Most science fiction authors are content to invent a fictitious technology and let it be. I'm to much of an Engineer. While I'm happy to let Magic be Magic where I know technology can't solve a problem, I draw the line at propulsion systems.
Well, at least propulsion system for Project Gilgamesh. My later book, Iliad 07 is going to utilize a completely magical reaction-less drive system. But that is a century (or more) in the future, and they technology is required to reach other star systems. Project Gilgamesh is set in 2001 (albeit in an alternate timeline). Granted, they have about a century on our rocket technology because they skipped the airplane, the automobile, and WWII.
Every ship, except for a few exceptions that the plot demands, is powered by technology that is at least on the drawing board of our rocket engineers today. I do take a few logical leaps past current prototypes to speculate how they would be deployed in production. But odds are, while I may be wrong on a figure or two, the engines that make ships go around the Solar System in my story are plausible.
An interesting side effect is that there are a lot of complications that are introduced by limitations on technology. For instance, even though the people in the Sublight world have mastered fusion, translating fusion power into motive force is still a problem for them.
In this world, the Great War (1904-1918) saw the introduction of nuclear fission bombs and medium-range ballistic missiles. The Central Powers and Allied Powers fought to a stalemated, and the great Plague of 1918 led to an armistice that never culminated into formal peace treaty.
This led to a century long cold war. Nuclear stalemate meant that land conquest was more or less a thing of the past. So the great powers started competing to conquer space. The moon was the first race, and the Central Powers (now consolidated into the Empire of Krasnovia) were the first to land, the first to settle, and the first to build a military that could control the entire surface. By 1955, Krasnovia controlled the entire moon.
In their world, Fusion developed first as a form of propulsion. The first controlled fusion reactions occurred in 1958, with production engines on spacecraft flying by 1962. Fusion engines allowed heavy cargos to be hefted to orbit, and it shrunk the flight time to the asteroids to weeks of direct flight instead of years of gravity assists and drifting.
Krasnovia was caught flat footed by the development of the fusion engine. This gave independent nations, large corporations, and even a major religion or two the means to establish a foothold in space.
While the systems would improve in efficiency over time, all fusion drives operate on the same basic principle:
Basically it works by detonating pellets of nuclear material inside a cloud/pool of propellent. The nuclear explosion causes the propellent to flash to a plasma, and fly out of a nozzle at tremendous speeds. By altering the amount of nuclear material and propellent one can produce either an engine with enormous thrust (but low specific impulse) or low thrust and extremely high specific impulse.
More propellent: more thrust, but it flies out at a lower speed. Perfect for lofting payloads into orbit. Less propellent less thrust, but what flies out is at a tremendous speed. This is useful for interplanetary flight.
But even with all of this technology at out disposal we still run into every rocket engineer's lest favorite friend: Tsiolkovsky's rocket equation
Fusion power doesn't get you past the tyranny of mass. It just lets you battle with an extremely energetic fuel. Changes in speed for a spacecraft require a change in mass. For fusion drives we lob measured portions of water out a pipe at a fraction of the speed of light. But we still have the problem that every kilogram of vessel requires several more kilograms of propellent to move it. And those kilograms of propellent require kilograms of propellent, an so on.
I have prepared spreadsheets and flight plans for several different size vessels with a range of engine performance. But even with all this high-tech wizardry, Ships still need to carry around a significant portion of their own mass in propellent. And it's not very fuel efficient to carry around a bunch of extra.
If you want to review my math, look on the "Performance" tab if this spreadsheet
In summary, a vessel with enough internal volume to support a town of 2000 people weighs in at 6.43E+10 kg. That's just the hull, equipment, soil, radiation shielding, etc. That doesn't include cargo, spare parts, or fuel. That is about 48 times the mass of a fully loaded Nimitz Class aircraft carrier. Cargo and fuel for the reactors adds another 2 Carriers worth of mass. Propellent sufficient to get the craft up to a cruising speed of 0.75 Au/day (1300000 m/s, or 2900000 miles/hour) has a mass of 9.80E+09 kg. The amazing thing that, as much mass as that is (about 10 Nimitz Carriers), that's only about 15% of the ship itself (without propellent).
By comparison, the Saturn V rocket had a total mass of 2.97E+06 kg. For that much rocket and mass, it could only propell 1.40E+05 kg to orbital speed (7500 m/s). The rocket was 95% fuel and expendable stages. Fusion propulsion puts performance into a realm more like a seagoing vessel. Fuel is a consideration, but not quite as much of an obstacle.
