Human spacecraft design

Human starships are rudimentary compared to their Covenant counterparts, having much fewer exotic technologies at their disposal. Since the Human-Covenant War, the UNSC has been working on closing the technological gap between the two civilizations, with UNSC-built vessels often seeking to circumvent Covenant advantages through various lateral solutions.

Early days (21st-22nd century)
Military spacecraft as of the Interplanetary Wars were divided into four generations retroactively created by historians:
 * GEN1 (pre-2110s) were considered experimental vessels, with ship classes being limited to less than six vessels each, and significant modifications made to each sister. They were usually limited to one ship, and all were designed as short-ranged ships with some sort of weakness (usually propellant/engine limitations) that prevented them from going too far. Diverse in shape and design, though most followed the design of a handful of pioneer designs.
 * GEN2 (2110s-2140s) were the production vessels, with modular components allowing for far faster and cheaper production. Most heavily-armed ships are still limited by range, but cruising patrols are possible. Many factions of the Interplanetary War began the conflict with GEN2-type vessels. While they existed beforehand on some vessels, nuclear-powered drives became standard on combat vessels during this era, largely due to the still-ongoing East/West arms race.
 * GEN3 (2140s-2160s) saw the rise of increased diversity. Two separate lines of shipbuilding theory were popular; the major powers favored large, multirole designs that combined firepower, range, and carrier capabilities in a single force-projecting package, while others favored smaller, more difficult to detect vessels that were intended to secure holdings.
 * GEN4 (2160s-onwards) were the ships built in the Interplanetary Wars.

The first orbitors descended from experimental military spaceplanes and reusable rockets, though some nations built their combat vessels to be space-only from the outset, relying on external means for orbital transit. While politically attractive to some nations, spaceplane-style spacecraft were highly limited by their generalist design philosophy and weighed down by extra mass, features and systems that dedicated exoatmospheric craft could use for combat purposes or eschew altogether. As the scope of military space operations grew and the dynamics space combat became increasingly complex, aerodynamic, atmosphere-rated combatant vessels quickly fell by the wayside, as they were outperformed in virtually all areas by dedicated space-only vessels. Not only that, as combatant vessels grew in size and mass, the costs for surface-launched missions became prohibitively expensive. By the turn of the 22nd century, virtually all orbitors were built in space exclusively for space operation, with planetary transit provided via separate shuttles or rocket vehicles. Despite their lackluster performance compared to their successors, the streamlined "Golden Age" orbitors (particularly those resembling the traditional image of rocket ships) were frequently romanticized in media, their supplanting by less glamorous yet more practical designs being seen as an end of an era of sorts.

As they largely relied on relatively inefficient chemical rocket engines, first-generation warships (then largely consisting of orbitors) were highly limited in their range and maneuvering capabilities, often using additional booster sets and supplementary propellant tanks on a mission-specific basis. As such, the first-generation orbitors were more like semi-mobile platforms than true combat vessels by the standards of later warships, and indeed many were regarded as more akin to armed satellites even at the time. The early orbitors' reliance on chemical engines prompted many nations and political blocs to build an extensive network of orbital fuel depots around Earth, in addition to the already extensive civilian infrastructure. From early on, many orbitors incorporated at least a single ion engine, usually powered by a deployable solar array used to charge a set of power cells outside combat maneuvers. However, the age of the non-nuclear orbitor was short-lived. After China adopted nuclear reactors on their warships, other governments soon followed suit.

Before the 22nd century, dedicated military missions to the other planets were exceedingly rare, singular events only undertaken by powerful nations or multinational military alliances, such as NATO.

By the second generation, most orbitor classes had eschewed chemical main engines for either a nuclear-powered ion drive or a closed-cycle nuclear thermal rocket. While these early NTRs provided superior thrust for combat maneuvers, along with roughly doubling the propellant efficiency from chemical engines, they were also mass-intensive, and ultimately only a marginal improvement over conventional rockets for long-range travel. Largely derived from old, long-discarded technology, solid-core NTRs were always a stopgap solution deployed when the demands of space combat began to grow beyond the capabilities of chemical rockets. While everyone always saw them as a stepping stone on the way to more powerful drive technologies, some nations opted to skip them altogether in favor of more economical ion drives, particularly on ships designed for long-range and/or high-endurance missions. As their low thrust made them virtually useless for combat maneuvers, ion drives were usually supplemented by secondary chemical engines, which were used in combat or to provide additional thrust for long-range missions. The limitations of early nuclear rockets would not be overcome until the late 3rd generation, which saw the development of the first viable open-cycle nuclear rockets along with various intermediate designs, such as the limited-issue nuclear lightbulb engines. While such drive technologies had been contemplated for over two centuries at that point, the political will to develop them had remained lacking until tensions between the various interplanetary factions reached the boiling point, and a functioning implementation of fusion drive technology still remained decades away. Even so, the adoption of full nuclear engines was controversial, earning some vessels in particular unflattering nicknames such as "Flying Chernobyl".

The mass budgets and limited radiation-shielding technology of the time also had a major effect on the physical design of second- and some third-generation vessels. It was virtually necessary (or at least the most economical option) to isolate the reactor from the rest of the ship with a flat radiation shield or "shadow shield", rather than encasing it in heavy shielding in its entirety; this limitation would not be fully overcome until around the advent of the third and fourth generations, which saw the rise of increased flexibility in reactor placement. Even then, however, many shipwrights preferred the shadow-shield method, as it freed up considerable amounts of available mass for other components such as armor and weaponry. Up until the third generation, most ships used a lightweight spine-based superstructure or hybrid designs featuring a central spine and boxed-in armored sections. During the war, the use of more efficient reactors and engines saw the emergence of the more mass-intensive, fully boxed-in structure on some ships to provide superior protection. The implementation of nuclear reactors and more powerful drives also increased the need for cooling. Consequently, large heat radiator panels and vanes dominated the design of most second- and third-generation warships.

Until GEN3 designs, spatial vessels were comparatively weaker and smaller than maritime designs because of cost, requirements of their role, and physical limitations (mostly mass). Ships of both groups typically had service lifespans of 20–40 years.

Many limitations and restrictions of early warfighting craft were also due to limitations imposed on them by previous 'enlightened' regulations from the United Nations, which would be undermined or repealed across the centuries. Weapons were one of the primary things that are curtailed to prevent the installation of WMDs, such as space-to-ground missiles and tungsten rods. UN-funded/owned vessels were probably eligible for increased weapons payload, and an early policy of encouraging cross-nation shipbuilding programs also allowed world powers to gain access to techniques or components that were typically kept secret by smaller nations. Lobbying for reduced restrictions and blocking of 'enlightened' acts that would have made it difficult to build powerful warships or armed civilian craft contributed to a low-profile arms race where some ship classes suddenly experienced massive size increases due to a recent repeal.

Power and propulsion
By the 26th century, most UNSC warships draw their power needs from the fusion drive itself, though they retain fission reactors as backups.

Thermal control
As for the 26th century, UNSC ships have transitioned from external heat radiator panels to using an advanced, highly efficient radiative material embedded into the hull plating of the ship. In addition, modern fusion drives are far more efficient than their early predecessors in cooling themselves via transferring most of the heat of the reaction into the superheated exhaust. In addition, many UNSC ships still have external radiator panels that are normally folded into the hull but can be extended during combat or sustained drive burns. Civilian vessels remain more eclectic in their means of heat management, and many cargo vessels in particular retain conspicuous radiator panels.