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 fewer 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 combat 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. This coincided with the buildup of orbital industry and launch-assist systems around Earth during the Golden Age of Space Colonization throughout the 21st century, which gradually enabled increasingly ambitious construction projects in space. By the turn of the 22nd century, virtually all orbitors were built in space exclusively for space operation, with planetary transit provided via separate launch 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 despite media controversy.

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. Largely derived from old, long-discarded technology, the first nuclear rockets were always a stopgap solution deployed when the demands of space combat began to grow beyond the capabilities of chemical rockets. In the early days, some nations also used nuclear-powered ion drives on space-only craft, particularly ones designed for long-range and/or high-endurance missions. While more economical than nuclear rockets, ion drives' low thrust made them virtually useless for combat maneuvers, and they were usually supplemented by secondary chemical engines, which were used in combat or to provide additional thrust for long-range missions. The second and third generations saw the development of more efficient and lightweight designs for atomic drives, making them virtually the norm for combat vessels. 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.

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.

The end of the Interplanetary War saw a dramatic halt to this age of rapid evolution and progression. Although the Navy was still interested in experimenting with new technologies and concepts, the simple fact was that they had more than enough ships to secure the entire Solar system. Too many, in fact, and over the 2170s and 2180s they decommissioned most of their inherited fleet. Only the best and most modern were chosen to be laid up in dockyards around Earth, while the rest were scrapped, sold in a demilitarized state, or used in weapons tests. The only ships they procured afterwards were based on late GEN4 designs, and even these only had incremental improvements whose main benefit was standardizing the components, weapons, and capabilities of the Navy. Still, a handful of prototypes were ordered, ranging from fully autonomous starships to the so-called "maw ships", few of which survived the 23rd century.

Since the 2340s, the advent of viable fusion engines and their proliferation in the commercial sphere introduced the need for the UNSC Navy to modernize their fleet. Though the UNSC no longer had peer opponents on the scale of the Interplanetary Wars, various rogue actors persisted throughout the Sol system, along with a pervasive sense of paranoia over cliques arising from within the UEG to challenge the prevailing order. The rise of a handful of megacorporations operating with limited oversight in the untapped frontier of the outer system was a common source of this tension, particularly in the late 23rd century, even more so than the occasional insurgent faction. This was exacerbated by the hotspots of the solar economy shifting to the outer planets for fusion fuel mining.

Although fusion power brought about new possibilities in almost every area of ship design, the technology was also immature, and underwent several iterations in a relatively short period of time before settling on a relatively stable paradigm by the advent of the 24th century. Fusion-powered ships were faster, more efficient, and had a far higher power output than before, ushering in other advances such as enabling the construction of larger and more massive vessels. However, early fusion drives were themselves heavy, cumbersome, and required additional cooling, which limited their adoption on small ships. The iterative nature of the technological progress in this era gave rise to several intermediate designs, some of which were rendered obsolete overnight as they were supplanted by new technologies. These technologies were never fully tested in combat until over a century later, with most crises of the 23rd and early 24th centuries being ground-based, highly localized in scale or unconventional in nature.

Colonial Era and the Insurrection
Since the advent of the Shaw-Fujikawa Translight Engine in 2291, most efforts in ship design once again focused on peaceful exploration, much as they had during the early Solar Golden Age. Although the two centuries since the Interplanetary Wars had seen incremental improvements in warships, these new technologies would only be tested in action during several engagements of the Inner Colony Wars of the late 24th century. While the following Pax Humana period of the late 24th to the mid-25th created another lull in warship technologies as most advancement focused on exploration and commerce, this would change as the second half of the 25th century dawned. The rise of the Colonial Military Administration as a rival to the UNSC brought about a renewed urgency to improve ship design as the tensions of the UNSC-CMA Cold War mounted. However, these technologies would only see action as the brushfire wars of the Insurrection flared up in the 2490s, with the UNSC facing down ex-CMA vessels and other hardware in the hands of the well-equipped Secessionist Union.

