The Subluminal engine — a starship engine that cannot exceed the speed of light — is one of the staple technologies of modern spacecraft. Most are, when distilled to their bare components, very complicated rockets: they produce powerful streams of particles that shoot away from the ship and propel it forward. The most common form of superluminal engine is the fusion drive. It has become the standard by which other drives are measured, providing a reasonably powerful, fairly efficient and mostly safe method of producing thrust.
A fusion drive is a controlled fusion reaction with a partially open reactor. A magnetic nozzle controls the exhaust from the reactor, varying the levels of thrust produced. Fusion drives can produce a great deal of thrust in a short amount of time, potentially generating forces that would crush the occupants of a craft if the pilot is careless. They also rely on a cheap, highly abundant source of fuel: Hydrogen.
Unfortunately, if a fusion drive is damaged it can destroy a ship as reactor containment fails. Some alternate propulsion systems, such as chemical drives or luminal drives (drives that use the force generated by photons to create thrust) are far safer in this regard, but are considerably less efficient or far more expensive to operate. Other propulsion systems, like antimatter drives, can provide considerably more thrust with far more efficiency — but they are generally considered temperamental, and when they break they do so with a finality that can be seen planetside.
Most modern subluminal engines can reach speeds approaching the speed oflight given enough time and acceleration, but this is rarely done. The time required for acceleration and deceleration during in-system travel makes reaching those speeds impractical — the only time it would be feasible would be for intersystem travel, when superluminal drives are a more practical alternative. Generally speaking, most ships rarely approach even a tenth of the speed of light, which is usually sufficient for in-system travel.
Taken from An Informal Guide to Known Space, Edition XVI, McPherson/Sovitt Publications.