Standard Sci-Fi Fleet: Well, most of these classes exist – although it is particularly important to realize that the Empire alone fields literally thousands of specialized class vessels that don’t fit neatly into any of these categories, and that to a certain extent, trying to shove everyone’s ship designs into the approximate same paradigm is an exercise in futility…
Ignoring the permanent city-ships, and starting with the military classes, we have, first, the regular fighting-ship classes. These begin with the frigate and destroyer (including the latter’s stealthy recon variant), small and fast ships used in “wolf-packs” for scouting, escorts, and screening elements, but which don’t themselves have the resilience or firepower to stand up in the wall of battle.
The middleweight combatants, and the most maneuverable/versatile, are the cruisers and battlecruisers, which also serve as screening elements for heavier ships, but are more often seen as the standard patrol and task-force element, often operating in flotillas (a cruiser wing with a battlecruiser or two thrown in for stiffening) or even independently (especially the battlecruisers). And since this type of operation (power projection, anti-piracy patrol, general keeping the peace of the spacelanes) is the bread-and-butter of the Powers and their naval forces, most navies, the IN included, field more cruisers and battlecruisers than just about any other type of starship.
These are also classes that come with a large number of variants. Most recognized among the cruiser classes are the assault cruiser (optimized for planetary assaults, i.e., heavy on the ship’s troops and capable of launching drop shuttles and drop pods into atmosphere; some of these are aerospace cruisers, which air fighters can sortie from before there’s an orbithead established); the diplomatic cruiser (a big stick to transport the softly-speaking); the point-defense cruiser (the one type of cruiser you might see in the wall, designed specifically to augment the point-defense of other ships); and the interdictor cruiser (specializing in the volume-security mission, which is to say, to chase down, capture and board other starships). The primary battlecruiser variants are the command battlecruiser (optimized to carry the admiral commanding a CC/BC task force) and the carrier-battlecruiser (which carries AKVs – see below – as well as its internal armament; this is the type of BC usually found operating alone, due to its significantly enhanced operational envelope and capabilities).
Then we come to the actual ships of the wall, battleships, carriers, and dreadnoughts. The battleships are the mainstays of the wall, large and slow vessels mounting heavy, long-range firepower for fleet engagements; and the carriers, even larger vessels, carrying an extensive complement of AKVs (autonomous kill vehicles, the missile/attack-drone fighter-interceptor hybrids described under Space Fighter, to swarm and destroy enemy starships at sub-”knife fight” range – i.e., hopefully inside the minimum effective range of their point defenses). The dreadnoughts are effectively “super-battleships” built on carrier hulls, used in relatively small numbers to stiffen the wall.
Superdreadnoughts are either dreadnought-class vessels built on even larger hull frames, or regular dreadnoughts with only battleship armament, using the extra internal volume to hold specialized systems; common examples are the command superdreadnought which houses the admiral in charge of a large task force or fleet; the information-warfare superdreadnought; the loadout-heavy mauler superdreadnought, the anti-RKV superdreadnought, etc., etc.
At the top end of the regular classes, we have the hyperdreadnought – taking the design principles of the superdreadnought classes even further – of which the Empire fields three, each unique within its class; Invictus, Imperiatrix, and God of War. In order, they are the home of Admiralty Grand Fleet Operations, the Imperial Couple’s personal flagship, and the literal embodiment of the archai/eikone of war. Any one of them turning up on the battlefield would have implications that, by and large, no-one wants to think about thinking about.
Less regular military classes include the starfighter, a frigate-sized mini-carrier with four to eight AKVs clamped to its outer hull, used primarily for covert operations and commerce raiding; the fleet carrier, a giant (and not itself offensively armed) lighthugger starship on the lugger model (see below) whose purpose is to ferry naval task forces to systems not connected to the stargate plexus; the fluffships – whose design is implicit in their name – that police systems for debris, ricochets, and misses after battles; and the relativistic kill vehicles for practicing MAD on an interstellar scale with giant lighthugger missiles capable of shattering planets, given a good run-up.
