In Federation starships, the impulse drive was essentially an augmented fusion rocket, usually consisting of one or more fusion reactors, a driver coil assembly, and a vectored thrust nozzle to direct the plasma exhaust. The fusion reaction generated a highly energized plasma. This plasma, ("electro-plasma") could be employed for propulsion, or could be diverted through the EPS to the power transfer grid, via EPS conduits, so as to supply other systems. The accelerated plasma was passed through the driver coils, thereby generating a subspace field that improved the propulsive effect.
In 2267, Science Officer Spock calculated the explosive force of an overloaded Starship-class impulse engine to be 97.835 megatons. The Enterprise was fast enough to stay ahead of the Planet Killer under "emergency impulse power", but doing so would exhaust their fuel supplies in less than a day. (TOS: "The Doomsday Machine")By the 2270s, impulse was capable of sustaining warp 0.5 without the warp drive even being on-line. (Star Trek: The Motion Picture)
The Type 15 shuttlepods were equipped with two long range 750 millicochrane impulse driver engines, also known as impulse nacelles, which were located on either side of the craft. (TNG: "In Theory", "Descent" display graphic)
Aboard the Galaxy-class starships, there were a main and saucer impulse propulsion systems. In 2368, on a schematic of the USS Enterprise-D (viewed at Commander William T. Riker's station on the Enterprise-D bridge), the location of the main and saucer impulse engines (port and starboard) were labeled. (TNG: "New Ground") Later that year, after the computer systems of the Enterprise-D were disrupted by a energy wave from a Satarran starship, Geordi La Forge accessed the basic system directories of the Engineering Department. According to the department, the main and saucer impulse propulsion systems were off-line. (TNG: "Conundrum")
Several ships, including Jem'Hadar fighters, Galileo-type shuttlecraft; Danube-class runabouts, the Delta Flyer II and Class V planetary probes use impulse thrusters for sublight propulsion. (Star Trek V: The Final Frontier; DS9: "Rocks and Shoals", "The Assignment"; VOY: "Drive", "Blink of an Eye") Magnetic constrictors were a component in the Danube-class impulse thrusters. (DS9: "The Assignment")
Parts of the system
- Avidyne engine
- Driver coil
- Impulse capacitance cell
- Impulse control system
- Impulse deck
- Maneuvering jet
- Impulse manifold
- Impulse nacelle
- Impulse reactor
- Impulse response filter
- Impulse thruster
- Primary drive coil
- Radioactive disposal vent
- Torque buffer
Early drafts of the Star Trek: Deep Space Nine episode "Emissary" included the proviso for vessels that wanted to use the Bajoran wormhole had to stop at Deep Space 9 to have energy buffers installed on their impulse engines to mitigate the effects on the Prophets. Judith Reeves-Stevens and Garfield Reeves-Stevens suggested several reasons why this aspect was dropped: "If, to pass through the wormhole into the Gamma Quadrant, a ship needs special upgrading available only at DS9, then how could ships from the Gamma Quadrant ever come through from the other side? Part of Deep Space Nine's appeal is that unknown Gamma Quadrant aliens can emerge from the wormhole at any moment. With this condition still in place, they'd never make it. The elimination of this idea also removes the nagging inconsistency of entities with godlike powers being subject to destruction by mere human technology. Besides, if they exist in all times at once, why didn't they see this coming? Better they should just be disturbed by the human concept of linear time, rather than be threatened by human machines. Then their decision to stay out of sight becomes an emotional one that is not subject to human intervention". (The Making of Star Trek: Deep Space Nine)
Dialogue from several episodes, including "Where No Man Has Gone Before" and "The Doomsday Machine", suggests that the impulse drive could be used in some manner to propel a ship at faster than light speeds, albeit with a lower maximum speed and a higher rate of fuel consumption than the main warp drive. This could possibly make the relationship between impulse and warp drive analogous to the relationship between electric and combustion engines in a modern hybrid car. It would also explain Scotty's remark in "Balance of Terror" about the Romulans' power being "simple impulse" and allow it to be reconciled with their clear capacity for interstellar travel.
In The Motion Picture, The Enterprise traveled at warp 0.5 from Earth to past the planet Jupiter, a distance of (at a minimum) 390,674,900 miles, in 1.8 hours, making that speed approximately equal to 97,026 kilometers per second (217,041,611 miles per hour), or roughly 1/3 light speed. The difference may be explained by differences in orbital precession between the two planets at the time, or, as with warp drive, there may be other variables involved.
According to Jo'Bril in the episode "Suspicions", 'I am one million kilometres from the star's corona. Proceeding at three quarters impulse. I should reach it in approximately three minutes.' That is approximately 12,400,000 miles per hour or 5,543 kilometers per second. ¼ impulse for the shuttle could be estimated at 1,852 km/s.
The 'Star Trek Voyager Technical Manual' page 13 has full impulse listed as ¼ of the speed of light, which is 167,000,000 mph or 74,770 km/s. ¼ impulse for Voyager would be 18,665 km/s. Voyager's ¼ impulse is 10 times faster than the shuttle's. Therefore, the term "¼ impulse" isn't a fixed speed as much as it's ¼ of the full speed of the impulse engines' maximum.