Fusion drive

A fusion drive, also known as a fusion engine, fusion thruster, as well as a fusion torch or torchdrive in its more developed incarnations, and formally categorized by the UNSC as a direct fusion engine (DFE) is a type of spacecraft propulsion system widely used by humanity.

Design and function
In its basic form, a fusion drive comprises a nuclear fusion reaction chamber paired with a magnetic thruster nozzle. The constant fusion reaction generates superheated plasma, which is channeled into the magnetic nozzle and expelled out at relativistic velocities, providing thrust. Some drive designs, most prominently military ones, are capable of enhancing their rate of thrust for combat purposes by adding separate water or hydrogen reaction mass to the plasma jet. Modern drives use a complex array of adjustable magnetic thrust-vectoring plates arranged in a circle around the nozzle, which enable a high degree of control over the thrust rate and as well as its direction. The drive's main thrust can be angled to produce torque, allowing the ship to be turned independent of its reaction control thrusters.

The fusion drive often also serves as the ship's main power plant, generating both fusion products for direct thrust and electricity to power the ship's systems, with these representing a fraction of the power required to move the ship. Ships equipped with recent drive models, which move the reaction outside the vessel, may be forced to rely on a separate reactor to have a reliable power supply for the various energy-intensive subsystems such as the MAC gun and other coilguns as well as gravitic arrays and later dispersal field generators.

The exact configuration of fusion drives varies by design and manufacturer. Some drives are designed as self-contained units, including both the fusion reactor and a single nozzle. Others use a single reactor core (or "vent core") to maintain the fusion reaction, with the resulting plasma being channeled into a series of exhaust nozzles. Due to the immense temperatures involved, the plasma is entirely contained within magnetic fields and never comes into direct contact with physical material. Heating is still a concern, and without elaborate cooling systems, a fusion drive would turn the ship it is mounted on into slag in a matter of seconds.

Modern fusion drive cores are relatively safe, and virtually cannot cause an explosion under standard circumstances. Upon total failure, the reaction is designed to instantly shut down, and it takes deliberate action to induce a chain reaction known as a "wildcat destabilization", which causes the fusion core to go supercritical and violently explode. However, it is still possible for a malfunctioning or damaged drive core to endanger the ship. In the event of critical thermal overload or failure in the magnetic containment fields, the fusion reaction may have to be forcibly terminated. In classical shipboard slang, this is known as "frogblasting the vent core" and involves the rapid expulsion of all drive plasma from the ship. The origin of the phrase is unclear, with the "frog" part being particularly nebulous. Some speculate that it has to do with the sounds produced by a drive core being flushed in early vessels, while another theory holds that the colloquialism originated as a simple nonsense phrase used by sailors to confound fresh crewmen and was only given its current meaning retroactively.

History and development
Early Sangheili and Covenant ships used fusion-based thrusters before the discovery of the exotic repulsor engines the Covenant would later be known for. Native Kig-Yar vessels also utilize fusion-based drive designs along with nuclear fission and miscellaneous electric drives.

Though humanity had functioning fusion-based power plants by the late 2100s, various technological challenges involved meant that the use of direct fusion propulsion on a spacecraft only became viable in the 23rd century. The advent of the fusion engine had a massive impact on the dynamics of interplanetary travel, enabling ships to reach their destinations much faster than the fission-based rockets or ion drive designs in use at the time. The combination of efficiency and high thrust also meant that ships' mass budgets could be considerably loosened. The fusion drive made the outer worlds of the Sol system much more reachable than they had ever been: by the second half of the 23rd century, missions all the way to the Kuiper Belt and back were possible in a matter of weeks, a significant improvement over the years-long journeys they had previously been. This era also saw the beginnings of the exploitation of the outer ice giants for hydrogen and helium fuels.

The developmental history of fusion drives is often generalized as a series of generations; the first being the initial fusion engines used since the mid-24th century, the second being the more efficient and varied types into service in response to the Inner Colony Wars and the following century, the third during the Insurrection, and the fourth in the later years of the Human-Covenant War. The generations are rough categories and by no means represent singular models of drive; each generation was punctuated by numerous incremental improvements and divergent technological lines between different manufacturers.


