Not all factions use every classification of vehicle. For exmample, Dweller naval strategists focus on frigate-sized vessels with cruisers forming the bulk of their larger fleet.
Strike Craft
Strike Craft are defined as the smallest space combat vessels, and more specifically – those that are not capable of long-duration independent action. When operating on their own or in small groups, their primary purpose is to interdict and defend against other strike craft, and in larger numbers they become a potent threat to larger vessels.
Fighters – The smallest combat spacecraft classification in any doctrine, the fighter’s primary purpose is to deal with other strike craft and attack other small targets. Fighters are often comparable to aircraft that function in the interceptor, air superiority, close air support, and multirole combat fighter roles. There are exceptions to this, such as the Terran Alliance’s use of heavy missile-laden strike craft to take out larger capital ships, and the Proximan Suzerainty’s use of one or two-man ramming fighters used for boarding operations. A fighter has extremely limited combat linger time, both in terms of fuel and ammunition stores, but this sacrifice in endurance increases the vehicle’s agility and therefore survivability. As such, fighters are never seen operating independently, they will always be used near a base or a vehicle that has the ability to maintain strike craft.
Swift Assault Craft (SAC) – Larger than fighters, but smaller than frigates, and usually incapable of independent FTL travel (unlike scout ships, which are usually capable of independent FTL travel), the swift assault craft forms the main firepower of a strike craft-focused doctrine. SACs are comparable to large multirole combat aircraft, bombers which can also function as gunships, or bombers that have extensive anti-air defenses installed, or (from a naval perspective) fast attack craft such as torpedo boats, patrol boats, and missile boats. Whereas most fighters’ primary role is to deal with other strike craft, SACs function as anti-frigate vessels when operating in groups. SACs can also be designed for anti-fighter operations, mainly through the use of turreted and firing arc-based weaponry to deal with the fighters’ innate agility. SACs have some capability for extended operation, but they still require regular refueling and rearming. SACs are relatively uncommon amongst most interstellar powers, they are a niche role fielded by powers who require more firepower in non-FTL capable vessels, usually due to costs involved in manufacturing those smaller FTL-capable vessels.
Auxiliary Craft
Larger than a strike craft, yet smaller than a frigate, vessels that fall into this poorly-defined area fill a variety of roles.
Courier – While FTL travel has clearly been perfected by interstellar powers, nobody has quite managed to develop a means of faster-than-light communication that is both reliable and high-bandwidth. Telepathy is practically instantenous and reliable, but its bandwidth is terrible. Quantum Nonlocality methods of communication can offer high bandwidth (at obscene prices), but they are less than reliable (requiring constant maintenance and diagnostics), and they suffer from notable transit times when dealing with insterstellar distances – 22 minutes for the 4.2 LY distance from Sol to Alpha Centauri, for example. Consequently, all interstellar powers utilize courier vessels to move low-priority and large communications from place to place when dealing with interstellar communication. Couriers tend to be small, cramped, with limited cargo space for large parcels, and maybe room for a single passenger in addition to their crew. However, what they lack in creature comforts and carrying capacity, they make up for in raw speed. Whatever their method of FTL travel is, courier vessels always have the largest possible engine that can be crammed into their hull. Couriers are armed with only the lightest of weapons, as they are designed to use speed and agility to avoid combat.
Examples: Hermes-class Courier (Terran Alliance), Suzerain’s Voice-class (Proximan Suzerainty)
Runabout – Comparable to seagoing motor yachts, runabouts are small multipurpose FTL-capable starships not intended for combat use. They can serve as interstellar shuttles, cargo vessels, scout craft, tenders to larger vessels, or even as light fire support. Runabouts usually have a crew of three or four, but could conceivably be operated by a single person depending on the technology used on the ship.
Examples: Hecate-class Runabout (Terran Alliance), Janus-class Runabout (Terran Alliance), Pizzuto Industries PS-Series with PSJ Upgrade (Terran Alliance Civilian)
Scout Ship – Scout vessels are often romanticized in the media and fiction of interstellar powers, due in no small part to their role in filling in the empty space on any star chart. Scout ships are FTL-capable, and outfitted for extended unsupported operations in the void. Only slightly smaller than frigates, they trade firepower for well-tuned scanners, hangars holding auxiliary craft to meet a range of needs (up to and including strike craft), and a broad crew base instead of only soldiers. Scout vessels come in two varieties, standard and deep-range. The dividing line between the two is poorly defined, but usually deep-range scouts have twice the endurance of a standard scout and a larger crew capacity. A deep-range scout might make an initial survey through an area by itself, and then it will later be followed up for a more intensive survey by a wing of standard scouts with a few support vessels.
Examples: Anders-class Standard Scout (Terran Alliance), Shackleton-class Deep-Range Scout (Terran Alliance), Zoi’Yav’Mil-class (Dweller Empire)
Frigates
Frigates are the smallest vessels which are expected to be FTL-capable and intended to see combat, save for a few one-off modifications of SABs, the occasional scout ship which has weaponry above the standard outfitting for that vessel type, and FTL-capable Corvettes. In some doctrines, frigates form the backbone of the fleet’s firepower, while in doctrines favoring larger vessels the frigate is delegated to escort duty to protect convoys, supply ships, and othe vessels.