Deck layouts
Up until the 25th century, the majority of human ships had a "stacked" deck layout, with their decks oriented perpendicular to the main drive's thrust axis, more akin to a skyscraper than an oceangoing ship or airplane. In the early days, up until the 24h century, there was usually little distinction between "up" and "down" aboard spacecraft anyway, as only a minute portion of any space mission would be spent under thrust that would provide a consistent and appreciable semblance of gravity. Instead, dedicated rotating sections would be used to provide spaces with gravity on long journeys. As constant-acceleration fusion engines became more common, however, significant portions of an interplanetary journey could now be spent under noticeable gravity. By the mid-25th century, after the advent of zenostium-based artificial gravity plating, UNSC ships slowly began to transition to a thrust-parallel deck layout scheme. This was deemed to be more efficient for the elongated profile of UNSC vessels, and the exotic technology of the gravity plating was found to function more reliably when oriented perpendicular to the main thrust. UNSC ships maintain only a single axis of gravity and deck orientation across the entire ship, both due to the limits of the gravity plating technology and to avoid disorientation. However, maintenance access ways often lack artificial gravity and may be arranged in directions counter to that of the gravity plating. The transition to artificial gravity would not be complete until well into the 2500s, and military auxiliary vessels or older civilian ships lacking gravity plating remained a common sight even by the mid-26th.

Bridge
The command room of a ship is known as a bridge, usually combining the functions of a nautical bridge and a Combat Information Center (CIC). Historically, naval tradition has dictated that the bridge provide an unobstructed view to the vacuum outside, even though virtually all spacecraft steering is conducted based on sensor feeds. Due to the obvious weakness of an exposed bridge to enemy fire, the persistent tradition has been changing since the Insurrection, with the shock of the Human-Covenant War finally shaking off the remains of the old doctrine. All ships launched during or after the war have their main command bridges buried deep within the hull. An external observation deck with helm controls and various optional bridge functions is typically still maintained, however, with some captains preferring to use this outside combat situations.

Atmospheric operation
Other than shuttles and dropships, spacecraft designed to land on planets or operate in an Earth-like atmosphere at all have been rare until recently. Dedicated starships are usually built in microgravity (or on small, airless bodies, including Luna) due to the challenges involved in surface-to-orbit transit. There been craft designed to operate both in atmosphere and in space since the early days of space travel, though mass costs on surface-to-orbit transit as well as the challenges of interior engineering limited their size and tonnage. The decidedly unaerodynamic design of most UNSC starship hulls, along with various space-exclusive subsystems such as fuel storage or engines also make in-atmosphere flight non-viable for most of them. Modern atmosphere-rated ships are equipped with a shaped aerodynamic energy shield or "virtual fairing" to reduce atmospheric drag and protect delicate external components from aerodynamic heating. This is necessary particularly during re-entry and the climb to orbit due to the high velocities involved. However, virtual fairings are effectively useless for protecting from weapons damage.

Atmospheric operation is largely limited to shuttles, dropships, aerospace fighters and landers, as engineering ships for both atmospheric and orbital flight places additional restrictions on performance. Some modern UNSC ships such as specialized frigates and transport ships are able to operate in an atmosphere horizontally, similar to an airplane, and even landing on a planet in such a way; this has not been possible until very recently with the development of reliable contragravity systems. Even so, such systems have their limits, and they are found exclusively on ships that benefit from atmospheric operation enough to justify the added cost and mass of the gravitic engines. This may change in the future, however, as the UNSC's understanding of Covenant repulsor engines improves.

In prior centuries, the larger starships that could land on planets largely did so vertically like a conventional rocket. Even today, UNSC ships on the smaller end of the scale such as frigates are able to perform emergency landings this way, balancing on the fusion flames of their main thrusters; naturally, landing in such a fashion is only viable in uninhabited areas. To return to orbit, ships usually require extra propellant tanks and booster packages. With ships larger than frigates, the cost and unwieldiness of such arrangements is enough to make such craft space-exclusive.

Power and propulsion
Nuclear fission remained the primary power source for human vessels until the revolutions in fusion power that characterized the 23rd and 24th centuries. By the 26th century, fusion is ubiquitous on UNSC warships, but still not universally adopted in the civilian world, especially out in the Outer Colonies and on smaller vessels, many of which are still powered by compact fission plants. As of the early 26th century, compact and affordable fusion reactors were only starting to enter the civilian market; by the second half of the century, advances in fusion reactors during the Human-Covenant War had made them far more commonplace. Most fusion-powered craft draw their power needs from the fusion drive itself, though they retain fission reactors (in some cases, secondary fusion plants) as backups. Modern warships with power-intensive systems such as MAC guns often employ a dedicated fusion plant to reliably power the weapons.

For essential electronics and life support, ships maintain both deployable solar arrays and, in the event sufficient starlight is unavailable, fuel cells and radioisotope thermoelectric generators which can sustain the crew for a limited amount of time even if the primary fusion and secondary fission plants fail. In such an event, crews are instructed to enter cryosleep (assuming cryo-facilities remain operational) to conserve resources. Modern cryotubes are likewise equipped with small radioisotope batteries capable of powering them independently for several years in the event the pods are ejected from the ship.

