How does one travel billions of kilometers? A big, effective, and barely controlled boom.

Going Slow

Conservation of momentum is one of the most frustrating laws in physics. In short, you can’t move forward without tossing something overboard behind you: reaction mass.

There is fuel, and then there is propellant. Fuel provides the energy needed to burn the propellant hot, which makes it far more effective at driving a vessel onward than simply dumping it cold. The rocket equation determines the ∆v (delta-v; or change in velocity) available to every vessel. Thrust power and fuel efficiency is determined by this equation’s variables: exhaust velocity, propellant flow rate, the vessel’s wet to dry mass ratio, and individual engine efficiency, of which there can be significant variation. Pilots and navigators must always keep their ∆v budgets in mind, particularly in combat, where fuel efficiency is sacrificed for on-demand maneuverability.

Erudium-induced nuclear fusion is the primary power source for every voidfaring vessel in The Bary. Fusion power plants force rare, light gases like deuterium together, liberating huge amounts of nuclear binding energy. Simultaneous with the energy release, the reaction creates heavier, typically charged elements. Charged products are extremely useful because they can be magnetically directed.

Nuclear physicists achieved stable, net energy production from fusion some 250 years ago. Novani scientists harnessed newly discovered erudium inductors to squeeze and confine plasma using the z-pinch method, and this basic reactor design has remained venerable. But civilization is always on the move. For voidborne power and propulsion technologies, various engineering constraints come into play that don’t exist for a landed reactor. Fusion propulsion thus presents itself in two very different ways.

The first problem engineers had to tackle was reactor weight, because every kilogram of machinery subtracts from a vessel’s thrust-to-mass ratio. Magnetic confinement is the only way to keep charged reaction products from embrittling the reactor core wall. Magnetic bottles are heavy, and they become heavier at cubic rates with the increased output of a reactor. The right plasma confinement equipment for the task can easily exceed two-thirds of the power plant’s total mass. There are other components that cannot be lightened, such as biological shielding. For a landed reactor, this is no problem, but it presents an issue for air and space travel. Mobile reactor output is limited by a vessel’s mass budget and significant efforts have gone into reactor miniaturization in the past century and a half.

A vessel’s choice of fuel and propellant depends entirely on its proposed use or mission profile. In a dense atmosphere, such as upon an inhabited world, a voidcraft's mass ratio, aerodynamic profile, and flight regime are important factors in performance. If the rocket exhaust is too hot, surrounding air pressure will cause explosive blowback near a fragile magnetic nozzle, which is unsafe for flight.

Thermal Arcjets

Thermal arcjets sacrifice some of the power generation capability of microfusion reactors—relatively lightweight devices which run in pulses rather than continuously—to extract more work out of each kilogram of propellant. This property makes the thermal arcjet popular for those voidcraft that demand a high thrust-to-weight ratio, such as fighters, skiffs, clippers, and cutters. The arcjet and its microfusion reactor are far less efficient together than a pure vacuum drive, but they are not as massive or unwieldy as a dedicated magnetic confinement loop.

A thermal arcjet is a nuclear thermal rocket design. A microfusion reactor harnesses inertial electrostatic confinement to create pulses of electrothermal power. Liquid heat exchangers then preheat the propellant. Downstream of the reactor, the drive is fed this preheated gas. Upon nozzle ingress, an electric arc briefly ionizes the gas, turning portions of it into plasma. The doubly heated propellant grants impressive exhaust velocities from 250 m/s at low gear (high exhaust volume) to 10,000 m/s at high gear (low exhaust volume), with only a single small magnetic bottle.

But the true advantage of the thermal arcjet is that it can be made safe for nearly unlimited atmospheric flight, as the exhaust products are hot gases not interior to the reactor core. Air can be scooped and heated as propellant, granting an aircraft or voidcraft in atmospheric flight extreme range in suitable environments. Arcjets must not run too hot, however, as they can cause both gas dissociation (pollution) and ablate the heat-resisting material layered upon a mechanical thruster nozzle. Maximum specific impulse is on the order of 15,000 seconds in atmo—limited entirely by reactor fuel stores—and just over 3,000 seconds in vacuum with onboard propellant reserves. These numbers are more than enough for a hypersonic transport or trans-atmospheric strike craft.

Shear Drives

When a corporation or navy needs to move 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 solution and its derivatives are the most advanced and terrifying propulsion systems available. Voidfarers colloquially refer to such monstrous engines as shear drives.

