Sublight Propulsion Primer
In the world of Stardust, the Tsiolkovsky rocket equation determines the ∆v (delta-v; or change in velocity) of every spacecraft. At sublight speeds, all spacecraft are driven by various rocket designs with limited reaction mass. Thrust is determined entirely by the rocket equation: exhaust velocity, fuel density, fuel flow rate, the ship’s wet/dry mass ratio, and individual engine efficiency, of which there is significant variation. Pilots must always keep ∆v budgets in mind, particularly in combat where fuel efficiency is sacrificed for maneuverability.
Nuclear fusion is the primary power source for every spacefaring vessel in The Bary. Fusion power plants react light elements like deuterium together, liberating huge amounts of energy, and creating heavier, typically charged elements in the process. Scientific breakeven was achieved hundreds of years ago by Novani scientists with the aid of experimental erudium inductors using the z-pinch method, which has remained venerable. But there is a difference between scientific breakeven, engineering breakeven, and economic breakeven when considering the net energy gain factor and support structure complexity. Additionally, due to engineering constraints and individual environmental applications, fusion propulsion presents itself two very different ways.
Magnetic confinement is the only way to keep charged reaction products from attacking and embrittling the reactor core wall. Superconducting magnetic bottles are heavy, and they become heavier at cubic rates with the increased output of the reactor. The right plasma confinement equipment for the task can easily require 3/4ths of the power plant mass. For a landed reactor, this is no problem, but it presents an issue for air and space travel. Mobile reactor output is limited and great efforts have gone into miniaturization.
A vessel’s choice of rocket and propellant is entirely dependent on its use or mission profile. Vacuum drives can be as powerful as materials allow for. In a dense atmosphere such as upon an inhabited world, a vessel's dry mass and aerodynamic profile are important factors in maneuverability. If the exhaust is too hot, surrounding air pressure will cause explosive blowback near a fragile magnetic nozzle, which is unsafe for stable flight.
Thermal arcjets attempt to squeeze the most out of onboard fusion reactors with the least additional benign engine mass, making them popular on small craft that demand high thrust-to-weight ratios such as interceptors, bombers, clippers, and cutters. The arcjet system is far less efficient than a pure vacuum drive, but it is not as massive or unwieldy as a dedicated vacuum engine with additional magnetic confinement systems.
Divorcing the exhaust from the core is the key to miniaturizing a fusion engine. A pulsed, “microfusion” reactor working under inertial electrostatic confinement parameters is best suited for this purpose. The microfusion reactor is used to generate electricity and to thermally preheat a propellant with liquid heat exchangers. Downstream of the reactor, the drive is fed the preheated gas and, upon nozzle ingress, zaps it with an electrical arc. The doubly heated propellant grants impressive exhaust velocity from 250 m/s at low gear (high exhaust volume) to 10,000 m/s at high gear (low exhaust volume), without the need for heavy magnetic bottles.
But the true advantage of the arcjet is that it can be made safe for atmospheric flight, as the exhaust products are hot gases not directly from the blazing fusion reaction interior to the reactor core. Air can be scooped and heated as propellant, granting a vessel very long ranges in dense atmospheres with suitable chemical compositions. Arcjets must not run too hot in an atmosphere, however, as they can cause both gas dissociation and blowback around the thruster nozzle. Maximum specific impulse is on the order of 10,000+ seconds in atmosphere and over 3,000 seconds in vacuum with onboard propellant reserves, more than enough for a hypersonic transport or medium-range strike craft.
When a corporation or nation needs to move its freighters or flotillas about in deep space, and lower thrust-to-weight ratios are acceptable in the pursuit of long-haul efficiency, the shear flow z-pinch and its derivatives are the most advanced and terrifying propulsion systems available.
A differential “shear flow” plasma geometry provides stability in high volume plasma chambers and allows the reaction to occur continuously rather than in the regular oscillations common to microfusion.
A magnetic bottle can shape one end of the plasma for direct propulsive use with enormous exhaust velocities, ejecting charged particles beyond 100 km/s. On the other end, the engine generates electricity directly from the fusion products by feeding some plasma into a magnetohydrodynamic generator. Unlike pulsed microfusion devices, a shear drive ejects its plasma as propellant.
