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In a number of Science Fiction Series such as Battlestar Galatica, Star Wars and Stargate spaceships are mentioned to have something called a Sublight Engine. in an episode of Dark Matter i remember someone mentioning that when the ship was disabled both the FTL and Sublight Engines were down so at least in this case a Sublight Engine seems to be separate from a FTL Engine.

I am wondering what exactly is a Sublight Engine, particularly the real life basis all these series seem to adopt/adapt from.

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    FTL = faster than light :: Sub Light just means below the light speed threshold. They are just engines that will bring you any speed less than light speed. In the shows that you reference the Sub light engines tend to be thrust based (just like our rockets and Ion drives) that push the ship physically. The FTL tend to be warp/hyperdrive/other which allows the script writers to get around the fact that (according to our current knowledge) nothing can go faster than light.
    – user001
    Commented Sep 15, 2015 at 9:40
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    The term sublight in this sense has been used in science fiction since 1950 if not earlier.
    – user14111
    Commented Sep 15, 2015 at 10:03

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Short Answer

A Sublight Engine or Sublight Drive is the engine used to propel a starship through space at speeds below the speed of light, hence the term.

Slightly longer answer

The name is, as you suggest in your question, used to differentiate between the engine(s) used for Faster than Light (FTL) travels and the ones used for, so to speak, standard speed travel.

While the exact function and actual speeds of the different sublight systems differ between universes, their common features is that they are all used to travel in space at speeds below that of the light.

References

Star Wars Sublight drive

Stargate Sublight enigine

Battlestar Galactica Propulsion

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In the shows you reference, the ships have some form of FTL (faster than light) engines, that allow the vessel to travel faster than the speed of light. In sci-fi, this usually requires a dedicated FTL engine, meaning that when the ship wants to travel slower than the speed of light another, different, engine is used. The 'sublight' engines are the engines that the ship uses when travelling slower than the speed of light.

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STL drives use some form of propellant to create a reaction thrust. What the propellant is will depend on the size of the ship and the range/endurance. Typically a STL drive is a fusion reactor with a hole drilled in one of the sides to all the fusion/plasma to escape. Other options are an ion drive (charged electrons), nuclear pulse or good old fashioned rocket.

FTL will either warp space scrunching it up in front of the ship to make it shorter (ST), fold the fabric of space to allow to points to touch each other (Dune), punch through to another plane and travel conventionally then punch back having taken an extra dimensional (B5) shortcut.

Star Trek also has sub-space whichi is used for faster then light "radio" waves. I don't recall any drives that operate in subspace in the TVs/films but a book might mention them. subspace is supposed to be dangerous.

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