Modular Starship Design (Theseus Ships)

In interplanetary travel, starship mass is a critical aspect of mission design and starship planning. Every kilogram of armor, subsystem or payload will decrease the ship's net acceleration and overall fuel efficiency as more reaction mass will need to be expended to propel the ship to the same distance. Massive jack-of-all-trades ships like the ones seen in 20th and 21st century popular fiction are seen as excessive wastes of fuel and mass just to plan for every eventuality. Even in the age of fusion drives, mass efficiency is an important variable for reducing mission costs and maximizing ship performance.
As a result, most starships in the current age tend to incorporate some degree of modularity. Many freighters, for instance, are simply an array of hardpoints attached to a thruster and guidance control system. Rather than having a massive hull around the cargo, each attached container module only adds its own weight to the vessel.
Military designs are by far the most modular, as missions can range from direct ship to ship combat, to orbital constabulary patrol, to surface force projection. While most space forces will involve some interchangeable subsystems and weapon packages, none go so far as the Transnational Aerospace Security Alliance (TASA) which has designed an extreme degree of modularity into their warships.
Much like the modular rifles that rose to prominence at the turn of the 21st century, TASA ships allow a baseline vessel to be tailored to a wide variety of mission sets. Weapon pods, armor plating, passenger berthing, cargo storage, sub-ship hangars and sensors can be added or removed at shipyards, completely altering the specifications and capabilities of each TASA vessel. Even reaction mass can be added via external fuel pods which can be detached when empty, significantly increasing the acceleration range of a given vessel. Layers of armor can be installed for direct engagements or removed for patrols and expeditions.
Only the most essential core systems and superstructures are a constant against the changes. Even the propulsion engines and fusion reactors can be replaced upon the development of more efficient drive systems, greatly extending the operational lifetime of each class of ship. This has led to TASA warships being nicknamed "Theseus Ships" in reference to a thought experiment over the question of whether a ship that has been slowly replaced, piece by piece, is still the same ship. Much like the mythical Ship of Theseus, TASA ships often shed a large percentage of their mass over their lifetime. This extreme versatility has not yet caught on with other spacefaring superpowers like the Pacific Asia Coalition or the Confederation of Southern Nations - Confederación de las Naciones del Sur, giving TASA a unique advantage in maximizing the efficiency of their growing fleet.
One notable side effect of the Theseus Ship design among TASA vessels is that it hampers long range thermal and velocity based identification. Normally, observing the mass, velocity and exhaust plumes can lead to extremely accurate ship identification. The numerous possible permutations of TASA ship payloads obfuscates identification through this method. It is possible to determine the mass of a TASA ship, but identifying what mission the vessel has been modified for can be quite difficult without visual analysis, giving TASA a tactical advantage.