Both the power and propulsion systems were among the largest factors for the length of a given ship's service life. The cost and time of disassembling a ship simply to access them is prohibitive enough that powerplant overhauls are rarely done, with only the most important reserved this luxury. Over time, the effects of high temperatures, pressures, and magnetic interference will erode away at reactor insulation and integrity, requiring maximum power output to drop to maintain safety. The acuteness of the problem depends on how the ship is run - spacecraft that regularly run at close to maximum tolerance will wear out much quicker than those that remain within recommended levels.

Chemical rockets
A staple of spacecraft propulsion since the early days of space travel, chemical rocket engines were gradually supplanted as a primary form of deep-space propulsion by nuclear-powered designs beginning in the late 21st century. They continue to be used in reaction control thrusters and remain a common choice for staged surface-to-orbit transit.

Ion and plasma drives
Ion engines encompass a broad category of electric thrusters. Often considered an economy option with numerous applications in minor space technology, ion drives are versatile and simple, providing a much cheaper but far less powerful alternative to fusion engines or nuclear thermal rockets. Even by the 26th century, various types of ion drive remain in secondary uses, especially in the civilian world. Ion engines are usually powered by photovoltaic panels or small fission reactors. They are most common on automated satellites as stationkeeping engines, as well as various drones and mining craft that can afford to take their time to reach their destinations. Today, most ion-powered ships of any appreciable tonnage use plasma drives, or electrothermal thrusters descending from the classical VASIMR concept, powered by compact fission reactors.

Fission drives
Nuclear fission drives, nuclear thermal rockets (NTRs), or atomic drives in quaint parlance, are several categories of nuclear-powered spacecraft engines. Fission-powered drives had their heyday between the 21st and 23rd centuries as the standard drives on most combat vessels as well as various commercial ships.

Closed-cycle nuclear rockets use a nuclear fission reactor to heat a chemical propellant, usually hydrogen, methane or water, to provide thrust. Major subcategories include solid-core, vapor-core, and the obscure gas-core closed-cycle engines. Most warships used during the Interplanetary Wars had some form of closed-cycle NTRs as their primary mode of propulsion. 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 long been phased out of mainstream military use. However, as a mature and proven technology, they still have many civilian uses and are sometimes used as backup drives on otherwise fusion-powered vessels, including warships. Small vessels such as orbital fighter craft have conventionally used advanced lightweight NTRs up until compact fusion engines became affordable enough for mass deployment in the 26th century.

Open-cycle nuclear rockets, or atomic torches, are an archaic but powerful form of nuclear fission propulsion that never fully left the experimental stage. 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. Though a handful of experimental designs saw use during the Interplanetary Wars, many of these designs were notoriously unstable and accident-prone. Despite various experimental efforts attempting to revive the concept since, the technology was never perfected to the level of closed-cycle atomic rockets prior to the development of the fusion engine. A handful of insurgent groups have been documented as fielding homegrown open-chamber NTRs, but these have shared the volatile nature of their early predecessors.

Fusion drives
The dominant starship propulsion technology in the modern era, fusion drives generate thrust by expelling superheated fusion plasma combined with a chemical propellant through a magnetic nozzle. Modern fusion drives are in effect "torchdrives", combining high accelerations with high efficiency.

Attitude control
Human ships use a combination of thrust vectoring of the main engine units and attitude thrusters for maneuvering. Conventional thrust vectoring uses gimbaled engine nozzles to angle the drive flame, while modern fusion drives angle the shaped electromagnetic fields projected by their magnetic nozzles.

The reaction control systems for most modern warships use resistojet thrusters that draw on the same supply of hydrogen or water reaction mass as the main engines. In some applications, particularly in the civilian world, microsatellites, and thruster packs, monopropellant-based chemical rockets may be used instead. While largely replaced by non-toxic propellants in military and other high-end usage, hydrazine variants such as triamino hydrazine 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.

Surface-to-orbit transit
Advances in rocketry and power generation have made surface-to-orbit transit far more economical than it once was. However, escaping a terrestrial planet's gravity well and atmosphere is still far from a trivial endeavor. Large ships avoid entering atmosphere in the first place, with most spacecraft built in space for dedicated operations in a vacuum. Developed worlds still use launch assist systems such as mass drivers, skyhooks and space elevators wherever possible, as this makes orbital transit far more economical than the use of rockets. The use of such systems yields most benefits in civilian applications such as commerce.