Like any other fusion reactor, the shear drive requires a tremendous current to begin the z-pinch process, necessitating a backup energy source and masses of erudium inductors. A backup power plant of any design works for this purpose (often microfusion engines but also including radioisotopic generators and even solar panels) and may be used for full-stop and emergency power generation as well. The standard ESMES used to initiate the plasma pinch needs a considerable charge before providing for the cold engine start of a high volume reactor. After ignition, the drive itself can bleed energy into the system for a much quicker warm restart if the engine should need to be turned off, or if it should unstart.
Every Barystate takes advantage of the inertial electrostatic microfusion, thermal arcjet, and z-pinch shear drive trifecta for power and propulsion. But each civilization approaches fusion fuel in a different way, with different properties that play to strengths in production, reliability, repairability, or performance.
Deuterium + Tritium
A deuterium-tritium mixture is the most straightforward fusion fuel to ignite, but that does not mean D-T is always the easiest mixture to use. Tritium has a half-life of 12.32 standard years. Plus, the D-T reaction is highly neutronic and therefore highly hazardous.
These concerns can be mostly resolved by surrounding the reaction chamber with another tank containing liquid lithium-6. The Li-6 acts as a biological radiation shield. More importantly, the byproducts of the neutron activation of Li-6 include tritium. With some gas separation, the newly bred tritium can be siphoned and replace about half of the tritium lost in the reaction chamber and to regular decay.
D-T reactions are simple as far as fusion fuels go. They require only a short z-pinch length, need relatively low amperage leading to smaller ESMES systems and faster reactor start times, and the fusiles ignite at the lowest achievable temperature. D-T reactors are the most compact, least expensive, and have the broadest range of tolerances. When used in a shear drive, the thrust, specific impulse, and reliability make for a suitable all-around system, provided one does not mind the neutron flux or the slowly decaying fuel.
Deuterium + Helium-3
Ascending the ladder of complexity and ignition temperature, we find deuterium-helium-3 (D-He3) chambers. The Novani have favored this fuel mixture for centuries already, both resources being amply present among their worlds. The reaction itself creates no unwanted neutrons. However, Republican engineers had to overcome the challenge of deuterium side reactions, which do create neutrons. Modern Novani designs minimize these side reactions but do not erase them completely. To stave off slow neutron embrittlement of the engine wall, the helium-3 is spin-polarized for maximum stray neutron absorption. This modification reduces efficiency a bit but allows for a safer reactor to be built and operated with less radioactive shielding and successfully mitigates the aforementioned challenges when using this fuel mixture.
The Novani D-He3 fuel blend wastes very few fusion products. In a shear drive, it generates very good exhaust velocity, leading to high efficiency performance. This gives Novani vessels excellent range for deep space operation. But the Novani are also known for weaponizing their exhaust in the form of plasma weapons. Every use of this ordinance steals from the Δv budget.
The Geminese have all the deuterium they need in their two world oceans. A temperamental but more elegant fusion system exclusively uses deuterium in a fully catalyzed Triple Deuterium (DDD) reaction cycle. The opportunity afforded by this heavily mediated reaction is the simplification of the fuel source. Only deuterium needs to be harvested and stored.
Helium-3, tritium, a proton, and a neutron are created from the reaction. The engine coils eject the He3 and proton, contributing to thrust. The tritium reacts with the ambient deuterium almost instantly, creating an alpha particle and yet another neutron. A cylindrical blanket of ordinary water sheathes the reactor. Light water molecules are composed of an oxygen atom and two hydrogen atoms. As in a fissile nuclear reactor, the water acts as a moderator, slowing down neutrons. When these neutrons reach specific less-relativistic velocities, they will eventually be absorbed by the H2O’s hydrogen atoms. The engine now has more deuterium available in the form of heavy water. Deuterium can be separated by electrolysis and react again. Leftover from this process is diatomic oxygen that replenishes life support reserves.
The difficulty of this engine design is that the plumbing network complexity is substantial. The daughter products must be successfully shunted out the nozzle and—in the case of the light water catalyst—back into the drive where they can participate in new reactions, consuming the fuel entirely. A complex pipeline combined with a high neutron flux makes in-situ repairs impossible for all but minor faults.
