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 most being built to unique designs that were diverse in shape, size, and capabilities. Ship classes were limited in number (typically less than six), and even these had considerable variations because of the rapid progress of technology and shipbuilding experience. Nearly all were designed as short-ranged vessels with some sort of weakness (usually propellant/engine limitations) that further restricted their range. Firepower tended to be limited, at least when compared to later generations. However, later GEN1 ships did start to mimic the designs of successful 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.

Layout
Early military spacecraft were more akin to submarines in how their interior spaces were allocated and have continued to be quite cramped until recently to save space. The ratio of pressurized, habitable space to a ship's internal volume has been gradually increasing with innovations in various fields, most prominently drive design. On the earliest combat ships, the livable space was usually only a small and cramped section called a habitat. Most of the ship's volume would be taken by its superstructure, reaction mass, fuel, engines, weapons, and various subsystems, and any sections of the ship not accessed on the regular would be depressurized. During combat, most or all of the crew would be wearing pressurized suits, with technicians prepared to go EVA at a moment's notice to conduct repairs to the non-pressurized sections of the ship. Even to this day, nonessential modules are depressurized during combat on warships, which also serves as a counter-boarding measure.

Power and propulsion
Nuclear fission remained the primary power source for human vessels until the revolutions in fusion power that characterized the 23rd century. By the 26th century, fusion is ubiquitous on UNSC warships, but not entirely standard in the civilian world, especially out in the Outer Colonies and on smaller vessels, many of which are still powered by compact fission plants; by the mid-26th century, compact and affordable fusion reactors were only starting to enter the civilian market. Most UNSC warships draw their power needs from the fusion drive itself, though they retain fission reactors (in some cases, secondary fusion plants) as backups.

Chemical rockets
A staple of spacecraft propulsion since the early days of space travel, chemical rocket engines were gradually supplanted in deep-space travel by nuclear-powered designs beginning in the late 21st century. However, they have retained a niche in reaction control thrusters and remain a common choice for staged surface-to-orbit transit.

Ion/plasma drives
Often considered an "economy" option, ion drives are versatile and simple, providing a much cheaper but far less impressive alternative to fusion. Even by the 26th century, various types of ion drive remain in secondary uses, especially in the civilian world, as well as serving as backup engines on UNSC vessels.

Closed-cycle nuclear thermal rockets
Closed-cycle NTRs use a nuclear reactor to heat a chemical propellant, usually hydrogen, to provide thrust. Such drives had their heyday between the 21st and 23rd centuries. With thrust rates superior to those of ion drives but inferior to either open-cycle nuclear drives or fusion drives, closed-cycle atomic drives have largely been phased out in mainstream use. However, they still have some niches in civilian use and as backup drives on otherwise fusion-powered vessels.

Open-cycle nuclear drives
Open-cycle nuclear rockets, or atomic torches, are an archaic but powerful form of nuclear fission propulsion. On a surface level, they function much in the same way as modern fusion drives but leave behind a highly radioactive exhaust trail. On the other hand, when used to their full potential, they can rival and even surpass fusion drive thrust rates, which long struggled to achieve similar levels of thrust; however, they are far less fuel-efficient than fusion. Most prominently used during the Interplanetary Wars, they have been phased out of most uses as of the 26th century, though variant designs have been known to be used by some Insurrectionist groups. Many of these designs are also notoriously unstable and accident-prone.

Attitude control
Reaction control systems typically use simple, often monopropellant-based chemical rockets. While largely replaced by superior and non-toxic propellants in military and other high-end usage, hydrazine variants continue to see use in certain niches such as automated freighters due to its cheap and straightforward manufacture (especially in places like the Outer Colonies) and the possibility of dual use as an emergency power source.

Gravitic technology
Early on, most larger ships would include a spinning section to simulate gravity to stave off the effects of long-term microgravity exposure on the crew. In pre-fusion times, engine thrust provided only a fleeting semblance of gravity; the ion drives used for long-distance travel provided consistent acceleration for long periods of time, but this was so minor that the Gs felt by the crew were negligible. Chemical and fission rocket designs would provide multi-G burns, but these were very brief, lasting minutes at most. The fusion drive, in its modern form, provided more powerful thrust over longer periods of time, though by the time the drives reached the level of efficiency capable of applying a constant burn over the course of a trip, artificial gravity plating was already being developed and applied on ships.

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.

Life support
Cryosleep has been used since pre-fusion times to conserve life support and consumables on long journeys, most notably when large numbers of people (colonists or marines) needed to be moved across interplanetary distances.