Shear drives are a variation on landed reactor designs. A differential, sheared plasma geometry provides added stability in high-volume, one-way plasma chambers and allows the reaction to occur continuously rather than in the oscillations common to microfusion. A magnetic bottle can shape one end of the plasma stream for direct propulsive use with enormous exhaust velocities, ejecting charged fusion products well beyond 100 km/s. On the other end, the engine generates electricity directly from these products by feeding them into a magnetohydrodynamic generator. Unlike pulsed microfusion devices that simply heat propellant, a shear drive ejects its reactor plasma as propellant. This makes the fusion chamber simultaneously a reactor and a drive system.

Like landed reactors, the shear drive requires a tremendous current to begin the z-pinch process at gigawatt power scales. This necessitates a backup energy source, heavy confinement tooling, and masses of erudium inductors. An auxiliary power plant of any design works for generating an ignition charge of this magnitude (often microfusion reactors, but also f.ex. radioisotopic generators and even solar panels) and these sources may helpfully contribute to full-stop and emergency power as well. After ignition, the drive can bleed energy into the power banks for a much quicker warm restart if the reactor should need to be shut down, or if it should unstart.

Fusion Fuels

Every Barystate takes advantage of this technology trifecta for nuclear power and propulsion. But each of The Bary’s nationstates approaches fusion fuel differently, with unique recipes that play to strengths in production, reliability, repairability, or performance.

Deuterium + Tritium

Primarily Feronian

A deuterium-tritium (D-T) 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 short half-life of twelve and a half standard years. Plus, the D-T fusile reaction chain is highly neutronic, presenting extreme radiation risk.

Safety concerns can be (mostly) resolved by surrounding the reaction chamber with a tank containing liquid lithium-6. The Li-6 acts as a biological radiation shield. The byproducts of the neutron activation of Li-6 include tritium. With some very careful gas separation, the newly bred tritium can be siphoned and replace about half of the tritium lost in the reaction chamber and to its regular radioactive decay.

D-T mixtures are simple as far as fusion reactions go. They require only a short pinch length, need relatively low amperage leading to smaller ESMES systems and fast reactor start times, and they ignite at the lowest achievable temperature for net fusion. D-T reactors are the least expensive and have the broadest range of operational tolerances. When harnessed for voidflight, 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

Primarily Novani

Ascending the ladder of complexity and ignition temperature, we find deuterium-helium-3 (D-He3) chambers. The Novani have favored this fuel mixture from the outset, both resources 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 embrittlement of the reactor 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 without redundant radioactive shielding and successfully mitigates the challenges of harnessing this fuel mixture.

The Novani D-He3 fuel blend wastes very few fusion products—their microfusion reactors are quite fuel efficient. In a shear drive, it generates very good exhaust velocity, leading to high performance. This gives Novani vessels excellent range for deep space operation. But the Novani are also known for weaponizing their exhaust as directed plasma weaponry. Every use of this ordinance steals from the Δv budget.

Double & Triple Deuterium

Primarily Geminese

The Geminese have all the deuterium they need in their two world oceans. A temperamental but elegant fusion system uses either a double or triple deuterium reaction cycle (DD/D). The opportunity afforded by this complex reaction cycle 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 of two deuterium atoms. Depending on the confinement architecture, these products can all react with more deuterium. In a microfusion reactor, the products are optimized to produce the most thermal energy per pulse. In a shear drive, downstream reactions are most often optimized for the generator.

Aboard larger Geminese vessels or platforms, deuterium breeding is also possible. A cylindrical blanket of ordinary water may sheath 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. More deuterium is now available as heavy water. Like the tritium produced by Feronian reactors, this nuclear breeding takes time, and energy must be spent to split the liquid fuel into its working state, but it proves a useful feature for Geminese tenders and freighters to have. Leftover from fuel electrolysis is diatomic oxygen that replenishes life support reserves.

The difficulty of these designs is that the plumbing can be substantial. With the addition of a moderately high neutron background, in situ repairs are impossible for all but minor faults.

First-order deuterium self-reactions are more anemic per kilogram of fuel. On-demand thrust power is limited: Geminese vessels of all kinds are typically the slowest of them all. But deuterium is also highly compressible with modern technologies. The reaction chain does have many useful branches and endpoints that can be reconfigured, and energy per kilogram is competitive in high gear. By employing second-order reactions, deuterium alone proves the most versatile fuel for long-term energy production and storage.