Today, craft designed to travel to and from a planetary atmosphere such as shuttles, dropships, and certain aerospace fighters may be built to be capable of independent single-stage surface-to-orbit transit. Such craft have become increasingly common in the last century with advancements in compact fusion engines as well as the development of gravitic repulsor systems able to lighten the effects of gravity on a craft. Modern transatmospheric craft usually use hybrid engines which function as airbreathing jets in atmosphere and as fusion rockets in vacuum.

However, transatmospheric operation still requires compromises. Though generalist craft are useful and necessary for certain roles, craft specialized solely to atmospheric or space operation can be built to be far more efficient in their given domains. For one, hybrids of airbreathing jets and rocket engines are more mechanically complicated and rarely as powerful than either type on its own. A single-stage surface-to-orbit capability also requires a spacecraft to carry a considerable amount of fuel just to achieve escape velocity, which adds both mass and bulk. This is also why atmospheric fighters and orbital defense gunboats have historically been two separate lineages of craft, with spaceplane-type hybrids rarely excelling in either role. Orbital defense fighters (such as the Sabre) often lack the ability to reach escape velocity independently, relying on staged chemical rockets for STO transit when launched from planetside facilities.

Ongoing advances in gravitic technology are expected to revolutionize the surface-to-orbit transit paradigm in the near future. With the proliferation of gravitic mass-canceling engines in the commercial sphere, along with the ever-growing efficiency of such engines, craft capable of single-stage surface-to-orbit travel have become increasingly commonplace over the 26th century.

Sensors
Detecting, identifying and tracking enemy ships in space is a complex affair involving a suite of different passive and active sensors. Visible-spectrum sensors only useful at a short range, and in general, infrared-range telescopes are the primary means of detecting and tracking ships due to the heat emissions they emit, making them stand out against the near-absolute zero background of space. All sensors are limited by the "light barrier". As electromagnetic radiation travels at the speed of light, this already creates a lag on intrasystem distances where developments may occur minutes, even hours before the light from those events reaches a ship's sensors.

UNSC ships use a combination of shipboard and drone- or satellite-mounted sensors along with data from external local networks (military and civilian) where available, with dedicated AI algorithms used to process and sort the vast volumes of data being received. The use of AI allows predictive extrapolations of enemy ship movements even at long ranges, where light lag is a major factor. All warships carry an array of sensor drones and microsatellites for on-site deployment in order to ensure sufficient fidelity of this informational gestalt, as well as creating sufficient sensor redundancy in case the ship's onboard sensors are damaged or destroyed.

The rapid technological innovations brought about by the Human-Covenant War have seen many advances in UNSC sensor systems. In particular, the reverse-engineering of Covenant "hyperscanner" arrays in the second half of the war greatly improved the resolution and breadth of UNSC ships' sensor equipment. One of the most useful advances has been the use of faster-than-light information transfer technology to augment UNSC sensor platforms. Although the sensors themselves are still limited to the speed of light, sensor drones and microsatellites can now be equipped with superluminal wavespace communicators allowing them to communicate with the parent ship or fleet in virtual real-time across a stellar system. This greatly extends UNSC vessels' capability to respond to threats and other new developments in local space. FTl-capable sensor networks are useful even on relatively short ranges, where real-time data from can allow a ship to dodge incoming munitions more effectively than before.

Similarly, thanks to more precise slipspace drives, prowlers and subprowlers can now rapidly jump around a system to gather accurate real-time reconnaissance data. These developments, coupled with the Covenant's brute force superiority, have only increased the role of stealth and electronic warfare craft in the UNSC Navy. Advances in stealth have allowed the development of larger prowlers which are also able to carry additional sensor satellites.

Artificial gravity
Most human ships maintain some form of artificial or simulated gravity to stave off the effects of long-term microgravity exposure on the crew. The three main methods used to accomplish this are centrifugal spin gravity created in rotating "carousel" sections, thrust gravity provided by main drive acceleration, and most recently, paragravity achieved through the use of the exotic technology of gravity plating. Rotating sections and bursts of thrust gravity remained standard for much of human history until breakthroughs in paragravity in the 25th century. Before this, most deep-space vessels had one or more spinning sections in which the crew would spend most of their time on the ship. On some ships, these rotating sections could be folded in parallel with the hull during acceleration, as thrust would provide a semblance of gravity for the duration of the drive burn. This became increasingly common as drive technology improved, enabling longer sustained accelerations. By the 26th century, most human ships have partial or full paragravitic systems, though some older ships retain spinning carousel sections or thrust-perpendicular deck layouts.

