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 hypersonic 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 modern fusion drives used by the UNSC are based upon deuterium-helium-3 fusion, which is the cleanest type of reaction achievable with 26th-century UNSC technology in terms of waste radiation. 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. 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.

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. The first generation of fusion drives, known as torchdrives, were based upon deuterium-tritium fusion. These early drives were massive, cumbersome and required an unwieldy system of radiation shielding coupled with massive radiators in order to get rid of the excess energy absorbed by those shields in the form of deadly neutron radiation. As a result, early torchships were highly conspicuous and distinct from conventional vessels due to their massive shielding and radiator structures. Because the neutron bombardment wore down the components fast, the drives were also maintenance-heavy. But their main appeal was that they enabled much faster travel than traditional drives, using a constant-acceleration branchistochrone trajectory for interplanetary journeys. This was a revolutionary development in the 23rd century, and made the far reaches of the Sol system much more reachable by considerably shortening interplanetary journeys.

The problems of these early drives prompted various manufacturers to look into other avenues in drive designs and fusion fuels, the most promising alternatives being deuterium-deuterium and deuterium-helium-3. Drives utilizing both started to become commercially viable several decades after the first torchdrives entered the market, though deuterium-helium-3 fusion ultimately became the most common on human ships for various reasons; most prominently, deuterium-deuterium drives are not as efficient and though cleaner than D-T drives, still produce an uncontrolled neutron flux. While largely superseded by deuterium-helium-3 drives in the military, D-D still have many applications in the civilian sector as well as colonies lacking reliable H3 extraction infrastructure. Some colonies still use homegrown deuterium-tritium drives based on antique designs for similar reasons.

Modern fusion drives are far more complex than their early forebears, having developed considerably in terms of miniaturization, fuel efficiency, 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, to an extent, even its direction.