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. In its basic form, a fusion drive comprises a 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. Most drive designs add separate water or hydrogen reaction mass to the plasma jet to further enhance the rate of thrust. 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. However, many UNSC warships use a separate reactor to power the various energy-intensive subsystems such as the MAC gun and other coilguns as well as gravitic arrays and later dispersal field generators.

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, due to the various technological challenges involved, the direct use of fusion reactor plasma to generate thrust did not become possible until the mid-23rd century. While some pioneering efforts experimented with other fusion fuels, the current D-H3 reaction, already standard in fusion power plants at the time, quickly became favored as the mainline fusion drive power source. 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.

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 nuclear rockets or ion drive designs in use at the time. The combination of efficiency and high thrust also meant that the formerly-stringent engineering restrictions on ships' mass could be loosened considerably. 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 first "fusion torches" introduced into service in response to the Inner Colony Wars, the third during the Insurrection, and the fourth in the later years of the Human-Covenant War. In addition, 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 main types had come to dominate the market. By later standards, these early drives were very bulky, very fuel-hungry and required large external cooling systems. The limits on their fuel usage and thrust meant that constant-burn travel was not yet viable for interplanetary distances.
 * Second generation (2370s): First open cycle/hybrid core designs. Cooling needs were reduced due to open-cycle cooling, resulting in external radiator panels gradually disappearing on human warships throughout the 25th century. Propellant considerations and lack of inertial compensation still limited effective thrust rate.
 * Third generation (2500s): First fully open core-type engines with improved field shaping and use of spin-polarized fuel. Short constant-burn interplanetary trips possible at 1g thrust. Designed to operate in conjunction with paragravitic inertial compensation.
 * Fourth generation (2540s): External or pitched core-type drives, making full use of advances in magnetic field control to further increase the thrust rate and specific impulse while reducing the cooling needs enormously. However, this 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
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. Even by the mid-26th century, fusion drives were not ubiquitous outside the UNSC, with many civilian ships relying on fission rockets or fusion-powered electric drives due to the expense of fusion engines.

Ships equipped with such drives (along with modern paragravity-based inertial compensation) are able to cross most star systems within days. With well-established refueling networks, such as those in Sol and Epsilon Eridani, modern fusion engines can apply a constant burn over the course of the trip at multiple Gs. Typically, 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". Some drive designs, particularly after the development of inertial compensation, are instead optimized for 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, as this is 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 all have ongoing gas-lifting operations, with fleets of purpose-built shuttles near-constantly dipping to and from the atmosphere to scoop for helium fuel. Saturn and Uranus are Sol's most popular fusion fuel sources. In orbit, the shuttles unload their cargo on enormous gas tankers or "gas cyclers" that ferry the fuel to the inner system, or on local orbital fuel depots. 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 many in some of the Outer Colonies, lack a constant presence 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.

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.

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.