Combat Frigates – Combat frigates form the backbone of a destroyerless fleet doctrine. They are small ships, but typically capable of independent FTL travel. A combat frigate’s typical role is as a picket ship, or as part of a multi-ship patrol force. Combat frigates are too small to maintain a contingent of strike craft, but they usually have a shuttle or two for personnel transport and surface operations, since frigates are typically too large to make planetary landings. Combat frigates usually lack extensive multi-role capability, they are usually optimized towards a single method of offense and utilize the remaining space for defensive systems. The Medea-class frigate used by the Terran Alliance, for example, is practically built around two heavy railguns used for combat against frigates and larger vessels, but it has anti-strike craft weaponry installed for defensive purposes.
Examples: Medea-class Railgun Frigate (Terran Alliance), Medusa-class Torpedo Frigate (Terran Alliance)
Support Frigates – Support frigates are an important vehicle classification in a fleet doctrine that emphasizes frigate-class vessels. Strike craft have limited combat duration, and frigates are limited by ammunition and self-repair capacity. To fill these gaps, the support frigate provides resupply capability for both strike craft and frigates, and carries repair teams capable of making in-field repairs to other vehicles. Unlike combat frigates, support frigates have the capability to maintain a contingent of strike craft, allowing them to function as small carriers for patrol and picket groups.
Examples: Thoth-class (Terran Alliance)
Destroyers
A step up from Frigates in terms of size, firepower, and flexibility, Destroyers come in two varities – multi-mission, and purpose-built. Multi-Mission Destroyers are multirole combat vessels designed to take on a wide variety of missions and threat elements. Consequently, their firepower is diverse, but lacks a ‘solid wallop.’ Purpose-Built Destroyers, often referred to as <thing> Destroyers (where <thing> is the target they are designed to fight), trade flexibility in offense for the ‘solid wallop’ needed to counter a specific threat. Destroyers are typically heavily armed, and usually only twice the size of Frigates. Consequently, a Destroyer can bring a lot of firepower to the battlefield, but are low in crew comforts. Additionally, Destroyers usually have a limited ability to deploy and maintain strike craft, usually two vehicles of Fighter size. Like a Frigate, a Destroyer will also have one or two shuttle-type vessels for personnel transport and surface operatoins. Typically, Destroyers operate in groups of two or three.
Examples: Fist of Naicun-class (Proximan Suzerainty Cruiser Destroyer), Stanislava S Popov-class (Terran Alliance Multi-Mission Destroyer)
Cruisers
One step up from Frigates are Cruisers, ships that are usually designed to be operate independently, being the smallest combat ship intended to do so. Similarly to destroyers, cruisers have the ability to field some form of strike craft to aid it in combat, typically four to six vehicles of fighter size. Cruisers can be used as heavy exploration vessels, for interdiction, as command ships for patrol operations, for heavy escort duty, and a wide variety of other roles. Some species produce more specialized cruisers which lack the flexibility of a standard cruiser but excel at a specific role.
Examples: Praetor-class (Terran Alliance Missile Cruiser), Vun’T’Vorl-class (Dweller Empire)
Capital Ships
Capital Ships are the largest warships fielded by interstellar powers, but the designation really is just an indication that "this is bigger than all those other types of ships." Capital ships are designed to function independently, as command ships for fleets of smaller vessels, as components of large fleets, or as powerful ships used in the defense of static assets such as planets and space stations. The hallmark of a capital ship is combat durability, flexibility, and endurance. Speed is most definitely not a charactestic associated with capital ships, as they are typically some of the slowest combat vessels fielded by interstellar powers.
Amphibious Assault Ship – Seldom seen among most species, these are purpose-built vessels intended for the transportation of ground troops. Unlike a standard transport vessel, these are armed with enough weaponry to pose a threat in a space battle, and have the ability to field strike craft as well. Typically they are similar to a Carrier in design, though the majority of their space is used to house embarked ground troops, along with support personnel and transports for those ground troops. Functionally, these could also be termed ‘Assault Cruisers,’ ‘Assault Battlecruisers,’ or even ‘Assault Carriers’ depending on their size and weapons loadout, as they function like amphibious warfare ships used for naval surface combat.
Examples: Iwo Jima-class (Terran Alliance)
Battlecruiser – Essentially larger cruisers, battlecruisers are characterized by being able to combat several threat types at once through the use of varied weapons. Most fleets are commanded from a battlecruiser. While in naval terminology a battlecruiser is essentially a bigger, more powerful cruiser, the designation has taken on new meaning for interstellar powers. Battlecruisers always have the ability to house and maintain two full wings of fighters, along with a wing of SACs.
Examples: Conqueror-class (Terran Alliance), Lagos-class (Terran Alliance)
Carriers – Carriers have one purpose: bringing strike craft to bear in a combat situation. They house large amounts of personnel, fuel and weapons stores, and an extensive array of fighter and swift assault craft. Their role is to transport strike craft over distances that the craft could not move themselves, and make sure these strike craft are repaired and resupplied for combat. Carriers contain very little in the way of anti-ship weaponry, but are equipped with anti-strike craft weaponry and other defensive systems.
Examples: Yuri Andropov-class (Terran Alliance)
Command Ship – Often built as a derivative of another vessel type, usually battlecruisers, command ships form the core element of a fleet of capital ships. They house the fleet’s commander and support staff, along with extensive sensor array networks and communications equipment. They are actually rather uncommon, most fleets are commanded from a more standard vessel. They should not be confused with a flagship, which is defined as the ship that the fleet’s commander is based on. A command ship is usually a flagship, but a flagship does not have to be a command ship. Command ships are usually the size of battlecruisers, but armed more like a cruiser, with the remainder of the space going to command and control purposes. Like all capital ships, they can maintain strike craft, usually a wing or two of fighters and several SACs.
Examples: Phoenix-class (Terran Alliance)