doubt that would produce the speeds to get near the travel times I am thinking of, but that's one of the things were it's best not to explain how it's supposed to work specifically. And since Hyperspace is only another dimension, you still need engines to move through Hyperspace once you entered it. Fusion power famously needs very small amounts of fuel, but fusion-heated plasma still would eat through large amounts of propellant. The important part here is that there's only one type of fuel for sublight and Hyperspace that needs regular refueling.
What you're hitting on here is the difference between fuel and propellant. With the exception of chemical rockets in which the fuel is also the propellant, these are two explicitly different things. The fuel is what you use to run the reactor, the propellant is whatever makes up the reaction mass you're throwing out the back end. This need not be the same thing at all. Now, in a fusion rocket you're still fusing the propellant, by putting in through the reaction and hurling it out the back end as super-high energy fusion product particles. However, the low amount of fuel needed to run the ship means that so long as it's not accelerating it can maintain station more or less indefinitely, with 'station' potentially representing a complex orbital pattern through a star system.

Note that there are many different types of fusion rockets and they use different types of propellant depending on which reaction you are using. It's perfectly reasonable to pick one, but which one you pick makes a massive difference in where and what you have to do to mine for fuel - for example whether you harvest it from rocky worlds versus gas giants - and how you build your ships (in particular different fusion reactions range from 'essentially zero' to 'oh god please make the burning stop!' in terms of the amount of dangerous radiation they produce). There's also the issue that spewing high-energy fusion products out the back end of your starship turns the drive into a deadly weapon capable of vaporizing basically anything you point it at.

I think a strategic doctrine that focuses around the possession of fuel stations instead of star systems could be an interesting new approach. Those stations might actually be heavily defended fortresses in themselves and not simply highway gas stations.
There's an interesting question of how the propellant gets from where it is mined/made (if you're using fusion propellants like deuterium, tritium, or Helium-3 it's actually most efficient to produce it in gigantic industrial nuclear reactors rather than mine it) to the propellant stations. Since this is a retro-future scenario, presumably giant space tankers of some kind are involved (the historical equivalent of relevance is probably the Collier). Such ships might have a very significant economic role, especially in areas with limited propellant availability of their own - something that might be quite common if most of the propellant is manufactured in giant factory complexes in the homeworlds.