In a DDD engine design, thrust-to-weight ratios are quite a bit lower due to the additional network complexity contributing to engine mass. Thrust power is more heavily limited: Geminese naval vessels are the slowest of them all. However, more electric power is generated (even after heavy water electrolysis), refueling is simple, and the ∆v is comparable to D-He3.
Helium-3 (He-3) can react with itself with significant effort. The margins for such an ignition are just about the thinnest, with only the arcane Terran technology retained by the Empyreans being more temperamental.
Helium-3 is abundant in varied places within the gas and ice giant laden Astrild system. Like with the Triple Deuterium cycle, bipropellant mixtures are forgone, leading to simpler refueling and a ∆v boost. However, unlike with DDD, there are no side reactions to worry about in a successful He-3 cycle. The products of a He-3 self-reaction are an alpha particle and two protons, all of which can be expelled as exhaust or siphoned for power generation. The reaction is aneutronic and completely bio-safe.
The Sibyleans have enough He-3 in their backyard to make the most of this resource, applying their ingenuity along the way. Scientists of the Kingdom are some of the best minds in The Bary and are highly collaborative by virtue of the government’s lavish research spending. Sibylean vessels run hot, but successful sustained He-3 fusion is one of the Kingdom’s proudest achievements.
Proton + Boron
At the top of the ignition parameter scale is proton-boron fusion. Sometimes called “thermonuclear fission” because a proton splits boron fuel into three alpha particles, little is known about proton-boron fusion. The Commonwealth are the only ones to use this fusion method.
Proton-boron fusion cannot breakeven thermally. Therefore, a standard shear drive cannot be fed this mixture. A pellet-fed inertial confinement or inertial electrostatic confinement system, such as a much-enlarged version of the microfusion devices found paired with thermal arcjets, must be the basis for a p-B11 reactor.
The exact ignition parameters of p-B11 thermonuclear reactions are a closely guarded secret by the Empyreans. The technology is patented, and only a few select (and obsessively warrantied) civilian vessels outside the Commonwealth have been allowed to use such an engine. Other nations suspect the technology was reverse-engineered from Terran engine blocks, all owned and guarded by the Empyreans.
Nevertheless, the drive’s drawbacks may make it unworthy of curiosity. Net electric power accumulated from the engines is reportedly much lower than the other known fusion methods. Moreover, although the exhaust products are three alpha particles and no neutrons, this also lowers exhaust velocity, reducing specific impulse. The result is a middling fusion engine with no standout qualities other than complete biosafety and the ease of repair and resupply by Empyrean engineers.
Cold propellant from fuel tanks may be directly injected into the exhaust nozzle for higher acceleration, lower specific impulse thrust in engines equipped with afterburners. Practically, this is limited to strike craft, SSTOs, and sometimes corvettes and frigates, as larger vessels with significant dry mass do not see much benefit. Onboard propellant reserves are necessary for any voidcraft to break orbit.
Attitude control works in one of three ways. On most vessels, basic control is provided by vectoring the drive’s exhaust nozzle on a slight gimbal. For thermal arcjets, a larger gimbal adds mechanical complexity, and for shear drives, more erudium coils are required for proper plasma confinement. Another solution is to shunt some of the preheat exhaust in differing directions along the ship’s center of mass before it hits the arcjet proper, but again the electromechanical complexity required to do this adds mass and thus reduces performance, making this method most commonplace on the mightiest of capital ships. Cold gas reaction control is the preferred method of re-orientation on smaller vessels like strike craft, SSTOs, and even corvettes. Atmospheric strike craft can use aerodynamics to their advantage and opt not to carry any reaction control mass.
Every fusion reactor from microfusion to the largest shear drive produces enormous amounts of heat in any ordinary operating condition. Thermal radiation measured in the high gigawatt to terawatt range makes stealth impossible outside a full reactor shutdown. Heat dissipation is particularly troublesome for vessels running active energy weapons.
Atmospheric flight can therefore be easier on a strike craft’s systems, aiding in cooling at subsonic speeds, or at supersonic speeds if an intake heat exchanger is used. Radiators are the only way to cool a voidcraft.
The extreme thermal properties of shear drives in particular make them a hazard to nearby ships in the exhaust cone. If the engine exhaust were precisely directed at short range, it would make an excellent weapon.