Thermal control
One of the primary limitations placed on starship design and operation is heat. Reactors, engines, weaponry, defenses, myriad other subsystems as well as the crew generate heat, which must be properly managed and removed from the ship in some way. In the vacuum of space, heat transfer via conduction or convection do not work, leaving radiation as the only option.

Since the early days, ships have maintained two or more sets of thermal control arrays, one for managing the low-power systems such as the crew and life support, while another manages the heat-intensive ones such as the engines and power plant(s); many modern warships also maintain dedicated cooling system for the MAC gun, major coilguns and even more sets of subsystems, as the radiators must be maintained at the same heat range as the systems they are cooling. Hull radiator arrays are compartmentalized into a number of sections, allowing the system to divert coolant from hotter parts of the system (usually reactor and engines, momentarily also weapons) to other parts under less load. As the radiator panel needs to be maintained in the same temperature range as the system it is cooling in order to work, life support and small scale electronics (which tend to be the least heat-intensive parts of a ship) usually maintain their own closed loops. Most of this sectioning process is overseen by "dumb" automated subsystems.

Heat management is one of the key tactical considerations of space warfare, as combat actions in space are particularly heat-intensive. Covenant plasma weapons are especially devastating against human ships due to the incredible amounts of thermal energy contained therein; even a grazing hit by a plasma torpedo may mission-kill a UNSC vessel as the heat transferred to the ship strains its thermal control system to its limit. Ships at risk of overheating must retreat or risk their crews being cooked inside their own hulls. This has led to the concept of a ship's heat budget, which is one of the major factors determining viable engagement duration, or at least an individual ship's ability to stay in the fight. Although this is against regulations, daredevil captains have been known to continue engaging the enemy until crew members begin fainting from the heat. While ships' ability to manage heat buildup has steadily increased over the centuries, heat budgets continue to be a concern.

Radiator panels
Since the early days of space travel, the staple solution to heat management has been to circulate a coolant fluid throughout a thermal control system encompassing the ship, which is then pumped through pipes embedded into large, double-sided, highly emissive flat panels jutting out of the ship's exterior. These panels then shed the excess heat as radiation, typically on the infrared range, though when engaged in sustained engine burns or combat operations, they might glow bright red; the hotter the radiators' operating temperature, the more efficient the heat removal. As the superheated coolant circulates through the radiators, its temperature is gradually lowered before it is pumped back into the systems being cooled.

Various radiator designs have existed, ranging from sets of fins to the large sail-like panels dominating the designs of early long-range vessels. Others had a hinged, fold-up design consisting of multiple panels. The radiator's ability to remove heat increases with the surface area of its exposed face(s); this meant that double-sided panels were the norm for centuries, and continue to be favored aboard civilian vessels. Heat radiators were always considered one of the primary weak points on warships, due to their exposed and fragile nature combined with their importance to a ship's operation. On warships, radiators developed during and after the Interplanetary War usually had some form of armor to protect the cooling fluid pipes, though this came at the cost of effectiveness. Since the early days, many shipwrights have employed secondary and even tertiary sets of radiators which could be folded out in the event the main set was lost or damaged, though mass limitations restricted redundancy.

Since the mid-to-late 24th century, UNSC warships have been transitioning away from separate radiator vanes. Instead, most of the ship's exterior hull is covered with plating consisting of thin, dark gray carbide compound tiles interlaced with lattices of microscopic fluid pipes. Although these are less effective than dedicated radiator panels as they have only a single exposed face, they are far less fragile as they do not present as obvious of a target as classical radiators, instead covering much of the ship's hull. However, many civilian ships continue to use conventional double-sided panels as they are cheaper, more efficient, as well as easier to manufacture and maintain.

The shift to single-face radiators was partly made possible by advances in fusion engines; whereas most early fusion drive models were incredibly heat-intensive, military-grade drives developed since the 2380s have been increasingly efficient at transferring most of the heat of the reaction into the superheated exhaust rather than the ship. This, in turn, reduced the strain put on the thermal control system. Like conventional radiator panels, hull-based radiator surfaces will glow red when under considerable strain (often in battle), though the sheer efficiency of mid-26th century systems means that this is typically only the case when the crew is in imminent danger of being cooked; UNSC warships that have suffered hits or near-misses by plasma torpedoes will usually glow red or even orange as their hull-based radiators struggle to remove the excess heat. The coolant pipe arrays are designed with multiple redundancies with each radiator tile compartmentalized as a distinct unit, so the loss of one section of plating will not compromise the entire system. Small leaks are automatically sealed and entire sections of piping can be cut off from the overall circulation network.

