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. Some 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.

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 traverse distances of multiple AUs within a matter of hours. 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 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. Deposits found elsewhere, such as the surface regolith of various lesser astronomical bodies (such as Luna), are generally regarded as too scarce to justify mining operations. Some manufacturers (particularly in the civilian sector) build their drives to utilize the less energetic deuterium-deuterium reactions for this reason, as deuterium is much more universally available.

History and development
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 also 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. With the abundant power supply of the fusion reaction, ships could often forgo the coasting phase entirely and simply apply a constant drive burn all the way to their destinations. This made the far reaches of the 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.