This sort of slow and steady sailing aligns with the Geminese inclination toward pathfinding and long-distance exploration. Castori sailors were the first to employ brachistochrone burns between two stars when they set off from Lux to the microstates of The Alizarin Twins some years before the Interstellar Congress. Geminese merchants were also the first to cross The Bary end-to-end.

Helium-3

Primarily Sibylean

Helium-3 (He-3) can react with itself with significant effort. The margins for such an ignition are just about the thinnest.

Like with the Double or Triple Deuterium cycle, fuel mixtures are forgone, leading to simpler resupply. However, unlike with DD/D, there are no side reactions. 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 He-3 self-reaction is aneutronic and completely bio-safe.

Helium-3 is abundant within the gas and ice giant-laden Astrild system. The Sibyleans can make the most of this resource, but they had to apply their ingenuity along the way; the technology was only developed after the Novani-Sibylean War. Scientists of the Commonwealth are some of the brightest minds in The Bary and are highly collaborative by virtue of the government’s lavish research spending. Modern Sibylean vessels run hot, but successfully sustained He-3 fusion is one of the Commonwealth’s proudest achievements.

Any Fuel

Primarily Empyrean

The schematics of exported Empyrean technology are closely held by The Conglomerate. Only a few select (and obsessively warrantied) civilian vessels outside Empyrean space have ever been allowed to operate Empyrean technologies: the Titan-class freighters. The Titans have been adapted to use any quality fuel source in their fusion chambers, but Empyrean thruster technology is shrouded in mystery. It’s known to be a nano-electrokinetic thruster of some description, because nanotubular carbon channels array the business end of the engine block.

A unique property of an electrokinetic thruster array is that it runs very cold when compared to fusion thruster designs.

Empyreans are masters of nanotechnology. But that technology may not be new. The other Barystates suspect many Empyrean technologies were reverse-engineered from Terran wrecks, the most complete of which are owned and guarded by Empyrean Keepers.

On-demand Performance

Cold propellant may be directly injected into an exhaust nozzle for higher acceleration, lower specific impulse thrust in engines equipped with afterburners. Practically, this is limited to voidcraft and their thermal arcjets, but is sometimes seen on corvettes and frigates. Onboard propellant reserves are necessary for any voidcraft to reach or break orbit, much less maneuver in a vacuum. These propellant reserves may be internal to the fuselage and/or provided by external drop tanks.

Voidborne attitude control works in one of three ways. On most vessels, basic control is provided by vectoring the drive’s exhaust nozzle(s) on a slight gimbal. For thermal arcjets, a larger gimbal will add mechanical complexity. For shear drives, more erudium coils are required for plasma confinement along greater exhaust arcs. Another method of re-orientation is to shunt propellant in several orientations distant from the ship’s center of mass using resistojets. The additional plumbing required to do this subtracts from the thrust performance available to a voidcraft, making this method most commonplace aboard larger voidframes. Simple cold gas reaction control is the preferred method of voidborne attitude control on the smallest voidcraft.

Heat Dissipation

In space, everyone can see you radiate.

Every conceivable reactor from microfusion to the largest shear drive produces extreme heat while active. Fusion thrusters create even more. Thermal radiation measured in the gigawatt and even terawatt range makes stealth impossible outside a full reactor shutdown, and this heat takes quite some time to radiate away. Active heat dissipation is evermore troublesome when energy weapons are in play.

Thermal control is achieved via skin or boom radiators, though liquid droplet radiators exist for some vessels that idle often. There also exist ejectable heat sinks for emergency use, but these are consumables that should not be relied upon.

Modern microfusion reactors are well-controlled, all things considered. Their pulsed nature ensures thermal heat flux is held steady in normal operation. Extendable radiators and ejectable sinks are used as backups. Most voidcraft can get away with skin radiators, thermal vents, phase change materials, superthermal foam, etc. But combatant voidcraft with their many weapon systems do regularly push this limit.

Thermal management for shear drives is a whole other beast. Every such vessel has a primary and secondary bank of extendable radiators for the extreme conditions they find themselves in regularly—thermal interreflection be damned. Warships may “strike their colors” by extending every available radiator at once. This is an internationally agreed upon signal, first used in skirmishes among the Geminese before their unification.

The extreme thermal output of shear drives makes them a hazard to nearby ships in the exhaust cone. As the Novani proved with their plasma weaponry, if the engine exhaust of a shear drive can be precisely directed in short range combat, it makes an excellent weapon.