Separated from the hull armor by a gap usually left open to vacuum, the radiator sheet also forms as the outer layer of a Whipple shield, designed as the first line of defense against incoming projectiles, debris and micrometeorites. Having the radiators in the outer layer of the ship's armor, which is in many ways sacrificial, is an obvious weakness. Still, shipwide radiator coverage with redundancies was ultimately prioritized over more effective, but distinct radiator panels that could be targeted independently. The development of superior refractory radiator materials allowing more efficient heat removal and the introduction of Muratovski effect heat sinks during and after the war has mitigated these issues somewhat. Most UNSC ships also retain external radiator panels that are normally folded into the hull but can be extended during combat, sustained drive burns, or considerable damage to the radiating surfaces of the hull.

Carbon-based outer hull armor materials developed in the late-war and post-war eras combine the heat removal properties of radiator tiles with armor designed to improve protection from energy weapons such as plasma torpedoes and lances. Due to the challenges involving thermal damage, post-war ships are designed with at least two redundant layers of radiator plate in most areas.

Liquid-droplet radiators
The other major heat-shedding option in use is the liquid-droplet or mist radiator, colloquially known as a sprinkler, which uses minuscule droplets of a specialized cooling liquid, conventionally a liquid metal such as tin, sprayed through the vacuum in lieu of a solid radiator panel. These droplets are released from a nozzle at the fore of the ship, and radiate out heat via their 360-degree surface area while they are exposed to the vacuum; they are then captured for recycling through a collector at the ship's aft. However, this is only viable when the ship is stationary or accelerating forward, as the cooling liquid is quickly lost into space during maneuvers. Advances in electromagnetic field control have enabled the use of EM-based funnels to capture the droplet streams from a wider area, increasing radiator effectiveness without requiring a large and cumbersome physical collector boom. Most UNSC capital ships maintain droplet-style radiators as a secondary option, or as the dedicated radiator array for their subsystems such as weapons. Droplet radiators have also long been common in civilian applications, particularly large interplanetary cargo ships, as they allow ships to shave off crucial kilograms of their total mass. Due to their design, their effectiveness increases with the ship's length. Advancements in energy field shaping have led to research into the possibility of developing far more efficient droplet radiators in which the coolant particles are cycled through shaped magnetic fields far outside the ship, then returned with minimal losses. One of the primary types of heat management system used by the Covenant actually functions via remarkably similar principles; such systems are usually found on unshielded (often civilian) vessels.

Wavespace heat sinks
Some ships, most notably prowlers, also employ heat sinks for storing excess heat before it must be released again, though these only work over sustained periods of time when no engines or weapons are engaged. Modern prowlers are even able to run their fusion plants and engines at low power settings by dumping the thermal emissions into wavespace via the Muratovski effect. As the technology has become more established over the 26th century, heat sinks utilizing the Muratovski effect have become increasingly common even on mainline warships. While they are not viable as a ship's sole heat-management mechanism, they still improve a ship's endurance and remove their reliance solely on conventional radiators.

Crew facilities and life support
The ratio of pressurized, habitable space to a ship's internal volume has been gradually increasing over the centuries with innovations in various fields. Early military spacecraft were incredibly cramped and have continued to be relatively so until recently to save space. On the earliest combat ships, the livable space was usually only a small and cramped section called a habitat or crew module. This would also be structured as a separate module or set of modules on some ships. Due to radiation, crew modules were placed as far away as possible from the nuclear reactor(s), which were usually placed at the back of the ship adjoining the engine. 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 kept exposed to the vacuum. 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. While the amount of pressurized space has grown, the basic principles governing ship design and mass distribution have changed little. Even to this day, warship interiors are heavily compartmentalized, with compartments separated by airtight blast doors. Nonessential compartments are depressurized during combat, which doubles as a counter-boarding measure.

On the earliest spacecraft, crew sizes were as minimal as they could be, and comparable to those on earlier submarines. Automation and remote-controlled drones were used as much as possible to replace human crew, especially for EVA work. Even as the UNSC's automation technology improved with time, human crews remained essential, not in the least because of various cautionary examples of large-scale automated defense systems failing or being subverted in the early days. As crews could not afford comforts like privacy, modesty, or separated bunking and shower facilities, naval and spacegoing organizations developed cultures that were both matter-of-fact about the human body and strongly egalitarian. While crew spaces would later grow, even on military vessels, the centuries-old customs stuck. While such liberal mindsets are common on Earth, Reach, spacer communities and many colonies, they are not universal, and have been known to present something of a culture shock as people from more traditionalist human communities enter UNSC service. Still, even these primordial vessels could not rely on the smallest crew possible, and often carried supplemental crew for certain critical positions that needed filling at all times, such as some specialized engineers and bridge officers. These "one-and-half" crew ships were popular in first and second-generation spacecraft of the pre-Interplanetary War period. These later proved to be vulnerable to being run down and losing their cohesion, which later informed the three- or four-crew shift structure later UNSC ships would come to use.