 * First generation (2230s): Multiple wildly divergent fusion drive designs were piloted around this time, though by the mid-24th century, two or three major design schemes had come to dominate the market. By later standards, these early drives were very bulky, very fuel-hungry and required large external cooling systems, limiting their use to large ships. The limits on their fuel usage meant that the use of a constant-burn trajectory was not yet viable for long-distance (i.e. cross-system) travel.
 * Second generation (2380s): First open cycle/hybrid core designs. Cooling needs were reduced due to open-cycle cooling; this would later allow large and vulnerable radiator vanes to be shifted out of use on warships. The second generation saw increased diversity in both civilian and military drives, and the rise of the first drives small enough to fit on fighter craft and even missiles by the mid-to-late 25th century. This generation also marked the "STO breakeven" wherein fusion drives could be made light, compact and high-thrust enough to allow economical surface-to-orbit transit.
 * Third generation (2500s): The third generation saw many advances in drive diversification and scalability, along with improved field shaping, thrust vectoring and the use of spin-polarized fuel as standard, which lowers ignition temperatures and mitigates neutron flux, reducing maintenance requirements. Most drives used in the 26th century are based on third-generation technology.
 * Fourth generation (2540s): Entering service during and after the Human-Covenant War, fourth-generation drives make full use of advances in magnetic field control to further increase the thrust rate and specific impulse, while the use of an external reaction containment system reduces the cooling needs enormously. However, the mode by which they operate also reduces the power available for the ship's subsystems, often necessitating the use of a separate reactor for weapons and other systems.

Modern fusion drives are far more complex than their early forebears, having developed considerably in terms of miniaturization, fuel efficiency, exhaust acceleration, open-cycle cooling, and magnetic confinement methods.

Usage and capabilities
Due to their high specific impulse and fuel efficiency, which enables sustained high accelerations, fusion drives are the most effective subluminal drive systems used by the UNSC, and standard on most warships. Commercial fusion torches are highly efficient, but their thrust rate is mostly limited to accelerations far lower than that of military drives. Still, with sustained burns, they outperform any other drive system. Even by the 26th century, fusion drives were not ubiquitous outside the UNSC, with many cheaper civilian ships relying on fission rockets or electric drives due to the expense of fusion engines.

With well-established refueling networks, such as those in Sol and Epsilon Eridani, modern fusion engines are able to cross short interplanetary distances in a matter of days. Though the length and intensity of the drive burns over the course of a trip varies greatly, a common means of fast long-distance travel is to use a constant-burn brachistochrone trajectory. The first half of the trip is used to accelerate toward the destination while the second is used to slow down, or match velocity with the destination; the ship flips around in the middle of the journey in what is known as "turnover" or "retrograding". On ships lacking paragravity plates, such a trajectory has the added benefit of providing a semblance of artificial gravity throughout the trip, provided a stacked-deck layout is used. Though many UNSC ships have transitioned to a thrust-parallel deck layout for various reasons, some continue to employ both thrust gravity and paragravity in tandem, complementing the former with the latter when needed.

Modern military fusion drives are capable of maintaining a constant burn at significant fractions of 1g for interplanetary distances; within the Sol system, for example, the journey from Earth to Mars can be covered in just over four days, and Earth to Jupiter in a week. In practice, however, the fastest option is only used when necessary, and most ships travel at cruise thrust of less than half g. Modern ships are also capable of shortcuts using in-system slipspace jumps, for example jumping from one of the Sol-Earth Lagrange points to a Sol-Saturn Lagrange point to reach the outer system much faster.

In some instances, high-thrust fusion drives may be used to perform shorter multi-g burns at the beginning and end of the journey, with a long coasting phase midway through. In combat situations, ships sometimes accelerate and maintain a high relative velocity as long as they can as they meet the enemy to maximize the effects of their speed when unleashing salvos from MACs and other weapon systems, only applying extreme retrograde thrust at the very end of the trip; such maneuvers are known as "lancer" tactics.

