Even in the age of FTL travel, spaceships keep relying on good old sublight engines for day-to-day operations, orbital manoeuvres and to match the relative velocity of their translation target. This list covers the engine types that have to carry their propellant, reaction mass and/or energy source onboard, which means they are subject to the tyranny of the rocket equation.

Two terms are being used to characterize the performance of such engines: specific impulse and thrust. Simply speaking, specific impulse is a measure of the efficiency of an engine, i.e. how fuel-efficient it is. Thrust is the measure of an engine's power, i.e. the maximal acceleration it can produce.

Engines are ordered from least to most complex and therefore expensive. 

1 — Rocket engines

Rocket engines are a bit of an oddity in that their reaction mass and their propellant are the same: the fuel they use is also the mass they exhaust to gain forward momentum. They're a tried and proven technology dating back to the pre-Low Age space era. They come in many shapes and forms, but all share the same fundamental flaw: their specific impulse is incredibly poor, meaning that rocket starships tend to carry most of their mass in fuel (often up to 90%). However, rocket engines have high thrust and do not emit harmful radiation, which makes them an excellent choice for surface-to-orbit vehicles. Most modern rocket engines use fuels deriving from oxygen and hydrogen compounds to streamline off-world refuelling by having rocket fuel being extracted from water ice.

Simple rocket engines can be manufactured by virtually any community, and rocket fuel isn't challenging to come by either. On heavily populated planets such as the Earth or Elora, “garage rockets” are not difficult to find. 

2 — Microwave Electrothermal engines

MET or Waterjets, as they are known and loved, belong to the broader family of ion engines which have very low thrust but very high specific impulse and are used for station-keeping, slow orbital manoeuvres or intra-system journeys where time isn't a factor. MET use microwaves emitters to ignite plasma which serves as propellant. They are compact, they can be clustered together and practically run on water, which makes them ubiquitous on mining equipment and small utility vessels. They cannot, however, be used in an atmosphere or perform pre-translation burns. Waterjets are often powered by onboard batteries or solar panels.

MET drives are almost as simple to manufacture as rocket engines, and arguably simpler to build in zero-g environments. MET operators are typically referred to as “water-navigators” or “waternavs”. 

3 — Variable Impulse Ion Thrusters

The ubiquity of MET means that regular ion engines that use noble gases as propellants are very seldom used. The exception is a specific type of ion thruster known as Variable Impulse Ion Thruster or VIIT. A VIIT unit can “change gears” so to speak, using either a low-thrust, high impulse mode or a moderate thrust, moderate high impulse mode. This makes such engines ideal for orbital tugs in low-tech systems as well as automated probes, but they lack the simplicity of a MET drive. Though they can use solar panels and batteries, VIIT units are sometimes powered by a compact nuclear reactor.

Building and maintaining a VIIT is slightly more complex than for a MET or a rocket engine but well within the capacities of most space-based polities. On a small ship, a VIIT operator might the second most important person after the navigator. 

4 — Nuclear Thermal Engine (Fission)

The bread and butter of many an FTL-capable starship, nuclear thermal engines can be understood as regular rocket engines where chemical reaction mass has been replaced by the heat from a nuclear fission reactor. They have moderate specific impulse and excellent thrust, which makes them good candidates for velocity matching burns and long-range sublight journeys. They are relatively simple to build, albeit several orders of magnitude beyond rocket or ion engines in terms of technical complexity. Their main drawbacks are that they rely on a local uranium supply for refuelling and require extensive heat dissipation and radiation shielding. They also can't be used in an atmosphere for obvious safety reasons. Regardless of these flaws, nuclear thermal engines are widely used on mass-produced ships and especially cargo vessels.

Nuclear thermal engines are a rather old technology, though it's not considered as archeotech given that the pre-Low Age civilization did not develop working prototypes. They are relatively rare, though they are more constrained by the availability of nuclear enrichment facility than manufacturing concerns. Recent advances in heat sink and shielding technology have allowed nuclear thermal drives to operate at a higher standard of efficiency, and despite the competition of fusion drives they remain dominant in lower-tech areas of settled space.

In Communal Space and the Traverse, nuclear thermal engines have to follow specific regulations as both their exhaust plume and their potential acceleration are enough to classify them as weapons. 

5 — Fusion engine (Moon/Selene Drive)

Fusion engines are the high-end, all-purpose engine type that equips modern high-tech vessels. They use superheated fusion plasma as propellant, with water injection for short boosts. Most of them use focused lasers to ignite small pellets of fuel, though a few designs also use magnetic containment. Fusion drives are torch drives, that is to say they have high specific impulse and high thrust. They are both extremely efficient and powerful, making them ideal for velocity matching burns, long-range sublight travels and fast manoeuvres. They also require less radiation shielding than nuclear thermal drives. Fusion engines usually run on helium-deuterium fusion. Their main drawback is their technical complexity. Fusion drives are very complex to manufacture and maintain, even for developed communities, which restricts them to high-end vessels.

The most well-known fusion engine design is the Selene Drive pioneered by the Moon Communes which uses laser ignition. This pioneering design has been constantly improved and iterated upon in the past century, making it the obvious choice for most spaceship manufacturers. Recently, however, a group of communes on Elora developed an open-source alternative based on magnetic containment. The Talasea Drive promises better performances but is trickier to maintain and as such hasn't seen common use yet.

Fusion engines are subjected to the same restrictions as thermal nuclear engines and then some, as they are the exclusive means of propulsion for military warships and guided ammunition. 

6 — Antimatter drive

The antimatter drive is the single most powerful engine used on human vessels. It relies on the energy produced by a proton-antiproton annihilation to produce a properly monstrous amount of thrust coupled with an insanely high specific impulse. It is the only engine that can enable a ship to reach relativistic velocities with reasonable acceleration times. The drawbacks are massive, however. Antimatter drives require very high amounts of shielding (preferably several kilometres) and use exceedingly rare fuel. In effect, with geometry drives in play, antimatter engines do not see practical use in human vessels.

There is only one known human ship using antimatter drives, the Moon Communes operated courier ship No Time To Stop, which is incidentally the longest design ever produced, totalling one hundred kilometres in length, most of it being used to shield the living quarters from radiation. As far as we can tell, antimatter propulsion seems to have been the method of choice for slower-than-light interstellar civilizations, as exemplified by the Sequence and their “light skimmers” capable of reaching up to 90% of c.

7 — Lazward drive

The Lazward drive is an exotic engine using battery-powered arcjet propulsion with magnetic quench enhancement used on small-sized vessels. While it is a recent development, it is only a combination of already existing technologies resulting in a moderate impulse, moderate thrust drive ideal for velocity matching. The Lazward drive is discussed in detail in its own article. 

All illustrations are NASA public domain works.