Quick Explanation of Processes and Terms for Ad Astra/Hard Near Future Scifi
Terminology:
MHD: A magnetohydrodynamic generator that produces energy through the utilization of the Hall effect, the generation of an electric current through a magnetic field. This can be used to process fossil fuels as an additional harvester by placing them next to the primary turbine. Still, things that produce masses of charged particles such as fission or fusion can reach a theoretical efficiency of nearly 90% by utilizing the highly charged plasma as it is being ejected.
Brayton Engine/Turbine: Effectively a normal turbine that uses a gas as the working fluid instead of water. The cycle is used for space applications due to higher operating temperatures, especially as radiator emissivity varies on temperature to the fourth power, making a 1.5x temperature increase lead to thermal energy being radiated away around the visible spectrum at x5 the rate, allowing for radiators to be shrunk - saving weight and reducing the primary target area of vessels in space combat.
Specific Impulse: For rockets and any time in the quest I use the measure, only the basic measure of impulse will be used. This is effectively the exhaust velocity of an engine divided by a perfectly spherical 10m/s when rounded for the sake of my sanity and giving clean numbers but is measured in seconds due to the roots of the term.
MPD/MPDT: A magnetoplasmadynamic thruster and effectively the most viable propulsion technology currently available for long-duration burns and interplanetary journeys. Efficiencies of up to eighty percent are possible with a varying degree of specific impulse allowing a variety of missions to be flown. This is a drive that can go up to 350km/s of exhaust velocity at significant power, leaving only the problem of thermal efficiency.
Pulsed Plasmoid: Less efficient than the other electrical thrusters but with a notable advantage of practically infinite scaling across the nozzle. It uses the production of ball lightning across the conducting nozzle in rapid pulses to create a high-velocity stream of propellant. Efficiency is worse compared to the MPD(72-75%) but the thruster can manage exhaust velocities up to 500km/s with an expanded degree of scaling.
Ion/VHI: Ion engines and Very High Energy Ion engines are theoretically the most efficient possible electrical thrusters but with the most intensive engineering constraints involved in them. High efficiency of conversion is paired with a comparatively dense propellant and exhaust velocities that can reach comfortably into the relativistic. Exhaust velocities of up to 600km/s can be reached along with efficiencies of nearly 95% if the engine itself can resist the massive currents placed upon it.
Remass: The shortened term for the reaction mass of a rocket, most of the time this is hydrogen but a few engines such as ion engines have different propellant requirements. This is what goes out the engine at insane velocities to push a rocket places.
Fissiles: The broadest term for fission rocket fuel, generally in the form of Uranium-233, Uranium-235, and Plutonium-241. All of the fuels listed are fissile, breaking each other apart and releasing a massive amount of energy in the process, powering either reactors or rocket engines.
SCNTR: A solid-core nuclear rocket operates on a similar principle to a high-temperature gas core reactor with a solid fissile fuel element that is brought to the point of criticality(self-sustained reaction). This produces heat of up to 3600K (approximately double the efficiency of a chemical rocket), heating hydrogen or methane propellant to then eject out of a nozzle. The big limitation with solid core designs is inherently the melting point of the entire system as there is only so hot that it can go without the engine melting itself.
LCNTR: A liquid core nuclear rocket gets around the inherent thermal limitations of an SCNTR by keeping the fissiles and moderator mass in a liquid vortex all while cooling the walls. This effectively allows for temperatures to reach 6000K (approximately three to four times the efficiency of a chemical rocket), greatly increasing efficiency in a purely thermal cycle. From this the super-heated hydrogen or methane is sent back out the nozzle, producing thrust.
GCNTR: A gas core nuclear thermal rocket takes vortex methods of heat management a step further. By moving to a gaseous core temperatures can be further increased greatly increasing the expansion of the propellant. This produces temperatures exceeding 10000K (Five to Twelve times the efficiency of a chemical rocket), but it does come with a major downside especially as temperatures continue to increase, the vapor pressure of the gaseous fissiles also increases, leading to leakage of fissile material into the remass. Meaning that the more efficient the engine operates the greater the fraction of hot fissiles released out the back of the nozzle.
Conventional Radiator: A series of panels of thin materials made to radiate out heat from internal spacecraft systems. These effectively take hot coolant and then chill it to a more reasonable temperature through radiation of infrared radiation, allowing it to be returned and cooled. The issues with such a setup are that there is no convection or conduction in space, requiring high temperatures to attain reasonable efficiencies.
Droplet Radiator: As long as a craft is accelerating on a single axis there is no real reason that the radiator surface needs to be a single coherent panel. By ejecting droplets of molten metal or salt and using them to radiate off the heat before capturing them again, massive weight savings can be accomplished by avoiding radiator pipes. The biggest limitation with droplet radiators is that the radiative fluid has a vapor pressure, limiting the use of liquid lithium to 1600K, Copper to 2000K (2.5x more efficient per area than lithium), and Nickel to 2600K (7x more per area then lithium).
