Fusion Jet

The fusion jet, more specifically known as the Pure Fusion Engine, is a breed of engine that has been developed independently by many different species and nation across the Celestial Grove. Fusion jets are medium in size but considered the most economical for interplanetary journeys. They have the lowest specific impulse of any other type of fusion engine still in use which means they are the least efficient. However, they produce comfortable amounts of thrust. They are ideal for thrust gravity. Due to these factors, they are the most numorous spacecraft engine in use.

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Design

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Fusion jets are pure fusion linear magnetic devices. To break that down, pure fusion means that all power is derived from nuclear fusion. Fuel and plasma within the engine are magnetically confined in a straight line. This type of engine is relatively simple, but manufacturing is very meticoulesly controlled to prevent harmful defects.
There are three stages in a pure fusion engine: compression, reaction, and exhaust. Each stage has a ring of superconducting electromagnets that guide plasma through a unit and out of the exhaust without any fuel coming into contact with the engine. The compressor section turns a slush fuel mixture into plasma and superheats it via magnetic compression. The reactor is a very small portion of the engine that heats the fuel to supercritical temperatures via RF ionic heating. The heaters are comparatively massive compared to the portion of fuel they are heating but this is because of the fact that fuel is moving fast and through the reactor once. The exhaust section is also magnetically contained. This allows for electrical power to be produced plus the injection of thrust boosters.

Size Limitations

Fusion jets are installed on ships of all sizes, but fusion jets themselves do not come in all sizes. There are practical limits imposed on their scale. Fusion jets are a smaller breed of fusion engine, but their magnets and ionic heaters can only be so small before they are unable to maintain nuclear fusion. However, if they are too big then incomplete fusion will occur which has drastic effects on efficiency. Larger engines need to be antimatter catalyzed or are hybrids of pure fusion and pure antimatter engines in order to conduct thorough fusion. Such versions of fusion rockets are expensive. Instead of using fewer large engines, pure fusion engines can be fitted together in pods for large ships.

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Fuel

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Modern pure fusion engines are deuterium (D) and helion (³H) engines. There are many pros and cons to this fuel mixture. For one, Helium-3 reactions occur at much higher temperatures which results in a substantially increased electrical load for fuel heating and containment. On the other hand, helion engines produce no radiation via unstable fuel and practically no stray neutrons which not only makes helion engines more efficient, but also saves tremendous amounts of weight as there are no heavy shielding requirements. Deuterium-tritium engines operate at a much lower temperature, but produce substantial amounts of neutrons. The neutrons are electrically neutral, so they can't be guided by the engine's magnetic fields and most of the reaction energy is locked in these particles. This means most energy is lost in all directions. To make matters worse, the particles are so energetic that they can penetrate through several feet of solid radiation shielding while "activating" isotopes of other atoms which can make them unstable. This means there will be additional radiation coming from internal systems on a ship.

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Thermal Control

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Energy is money in space. Energy wasted is thus money burned. D³H engines, the least wasteful fusion jet, have the benefit of all reaction products being electrically charged. This means that most energy is converted to thrust. But some is lost into the containment magnets. Instabilities in plasma flow can push on the superconductors which acts sort of like friction generating heat. One engine alone doesn't produce a large amount of heat, although multiple engines compound. Fuel bypass helps to cool the units significantly and removes it via exhaust injectants. However, liquid cooling loops are used to remove excess heat.

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Exhaust Characteristics

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Fusion jet exhaust is shorter, broader, cooler, but also brighter than fusion based antimatter drives. Due to a relatively low exhaust velocity, plasma disperses over a shorter distances while the lower temperature of the exhaust plume prompts a greater volume of plasma to emit visible light radiation. Without injectants, the plume appears like a ghastly purple flame but is clearly visible. Injectants significantly cool the exhaust so that most particles emit visible light thus making the plume appear as a very bright white with colored highlights in cooler regions. D³H also have a more obvious electron kick. Electron kick is a soft, conical portion of the exhaust originating from the very end of the engine. It is formed when electrons and photons, traveling at higher speeds, eject small amounts of matter from the plume. This doesn't degrade thrust, but can damage spacecraft if not contained. A physical or magnetic duct may be all that is needed to divert electron kick back into the plume if necessary.

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Injectants

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Pure fusion engines may be known for their high thrust, but high thrust isn't the default of these engines. Fusion jets naturally would struggle to exceed 2 m/s of acceleration. So many units have injectors built in. Injectors are small units installed in engine exhaust systems that pump fluids or powders into the exhaust stream. This increases exhaust mass. Higher mass equates to higher thrust and thus more acceleration. With this, engines can achieve close to 20 m/s of accel. under ideal conditions. However, most are optimized for around 8 m/s of accel. Typical injectants may be deuterium or helium bypass. However, other options such as liquefied heavy gas (oxygen, nitrogen, argon, etc.) or water may be used for higher than average thrust while heavy molten salts and oxidized metal powders are options for the highest performing engines.

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Unique Applications

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Trans-atmospheric Vehicles

Fusion engines work best in a vacuum, but they maintain some functionality in atmosphere. Shuttles intrinsically are equipped with pure fusion engines for spaceflight but they can sometimes use those engines in thin atmospheres to save weight by reducing heavy chemical fuels require for turbines. Fusion jets do not work well in dense atmospheres unless the vessel that equips the engine is designed for that environment. There are well known (and illegal) competitions involving flying specialized pure fusion engine ships through atmosphere. They include aircraft-like cooling systems and intakes that use free, heavy air as an injectant.

Missile propulsion

Fusion jets have some applications on missiles. Such an application is not common because such a frame has to be somewhat large. Fusion jets may be small fusion engines but they do have some size. Missiles that use fusion engines are more likely to be used in asteroid redirects instead of in combat like people would expect. In some cases though, pure fusion engines are used on anti-ship missiles. They have the advantage of being able to fly faster (making interception harder) or making fragmentation from a destroyed missile more dangerous.

Torching

Torching is a highly unusual but remarkable application of any spacecraft engine. It is basically the practice of using the exhaust plume of an engine to damage or destroy a target. Because the plumes are electrically charged and have temperatures in excess of 100,000,000 degrees, anything to come into contact with the plume is certain to take some damage. It is most notably deployed by militaries or pirates as a way to counter missiles but in rare instances it has been used to cripple spacecraft. Torching can also be a quick way to clear a large surface of anything for whatever reason.