Fusion drive

A fusion drive, also known as a fusion engine or fusion thruster and formally categorized by the UNSC as a direct fusion rocket (DFR) 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. The fusion drive generally 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.

History and development
Early Sangheili and Covenant ships used fusion-based thrusters before the discovery of repulsor engines, impulse drives and other exotic drive systems 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 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.

While not as powerful as those used today, these early fusion engines (then commonly known as torchdrives) 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. While full constant-burn travel at high accelerations was not yet economical in the early years, drive designs improved over the centuries with various breakthroughs. This made the outer Sol system much more reachable than they had ever been, and enabled missions all the way to the Kuiper Belt and back in reasonable timeframes.

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. 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.

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. Ships equipped with such drives (along with modern paragravity-based inertial compensation) are able to cross most star systems within days, with interplanetary trips taking mere hours in ideal conditions. 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; the ship flips around in the middle of the journey in what is known as "turnover" or "retrograding". In military contexts, ships sometimes accelerate much longer to maximize the effects of their relative velocity when unleashing salvos from MACs and other weapon systems, only applying extreme retrograde thrust at the very end of the trip.

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 also 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. In orbit, the shuttles unload their cargo on enormous gas tankers 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 out 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 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.

Most UNSC capital 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.

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.