Among civilian ships, there are also various recognizable classes of starship for different purposes:
For freight transport, for example, one can recognize both the immense grapeships (from the appearance of the external cargo pods) or megahaulers, which transport vast amounts of containerized cargo along the largest and most dependable trade routes, and their smaller cousins the haulers, smaller freighters which handle more volatile but still regular traffic everywhere, and are willing to handle breakbulk as well as containerized cargo, and of course the volatiles-hauling tankers; and finally, picking up irregular and speculative trade and filling in the gaps, the thousand different classes of free traders (and their somewhat more combative overlapping variants beloved of smugglers and irregular commerce-raiding privateers, the blockade runner and corsair.) For routine transportation of volatiles, ore, and other such bulk and fungible cargo, fully automated slowhaulers often take up the task.
For passenger transport, likewise, we begin with the luxurious highliners and liners - analogous to the megahaulers and haulers in size and usage upon routes, and their express cousins the fastliners. And then, for those travelling off the regular routes or seeking a more unique experience, a great many free traders are just as happy to carry passengers as they are to carry anything else. Of course, the relatively wealthy and privacy-desiring have the option to travel in their private yachts, as ever, and at the other end of the scale, steerage-class transport is available to the relatively indigent on any number of iceliners, ships – often used as colonization transports – designed for the specialized task of transporting bodies in cryostasis or nanostasis, and minds recorded on data substrate.
In more specialized uses, dedicated classes abound: when messengers, mail, and packets need to get there really fast, within the stargate plexus at least, engine-heavy couriers are on the job; wrecks, debris, and flotsam are salvaged by debris recovery vehicles; hospital ships provide medical services (and reinstantiation services) to military fleets and disaster or epidemic-struck regions; logistics ships provide repair and construction services wherever they’re needed; oilers and tenders provide fuel, supplies, and other necessities to other starships; science, research, and exploration are done in the ubiquitous, customizable service/operations vehicles; smelterships render down asteroids into usable metal and other elements; and tugs and their larger cousins, the antimatter-torch equipped superlifters, move ships, modules, materiel – and in the case of the latter, entire habitats, asteroids, and even small moons – to where they’re needed to be…
…and if we’re willing to classify flying cities that are as much drift-habitats as starships, then we must include the civilization-backup ships, preserving archives, museums, and mind-states in the far reaches, ready to flee news of existential disasters; All Good Things, ICC, spreading the good word of commerce to underdeveloped regions with its skymalls; the empire ships, massive floating conferences/exhibitions/showpieces/parties flying endless loops around the Imperial Core and its many distant exclaves keeping population, culture, and knowledge well-distributed; and the embassy ships, similar exhibitions paying diplomatic calls on foreign polities and recently contacted worlds, bringing religiosity to the fuzzy-wuzzies and suchlike.
For local transport, small craft abound. For freight, lighters scurry about transporting cargo ship-to-ship, ship-to-station, and ship-to-ground; for passengers, pinnaces provide the same service, and in moving about between local stations or habitats in a cluster, the automated commutersphere provides rapid transport. Skydivers skim gas giants for fuel; maintenance and construction are carried out by the ubiquitous workpod; and other myriad local functions are served by the flexible, customizable cutter.
All of these, of course, exist within the framework of the stargate plexus. Outside that, a different type of ship entirely is required – lighthuggers need much more powerful engines (antimatter torch drives, for the most part) to reach the high fractions of c that make interstellar travel practical, sophisticated particle shielding to survive it, etc., etc. Let us leave aside for the moment the shardcruisers (not true lighthuggers, but hybrid ships built to service outposts in the outer cometary cloud of star systems, whose longest-range examples fade into slow, short-range luggers); and also the starwisps, ultra-light – a matter of pounds – light-sail vessels propelled by lasers at their point of origin, carrying information, tangle, or the smallest probes across interstellar space.
These then divide into clippers – high-acceleration, relatively low-mass vessels carrying premium cargo and passengers at the highest possible speeds, including, in the limiting case, the private staryachts of the very wealthiest; and luggers, their relatively low-acceleration higher mass vessels carrying passengers and freight in larger quantity. Specialized classes of lugger include the shiphauler (designed to transport docked starships rather than cargo directly; the military fleet carrier is an example of this type); the seedship (carrying ecopoesis packages and a startup colony); and the linelayer (transporting one half of a stargate pair to its destination system).