Starship engines

Depending on purpose and manufacturer, any given space vessel may have up to four different classes of engines, listed below.

Manoeuvring thrusters
Used for basic attitude and reaction control, it is very, very uncommon to find any object in space, short of debris and dumped detritus, that does not possess manoeuvring thrusters. Typically low-powered and possessing little reaction mass, these small thrusters are mostly used for tweaking a craft's course through space, performing precision manoeuvres, and stationkeeping aboard orbital stations.

By far, the most common type of manoeuvring thruster is the venerable ion thruster, seen commonly aboard satellites used for communications and defence - typically, if such craft require anything larger, then something has gone terribly wrong.

Although antiquated, some cheaper craft make use of chemical thrusters using hypergolic fuels, that self-ignite upon mixture.

The standard for manned interplanetary spacecraft, however, particularly for Accord spacecraft, is the magnetoplasmadynamic thruster - feeding ionised gas through electromagnetic fields, and using the interactions to generate thrust. Although expensive on energy, these thrusters are highly fuel-efficient, making them particularly good for long-range missions.

Some heavier vessels, particularly military craft, make use of metallic hydrogen thrusters for reaction control. Typically formed at the extreme pressures found at the core of gas giants, metallic hydrogen possesses many fascinating quantities, with its fantastic energy density and specific impulse being the most pertinent. Their combination of efficiency and power make them ideal for hard combat manoeuvres.

Torch engines
Commonly referred to as "main engines" or "sublight thrusters", torch engines are the massively powerful devices used to accelerate spacecraft to interplanetary speeds without such journeys taking months or years.

Before the creation of torch engines, engines were sorted into two broad types: high-thrust and low-efficiency, commonly used for hard burns between planets in manned missions, producing incredible thrust but at abysmal efficiency; and high-efficiency, low-thrust engines, such as aforementioned ion engines, used for unmanned missions and manoeuvring purposes.

Torch engines, on the other hand, are both high-thrust and high-efficiency, commonly making use of nuclear reactions to heat hydrogen to metal-blistering temperatures. These are most commonly fusion reactions, although some extreme fission reactions can also be used, and antimatter is an expensive but highly effective alternative, used on the largest Commonwealth and Imperial warships.