An impulse engine (a colloquialization of internally metered pulse drive) was a nuclear fusion based propulsion system used for sublight speeds. Impulse engines were first used by Humans in the 21st century aboard the Nomad unmanned probe and then on the manned USS Lewis & Clark, the first ship to visit Saturn. (TOS novel: The Rings of Time) This technology was later used by the United Earth, and later the Federation. (TOS novel: Final Frontier; ENT novelization: Broken Bow) Each impulse engine consisted of one or more nuclear fusion reactors, a series of subspace field coils, and a vectored thrust nozzle to direct the plasma exhaust; which produced a high energy plasma which was vented out of the thrust nozzles to propel the starship. (ST reference: USS Enterprise Owners' Workshop Manual) The deuterium based fusion reaction of the impulse drive also would serve as a secondary power system for a starship and was utilized for powering internal systems. (TNG reference: Star Trek: The Next Generation Technical Manual) In the event of an emergency while at warp, the impulse engines could also be utilized to temporarily generate a Warp field to maintain warp speed for the vessel or the primary hull in the event of Saucer separation. (ST reference: USS Enterprise Owners' Workshop Manual)
- The Last Unicorn RPG module: Starfleet Academy Handbook implies that the first impulse designs were created in 2169, but the phrasing leaves it open to interpretion as to whether they were created or it was just a new design put into use.
In addition to a standard impulse drive several subgroups of impulse engine exist. A hyperimpulse engine, such as the drive system utilized by the Timeship Aeon, have exceptional speed, but with high power requirements. (Star Trek: Online; VOY episode: "Future's End") Another form of impulse drive is a combat impulse engine, which offer an edge in maneuverability but low speeds. (Star Trek: Online) Additional impulse engines are available through modifications; however, the traditional impulse engine has undergone little change since the 23rd century. (TNG episode: "Relics")