Human ships have used cryotubes since the Golden Age of Space Colonization 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. This technology emerged in response to the prohibitively slow space travel in the early days, with interplanetary journeys often lasting for months. Putting nonessential crew into cryosleep enabled the transportation of large numbers of people to remote planets, moons, and asteroids with minimal life support requirements. Since the advent of the modern fusion torch drive, cryosleep is no longer necessary on most interplanetary journeys, but remains standard for slipspace jumps.

Atmospheric containment
One of the technological advances brought on by the reverse-engineering of Covenant technology in the later years of the Human-Covenant War are atmospheric containment barriers, energy fields capable of holding in the pressurized atmosphere of a ship or station. These can be used in lieu of physical airlocks in vacuum-facing hangar bay doors, for example, though even by the post-war years they are largely treated as temporary airlocks or as an emergency measure in the event physical barriers fail.

Armor
Human warship armor is multi-layered, usually with multiple layers of plating sandwiched between the outer hull and inner bulkheads. At least one of these gaps is left open to vacuum, while other layers contain ballistic gel and self-sealing foam. The layered structure is designed to disperse the energy of incoming munitions, ideally breaking them up and spreading them over a larger area in the next layer, decreasing chances of penetration. Different areas have differing shield configurations based on priority and other factors. Much of the external hull is coated with tiles of radiator plate. The Covenant war brought many changes to the UNSC armor paradigm, shifting the focus of defense from ballistic impacts to directed-energy weapons. These presented additional challenges as they transfer prodigious amounts of heat to the hull. As external radiator tiles are boiled away by plasma impacts, removing that heat becomes increasingly difficult.

Shielding
Although the UNSC developed conformal, always-active energy shielding systems for various small-scale applications in the final years of the war, these were found to be less viable when scaled up to entire starships. The shield projector networks were not only technologically complicated and unreliable but also power-intensive; initial tests led to frequent power failures when the shields were operated concurrently with systems such as AI, MAC guns and coilgun networks. Covenant shields double as a highly efficient heat-dispersal mechanism, but integrating this functionality with UNSC thermal management systems was nigh-impossible, meaning that said systems would have to be rebuilt from the ground-up; this, in turn, would be impossible until human engineers learned to replicate the exotic materials and manufacturing methods used in Covenant power conduits, pushing their development forward by at least several decades. Worse, the early shields actually presented a heat-buildup problem, as the emitters would heat up over continued use, particularly when the shields were strained by impacts; this also led to some of the myriad mechanical failures with the emitters as their delicate electronics overheated.

Instead, the UNSC opted for a more active type of defense, consisting of layers of missile-borne plasma interception warheads, point-defense rounds enveloped in electromagnetic sheaths intended to disrupt incoming plasma bolts, chaff launchers, and finally, dispersal field arrays: a form of localized shielding designed to generate an intense electromagnetic gradient capable of disrupting and potentially dissolving incoming plasma rounds. As the field is generated both locally and for seconds at most, the power drain is intense but manageable with an attendant system of capacitors, compared to the much greater power usage of always-on shielding. Scaled up from earlier experiments with "proto-shields" or impellers during the second half of the war, dispersal field arrays became the UNSC's standard form of energy shielding since the war, though their obvious drawback is their lack of protection against physical projectiles. As they operate in timespans measured in fractions of a second, these systems virtually rely on AI and automation to function.

Over the post-war decades, dispersal field generators gradually increased in power, range, and reliability, eventually becoming a solid replacement for full shielding when used in conjunction with other active defenses and especially when their use is coordinated between multiple ships synchronized via local-scale superluminal comm networks, eliminating crucial microseconds of light lag formerly present in intership coordination. They can still be overwhelmed by multiple attacks at once, and some hostile post-Covenant factions eventually have come to favor a combination of projectile- and plasma-based attacks to reduce the effectiveness of dispersal arrays and anti-plasma point defense. Cyberwarfare attacks targeting the AI or the ship's computers and data pathways may also provide an effective defense, though even decades after the war, the UNSC's electronic warfare technology remains ahead of most known hostile Covenant factions.