When Naval Architects in this world design a vessel, they start with a rough idea of how large the vessel itself will be. Next, they select a cruising speed. That cruising speed, and mass approximation, determine how large a reactor is needed, and how much propellent is required to reach that target delta-V.
Cargo ships ships generally accelerate to cruising speed, drift to their target location, and then decelerate to match the orbital velocity of the target.
Naval warships have a 5 step process of accelerating to cruise speed, decelerating to patrol speed, tactical maneuvers, accelerating to cruise speed, and then decelerating to the speed of their home base. And thus, a Naval vessel winds up being one giant propellent tank. (They often employ light weight tanks for the outbound cruise that can be jettisoned to save on vehicle mass.)
But you see, by simply considering the problems of physics, I have suddenly unlocked a whole host of potential plot complications. Vessels require days or weeks to thrust to their cruising trajectory. Altering course mid-flight could be impossible, or require the vessel to sacrifice a timely return home. Space fleets would almost have to agree to a time and place of battle before hand, or risk buzzing by one another at incredible speed. Speeds that a missile or a ballistic weapon would struggle to match. These craft are, quite literally, faster than a speeding bullet.
The Project Gilgamesh is intended as a work to introduce the reader to the Sublight Universe. I feel that hobbling the characters with conventional physics for the first book will make the payoff when technology overcomes these limitations in subsequent books all the more satisfying. It also lets me build drama and tension around the uneven rollout of these technologies. I can also show technologies that people thought would solve a problem, but turned out to be a giant pain in the ass.
Book 2 will be set on a starship that is stuck halfway between Sol and another star system. With 20 years into the trip, and 20 years left to go. I don't want that ship to be perfect, nor the characters superhuman. So I'm going to have to buttress human tendencies with near-magical technology. While at the same time, commit to near technological stagnation. The problems in book two are rarely technical, but human.
There will be a Book 3 (and beyond). But I'm only sketching what will happen in those. I'd rather they build from the non-logical outcomes of Book 2 (be those on the ship we follow, or developments back in Sol.) Because, lets face it, people love surprises.
In the process of building several different classes of ship on paper, I have realized that the expected flight performance dictates the role of the ship more than anything. Ships on the scale of the Cézanne enjoy extremely efficient engines that enjoy an economy of scale. They can pulse their engines at an incredible rate, because there is so much mass to absorb the shock. There are also thermodynamic efficiencies at large scale that have parallels in our own experience with building industrial plants and power generating facilities.
I have rigged the math such that efficiency of this technology for smaller vessels drops off considerably. However, small ships zipping around at orbital speeds don't have to be very efficient. They also are more likely to be utilizing high propellent/high thrust mode. The Cézanne requires a burn time of 15 days to reach its cruising speed. An orbital tug has a burn time in minutes or hours.
Another item that I worked out is that mega-ships like the Cézanne can cheaply carry smaller vessels (and by "smaller" I'm talking about a combined mass of around half a Nimitz Carrier, or 50 Alreigh Burke destroyers). Basically it acts as a mobile base for smaller craft, reducing the distance they need to travel, and thus the cruising speed, and thus the mass of propellent.
This also explains why the Von Gogh class mobile settlements have the same engineering plant as ISTO's Capital ships. Which, themselves, are essentially a mobile launching pad for missiles, strike craft, and a flotilla of destroyers and frigates.
I had originally just put this scheme together in my head based on engineering gut instinct. Now, I actually have a mathematical foundation that drives this strategy.
As far as Krasnovian ships go, though, they are generally stationed on a planet or moon. Their ships are optimized to intercept incoming ships. While they have some cruisers and battleships, keeping those ships forward deployed is an expensive proposition for them. ISTO's logistic network is built around mining from planetoids with negligible surface gravity. While the Moon, Triton, and Europa have a surface gravity that is a fraction of Earth, lobbing tons of material into orbit is still a significant expenditure of energy. Better to keep the shipyards and hangers on the surface, and just let the ships do all the flying.
Why do the Krasnovians stick to planets and moons? Economy of scale. The amount of mass of virtually any resource on the moon dwarfs supplies that are to be found in the asteroid belt. And practically any resource in the asteroid belt is embedded in one crater or another on the Moon. It's all concentrated in one place that has a stable, constant gravity field. You can build structures on the moon out of local material. You can build road systems. The amount of material and wizardry to support 2000 people on the moon is a fraction of what is required to build a station in the belt. They have cities with millions of people. The biggest city in the belt is 100,000 (New Franklin / Perseus Station on Psyche).