Fusion drive

A fusion drive, also known as a fusion engine, torchdrive or fusion thruster is a type of spacecraft propulsion system widely utilized by humanity. In its basic form, a fusion drive comprises a fusion reaction chamber paired with a magnetic nozzle. The constant fusion reaction generates superheated plasma, which is channeled into the magnetic nozzle, accelerated to hypersonic velocities and expelled out of the nozzle, 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. Coupled with modern paragravity-based inertial compensation, such drives enable ships to traverse distances of multiple AUs within a matter of hours. Fusion drives are regulated due to their strategic implications; not only because of the high accelerations they are capable of, but the fact they could effectively turn any ship into a weapon of mass destruction. Due to their destructive potential, 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 reactions, 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 torches 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.

Fusion drives based solely on deuterium-deuterium reactions were developed a few decades later, at the beginning of the era of extrasolar colonization. These have a lower thrust but produce fewer waste neutrons. While now largely superseded by modern deuterium-helium-3 drives, they 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. The fusion drives of high-tonnage ships are often equipped with a ring of secondary nozzles to control the thrust rate.