Point defense
Point defense (PD) systems are a common staple on human starships, and are the only weapons legally permitted to be installed on civilian craft as well. Their purpose is more defensive, to defeat incoming missiles, rockets, and plasma torpedoes whose impact is certain. During the Interplanetary War, it was not uncommon to see ships using their PDS arrays as short-ranged secondary batteries, however the widespread adoption of armor by the 26th century has been a very effective deterrent against this practice. They are still used for bombarding ground targets such as tanks and infantry, however.

Most PD systems are made up exclusively of small-caliber, very fast-firing electromagnetic coilguns known as point defense guns, which are spaced around to maximize coverage. They have a caliber no larger than 150mm and a rate of fire that can exceed thousands of rounds per minute. Because of the high speed and erratic course of any threat that requires their use, they have to be autonomously operated, either from the turret or by an overarching fire-control network. Generally, both are used: the former provides redundancy while the latter best optimizes their fire. Their success is achieved not through accuracy, but through volume of fire, and a single turret may have four, six, or even eight barrels that can fire simultaneously, each of which may be able to elevate independently. To maximize the time they can sustain their barrages, point defense guns will have multiple autoloaders per turret, and complex cooling systems that take heat from the barrels directly to the radiators. Anti-munition missiles are an alternate system that can be used in conjunction with the guns. They are more surgical devices that intercept incoming weapons by detonating in their flight path, either destroying them outright or changing their course enough to avoid the ship.

Most point-defense ammunition prior to the Human-Covenant War relied on explosive fragmentation warheads to defeat missiles, with high velocity semi-guided slugs to destroy fighters. However, the threat of plasma and energy shields has seen a much greater variety being introduced, including magnetized, ionic, and even micro-missile shells.

Point defense systems are a common sight on both civilian craft and stations, as the threat posed by space phenomenon such as meteor showers and pirates means that they must be able to protect themselves if needed. However, UEG regulations require them to be owned only by licensed individuals, of which there are four distinct classes:
 * Class A: This permits the installation of light turrets up to a caliber of 25mm, with a reduced rate of fire. They can only be installed on autonomous turreted mounts: fixed mounts are strictly prohibited. This is the only license most civilians can apply for.
 * Class B: This grade allows for the use of medium guns with a caliber up to firerate-restricted 75mm, or unlimited for guns below 25mm. It can only be granted to commercial vessels exceeding 150,000 metric tons of mass.
 * Class C: Very heavily controlled, Class C vessels are allowed to use point defense guided missiles and rockets. This is generally given to mining ships.
 * Class D: Unrestricted license granted to ships either loaned to military logistics organizations, or for government-owned vessels. Guns up to 120mm (the legal limit for point defense guns) with unrestricted rates of fire, as well as unrestricted use in missiles, are permitted, which are removed when the ship returns to civilian duties.

Backing up these limits are a whole suite of other regulations. Not only are the grade of weapons permitted per license, but also the number of point defense systems they can have, which will vary according to the size and tonnage of the ship it applies to. They must be installed in autonomous turrets that have manual safeties and fire authentication, as the guns have been known to target high-speed personal transports as missiles. Their use is strictly regulated in specific areas such as close to inhabited space and interstellar jump points, and authorities can remotely engage their safeties if necessary, such as when on the final approach to spaceports.

Directed-energy weapons
Directed-energy weapons are rare on UNSC starships and are confined to niche applications. The obstacle to energy weapons becoming widespread is not so much that the UNSC cannot make them; lasers and microwave weapons have existed in niche applications for centuries. However, weapons-grade lasers are so heat-intensive (when used in a capacity that would actually be useful) that they will quickly overwhelm the thermal management system and cook the ship starting with its crew, making kinetic weapons simply more viable. Modern prowlers are equipped with a low-power pulse laser due to their low detectability, but these are highly specialized weapons mostly used for electronic warfare, such as eliminating the electronics on enemy vessels or spy satellites, and not viable as mainline defenses let alone offensive weapons. Since the war began, the UNSC has been experimenting with Covenant-style plasma and particle beam weapons, though little progress has been made on these tracks as the UNSC lacks many of the secondary technologies (starting with materials science and manufacturing methods) required to make the weapons work in concert with the rest of the ship's systems, along with the persistent heat problem.

Warships
Modern human combatant vessels are broadly divided into the following categories:


 * Cutter
 * Corvette
 * Prowler
 * Frigate
 * Destroyer
 * Cruiser
 * Battlecruiser
 * Battleship
 * Carrier
 * Orbital defense platform

These ship classifications have fallen out of use as naval doctrine evolved:
 * Archaic ship types


 * Orbitor
 * Base ship