Fission Reactors Explained: The basic explanation of a fusion reactor rests in the separation of a hot section with a power-generating section. For the sake of grossly simplifying the design viewing it as a set of two sections allows a lot of the concepts to be looked at in isolation. The hot section of any conventional reactor is where the fission takes place producing thermal energy in most reactors that is then transferred to other applications. The most important factor for this section is the general neutron efficiency, for the sake of simplifying things, it's a combined measure of the degree of fissile atoms that get hit by a neutron to undergo fission. This has a side case in terms of civilian and breeder reactors where it applies towards fertile atoms that can be converted to fissile ones (A Thorium-232 atom gets hit with a neutron converting it to Protractinium-233 which then rapidly decays Uranium 233). All of these reactions produce both heat and neutrons, with the former conventionally being used for power production while the latter represents a massive amount of radiation and is used to continue lighting off fission reactions at a sustainable rate.
This sustainable rate of reaction is sometimes moderated by a moderator. The actual moderator acts towards changing the energy levels of the neutrons flying around as different atoms have different cross sections at varying energy levels. Thorium for example has a large cross section (area of being able to be hit) by a thermal neutron(lower energy/slower) but a very poor one for fast neutrons. Thus, materials that slow down neutrons such as graphite or water are used to improve neutron economy by increasing the rates of impact on fissile materials. This as an aside also produces heat that can be harvested by the reactor coolant. This coolant is then pumped through the reactor, taking away heat from the core and effectively moving the energy towards some apparatus that can convert thermal energy to electrical energy. The methods for that are broad and varied. There is a small difference for some reactor designs in that charged particles ejected from the core themselves have a significant amount of energy, allowing harvesting outside basic thermodynamics (effectively that a device can only be as efficient as the thermal gradient, capping out at 45% for gases and less for water).
PWR/BWR: The conventional and old fission reactor that is used for power and as a moderately compact system capable of providing continuous power. It's the primary type used for planetary power generation with a few exceptions, delivering a good degree of efficiency in exchange for a relatively lower degree of mechanical complexity. Supercritical regimes are normal for you, improving the thermal efficiency to around thirty-five percent. It boils water to effectively spin a wheel, generally not used for anything but terrestrial applications.
HTGR: Increasing the temperature of the reactor massively by moving to a gas coolant while maintaining a solid nuclear core allows a lighter system along with an improvement in efficiency. This is the majority reactor type for colonies and some of the older stations as refueling is simple. The efficiency of the core is low, reaching forty-five percent but the temperature output to the radiators is closer to 1300K, allowing the use of liquid lithium in the secondary circuit outside of the turbine. This is generally seen as the preferred more reliable core for most colonies as radiators are far simpler with available ground or theoretical use in an atmosphere. These designs are similar to those used in SCNTR rockets, with a hydrogen coolant that is ejected rather than harvested for energy.
LCR: These reactors are built around a liquid core either used in a propulsive role or a lower temperature vortex used in power generation. Outside of a generation of shuttles that use it as a propulsion method most liquid core reactors are a technical side note as they have the downsides of a rotating drum while also being unable to use an MHD. These are effectively only going to be used on missiles and shuttles as a more compact propulsive engine that is more compact than a gas core reactor. The one advantage of a liquid core is in breeding and refining applications, allowing for mass production of isotopes and fissiles as liquid fuel elements are easier to separate.
GCR: Moving to a gaseous fuel element if not entirely a discrete plasma contained within a drum allows for the direct harvesting of energy from the reaction through the massive energy contained within. A steady flow of reactor fuel effectively passes through a heavily moderated and reflected chamber, becoming promptly critical and undergoing fission. Afterward, this fuel is effectively sent up through an MHD as it is hot, charged, and slows down in fission rate as it leaves the effective reactor that is more of an amplification chamber. This starts the primary harvesting of electricity from the material with a secondary Brayton cycle turbine on an alternative loop to cool the fuel down and harvest more energy, reaching efficiencies up to 80%.
Dusty Plasma: Effectively breaking with the concept of a reactor and one that harvests energy from heat the dusty plasma reactor represents a further and arguably final refinement to fission power. A magnetically confined core of fissioning micro-particles is kept between two plates of moderators with the sides effectively pinched off for energy harvesting. This causes a stream of particles to be sent on either one or both sides, slowing down the emitted particles in an MHD, and harvesting their charge allows for incredibly efficient power production ranging to 90% efficiency, halving thermal losses and radiator size. Further, these particles could just be released into space through a magnetic nozzle, providing direct relativistic thrust and driving a rocket forward.
Injection After-burn: While any of the above rockets could be operated at their maximum temperatures and generally would be for the sake of efficiency, every single one can "afterburn." By pushing more remass through the engine each can easily exceed their rated thrust at a cost of efficiency in a semi-direct scaling (ten times the thrust at a tenth the efficiency). The one partial exception to this is the dusty plasma as in a non-after-burning mode the rocket will be ejecting fragments instead of coolant. By pumping hydrogen or methane through it, a new cooling material will be introduced, effectively massively boosting thrust for configurations that support direct drive use.
