But that's not what I want to write about.
I think this 'creep' has extended so far that we've forgotten (or just didn't know), what all of those ship classifications even mean, or haven't taken a good look at whether a particular wet navy ship type even makes sense in space. The term 'destroyer' is perhaps the worst offender. We get destroyers in sci-fi that range from small escorts to titanic capital ships. (I'm looking at you, George.)
Since sci-fi Space is an Ocean models invariably build upon conventions established for such recent events as the Battle of the Jutland, the rock-paper-scissors paradigm of naval combat that dominated in that period may have been lost, and with it, just what exactly made a destroyer what it was and why. Battleships rule the waves, with armor that only another battleship could defeat, and large bore cannons which could in theory strike their targets over the horizon. If you wanted your Navy to compete, you needed to build battleships, which were crushingly expensive burdens. (Look back to the period for commentary on how the naval arms race prior to WW1 was driving countries to ruin for examples of this.) Then along came the torpedo, a powerful, relatively cheap weapon that could sink one of these behemoths, and could be deployed by relatively tiny, fast, and cheap swarms of torpedo boats. The Battleship, especially the post-Dreadnought models of the type, could not effectively engage torpedo boats with their big guns. Thus entered the Destroyer, short for Torpedo Boat Destroyer. It was much smaller and cheaper to build than a ship of the line, was fast enough to chase after its quarry, and had lighter guns that could track and sink the torpedo boats. Battleships sink Destroyers and other Battleships, Torpedo Boats sink Battleships, and Destroyers sink Torpedo Boats. Rock, Paper, Scissors. The Destroyer had a specific purpose, otherwise it would have never existed. Submarines being torpedo boats that could go underwater just meant that the Destroyer still had a reason to be as naval warfare technology moved on.
Does your Destroyer have such a purpose? It's something to think about. A Destroyer in a space setting could be a 'Fighter, Drone, and Missile' Destroyer, heavy on the point defense systems and acting as a consort to a larger and more important vessel. That makes perfect sense and justifies the class. If your Destroyer can stand in the line of battle, with a primary anti-ship armament and good protection, and even carry a few of its own fighters or troops along for the ride, you might want to reconsider its classification. Make it a cruiser, or, if it really is just a battleship in destroyer clothing, call it something that reflects that role.
For my own setting, I've adopted naval ship classes that seem to have a purpose. Here's a list and the reasons for their existence.
- Battleship - Space Control ship. Heavy armament and stout protection. If you don't have anything that can stand up to it available, you have to concede the space it can control. Thus, Battleships can take control of orbital spaces, or even entire solar systems without firing a shot if the defenders don't have a comparable amount of tonnage and throw-weight to resist them and the willingness to do so.
- Carrier - Interface Fighter and Strike Craft mothership. In my setting, there is nothing a Space Fighter can do that a drone missile carrier can't do better, but when it comes to planetary real estate, a fighter that can operate in low orbit as well as within the deep atmosphere has greater utility. They can be Johnny-On-The-Spot for close air support, ISR, and air superiority in a way that orbiting warships providing for these roles cannot. For one thing, in low orbit, a warship will only pass over a given location on the ground for about 10 or 15 minutes, maybe 4 or 5 times a day depending on the latitude and the inclination of the orbit. Because Interface Fighters and Strike Craft have a reason to be, so too the Carrier, to bring them to low orbit, recover them after their mission, and return them to service.
- Cruiser - A cruiser is more or less a scaled down Battleship. Battleships are expensive to build, man, and maintain, and if your setting has lots of places to go, the amount of space you can actually control will be limited by your ability to put something there that can take care of itself. The Cruiser, being smaller, is less expensive to build, man, and maintain, so you can build more of them to control more space, reserving the Battleships for the really important locations and to keep them available to mass for a decisive engagement. Wet Navy cruisers tended to be faster than battleships, but in a Newtonian physics based space setting like my own, one's 'speed' in the end comes down to propellant fractions, which can be the same for any given size or class of spacecraft. Instead, a Cruiser is built for long endurance independent operations, trading a little firepower and protection for the ability to maintain a presence somewhere and control space where a Battleship isn't worth sending instead.
- Assault Ship - A troop carrier. Their job is to transport ground based combat power to target worlds and deliver them with organic interface craft. They may carry interface fighters and strike craft as well as landers, they may even have their own space to surface weaponry for fire support, but most of their displacement is given over to housing troops, their vehicles, their gear, and their supplies.
- Tanker - Just what it says on the tin. The Tanker carries reserves of propellant and reactor fuel to replenish the fleet. Since propulsion within a system is by reaction drive, massive quantities of reaction mass get consumed, and it is the Tanker's job to keep the fleet fueled. Tankers usually carry harvesting craft to find sources of hydrogen, deuterium, and helium-3 and have their own processing plants to turn raw materials harvested into useable propellants/reactants. An army marches on its stomach, and so too a fleet maneuvers on what is in its tankers.
- Logistics Ship - A military version of a freighter, usually identical to existing civilian merchant classes, with perhaps a slight upgrade to armament, communications, and protection. Warships need spare parts, replacement missiles and other expendable stores, water, and food.
- Tender - A repair ship. These carry fabrication plants, raw materials, and technical shops to maintain, repair, and in some cases even rebuild parts of spacecraft to keep the fleet in the fight away from their home stations.
- Frigate - A scaled down Cruiser. You can build and operate 2 or 3 frigates for the same expense as a Cruiser, so you can at least 'show the flag' in more places, and in remote systems even control them. Most Frigates have a small organic troop contingent embarked on board as well as small landers to transport them, allowing them to occupy or provide security for outposts and small colonies. The Frigate is the Swiss Army Knife of the fleet. It doesn't do any particular job very well, but it can do a little of everything, and because you can build lots of them for the same price as a Battleship, your ability to at least influence large volumes of space is much greater than a powerful warship that can only be in one place at a time.
- Destroyer - Also called an Escort. The purpose of the Destroyer is to eliminate any missiles or small craft which threaten its consort, usually a Battleship, Carrier, Assault Ship, Tanker, Tender, or Logistics Ship. It does not have an appreciable anti-ship armament, and would be helpless against even a Frigate. They have to be fairly agile in order to provide coverage, or if necessary, position themselves to eat an anti-ship missile instead of letting their charge get hit.
- Corvette/System Defense Boat - A scaled down Frigate, usually optimized for an anti-ship role. Since a corvette/SDB is unlikely to have a direct fire armament that can seriously threaten larger warships, they typically incorporate a significant fraction of their weapons displacement as anti-ship missiles and will engage in packs. This gives them limited combat endurance, but offers a cheap way to punch well above their weight in an engagement. Think of them as the wet navy Torpedo Boat. (Ironically, Destroyers are not built to engage them, but a Frigate or another Corvette/SDB would serve admirably in this role.) Corvettes and SDBs may also be assigned to convoy escort duties, sometimes led by a Frigate or a Destroyer. Their job is to intercept commerce raiders away from the convoy, and destroy them with salvos of anti-ship missiles. The original wet navy Corvette was meant to operate in home waters in a patrol role, and with a fairly strong for its displacement anti-ship capability. A System Defense Boat is simply a Corvette without an FTL system, making them more in line with the original wet navy concept.