Fusion Propulsion
Nuclear fusion is the most popular way to power and propel any spacecraft in the Celestial Grove. For some species, they have been using it for thousands of years and thus the technology has been almost perfected in some places in the Grove. Over time, many fusion engines have been developed however only one gained supremacy. The fusion jet is the most popular. It is a linear compression engine that relies solely on nuclear fusion reactions for power instead of some other source such as an antimatter reaction. The engine itself forms a sort of middle ground between efficiency and thrust. All fusion engines have the benefit of being efficient compared to their chemical precursors. However, fusion propulsion is on the low end of thrust generation being only able to produce a comfortable amount of acceleration and nothing more. Only highly specialized spacecraft will get a massive kick out of a fusion engine.
Fusion engines are not unlike any other type of spacecraft engine. A fluid is propelled out of a nozzle at high speed which in turn acts back on the spacecraft to accelerate it. Fluids with higher mass provide more acceleration, but fluids of lower mass have much higher exhaust velocities which is related to high efficiency. As the propellants are burned or heated they increase in pressure which escapes through the rocket nozzle. Fusion engines work no differently.
Operation Principles
Fusion engines are not unlike any other type of spacecraft engine. A fluid is propelled out of a nozzle at high speed which in turn acts back on the spacecraft to accelerate it. Fluids with higher mass provide more acceleration, but fluids of lower mass have much higher exhaust velocities which is related to high efficiency. As the propellants are burned or heated they increase in pressure which escapes through the rocket nozzle. Fusion engines work no differently.
Propellants and Reactants
Fusion engine propellants are generally low mass particles traveling at a very high speed. Common exhaust products are the nuclei of hydrogen and helium atoms, nuetrons, and high energy photons. Neutrons are distasteful because they cannot be guided by magnetic fields and thus only waste energy. The most commonly used fuels are a mix of slush deuterium and frozen helium-3, also known as helion. A reaction between the two produces no harmful and wasteful neutrons. A cheaper option would be deuterium and tritium, isotopes of hydrogen, but that reaction produces a vast amount of incredibly energetic neutrons which can easily damage electronics and produce a significant radiation risk. Many engines also include injectors of some sort which either use fuel bypass or some other fluid that is injected into the exhaust stream to produce extra thrust. Injected fuel bypass doesn't undergo fusion in the plume but instead cools the exhaust significantly. This, ironically, makes it appear brighter because the exhaust would otherwise emit energy in the ultra-violet spectrum at the temperature it normally is at.Design principles
Reactor
The reactor of any fusion engine is its heart. It is the place where reactions occur. Reactor design can differ wildly, however, between engine types or classes. In a nutshell, all reactors are confined places where the fuel reaches its thermal apex and ignites to produce even more heat and energy. Confinement in this region is incredibly strong because not only does the fuel have to ignite but the force of a miniature nuclear explosion needs to be held at bay to protect the engine and spacecraft.Thermal Control
One of the biggest headaches surrounding nuclear fusion rocketry is the sheer amount of heat produced by the reactions. Magnetic confinement protects most of the reactor from destructive plasma, but the magnetic pressure exerted by reactions produces a lot of heat itself. This is problematic because superconductivity only happens in a small temperature margin. Exit the margin and the entire system catastrophically fails. Engines are designed with very complex pressurized molten salt cooling loops with 3D printed channels to maximize coverage. This system evenly removes heat from engine blocks. Heat from the engine loops may either directly connect to radiators or is exchanged with a larger cooling loop. Exhaust injectants help to cool the engines massively, but they won't carry the system.Exhaust Plume Injectants
High efficiency and low thrust are intrinsic of all fusion engines. Fusion jets, at the very least, produce enough acceleration on a ship to emulate planetary gravity. However, unbolstered thrust from common fusion engines would be hard pressed to exceed even 2 m/s of acceleration. In order to exceed that value, thrust needs to be increased. Increasing thrust requires increasing the mass of the exhaust which is partly fixed on reactor limitations. In order to increase thrust then requires injecting something directly into the exhaust plume.Use in Atmosphere
Fusion engines were designed for use in space and so using them in atmosphere may be incredibly risky if not impossible. In some very extreme cases, however, it has been done. Fusion jet equiped spacecraft are surprisingly the best (albeit still terrible) at atmospheric flight and hover. The plume itself can be used as a heat shield during entry and exhaust pressures are high enough to beat back any atmosphere. However, heat from the engines interferes with radiator systems and causes air to flow wildly which can overwhelm reaction control.
Plume Appearance
The exhaust plumes of fusion engines are generally similar in appearance. Exhaust plasma at full operating temperature is partially invisible to eye, appearing as a faint purple ghost to what species can see UVA and high frequency visible light. Plumes with injectants appear brighter as the cooler plasma emits vastly more amounts of visible light. Typically they appear white but have color around the edges where the plasma is not as dense. Colors such as purple and blue are common, but green, yellow, and red tones can be present depending on what sort of engine injectants are used. The shape and size of the plume differ a bit between engines. While one type of engine is exclusively referred to as a "jet", in reality all fusion engines have the jet nozzle that produces the same general plume shape. However, high exhaust velocities produce longer and sharper exhaust. DT fusion jets have the lowest exhaust velocity and thus a shorter, wider plume. Hybrid antimatter-fusion engines have incredibly long and sharp plumes. Each design produces a mushroom like effect regardless. The plasma rapidly cools and disperses into space which produces a sudden "mushroom head" behind the ship with low velocity engines, but at higher velocities the mushroom head is not as wide but is dispersed farther down the length of the exhaust plume.
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