In the wrong hands, each fusion drive is a potential weapon of mass destruction, capable of turning ships into impact weapons, scorching entire cities or space stations with their superheated exhaust, or being converted into improvised bombs. As such, fusion drives are categorized by the UEG as Destructive Propulsion Devices, and are heavily regulated. The use of fusion drives is strictly controlled near spaceports and other space-borne infrastructure, as well as in atmosphere. Most spaceports on developed colonies require incoming or outgoing ships to relinquish control to human or AI pilots while in the local vicinity to ensure security.

Fuels
Most human fusion drives are powered by fusing deuterium and helium-3, the cleanest and most efficient type of fusion reaction achievable with 26th-century UNSC technology. The only limitation on the use of such drives is the relative scarcity of helium-3, which is mostly found in abundance within gas giants and requires fairly sizable siphoning infrastructure to harvest en masse. Abundant helium-3 deposits do occur outside gas giants, though they are rare. It was once thought that Luna's surface regolith may become a lucrative source of fusion fuel, but after tentative efforts in the 21st century this was found to be uneconomical due to the scarcity of helium-3 there.

Well-developed gas giant systems such as those in Sol and Epsilon Eridani have established gas-lifting infrastructure, with fleets of purpose-built shuttles near-constantly dipping to and from the atmosphere to scoop for helium fuel, which is stored and refined in orbital facilities often based on moons. These facilities supply passing ships as well as the system's colony worlds via enormous gas tankers or "gas cyclers" that ferry the fuel to the worlds of the inner system as well as fuel depots and ports near interstellar jump points and other nodes of travel. By the 26th century, most of these steps were automated along with the ships themselves, often overseen by "dumb" AI, though a handful of high-end operations were run by dedicated commercial "smart" AIs along with regional STC. Less established systems, such as some of the Outer Colonies, lack fixed fuel mining and refinery infrastructure at the gas giants, with fuel harvesting instead being carried out by individual gas tankers carrying their own dedicated scoop-craft. Sol's first fusion fuel-lifters in the 23rd century also operated in this fashion, before the outer giant planets had permanent harvesting and refinery facilities established. Today, Saturn and Uranus are Sol's most popular fusion fuel sources.

Some large UNSC vessels maintain their own scoop-craft as contingencies, though these are inefficient at resupplying the ship's fuel needs in the long term, and larger fleets and battlegroups opt for dedicated refinery-ships for extended campaigns to extract both fusion fuels and reaction mass. Typically, a ship's reaction mass tanks have to be replenished more often than the fusion fuel itself. Most ships rely on liquid hydrogen as their standard propellant, though they can use water as well, with many ships even capable of sourcing it in situ from cometary ice if need be. This also makes up the majority of a ship's total mass. For utilitarian reasons, most modern ships draw on the same supply of propellant for their resistojet reaction control thrusters, though some ships opt for monopropellant rockets instead. The 26th century has seen efforts to use intense gravitic fields to manufacture and even store metastable metallic hydrogen, which would allow far higher propellant densities.

Most fusion drives have used the current deuterium-helium 3 fuels since the advent of the technology. Other contenders included the easier deuterium-tritium and deuterium-deuterium reactions, whose component fuels are also easier to come by; however, both have major drawbacks as they release much of their energy in the form of useless neutron radiation, which rapidly wears down engine parts and requires heavy radiation shielding. Some of this can be mitigated via spin-polarization, however. Because of the relative scarcity of helium-3 and the infrastructure required to harvest it, many fusion drives built for the civilian market in particular utilize the less energetic deuterium-deuterium reaction, as deuterium is much more universally available. However, this comes at the cost of performance, along with additional issues with radiation. Still, D-D drives (along with the even more archaic deuterium-tritium-based designs) are popular in the Outer Colonies in particular. Recent developments in spin-polarized fusion in the 26th century have also increased the viability of the deuterium-tritium reaction, which had mostly been relegated to fringe uses due to its high radiation output.

There is currently no serious challenge to the prevailing hydrogen-helium fusion economy. However, experiments with fully aneutronic proton-boron fusion have shown promise, and may eventually provide a more accessible alternative. Due to the established drive architectures and the amount of fixed infrastructure for the current fuel sources, however, D-H3 fusion is unlikely to become obsolete any time soon.