Impulse engine
An impulse engine or impulse drive was a common propulsion system used on starships used to create impulse power in order to achieve impulse speeds.

Technology
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. 

This technology remained largely unchanged from the 22nd century to the 2360s. ()

On August 12, 2152,  approached a black hole at impulse because of the extreme gravity. () 

The auxiliary impulse engine was an engineering component of the  starships in 2266. ()

In 2267, Science Officer Spock calculated the explosive force of an overloaded Starship-class impulse engine to be 97.835 megatons. ()

By the 2270s, impulse was capable of sustaining warp 0.5 without the warp drive even being on-line. ()

The impulse drive of Bajoran interceptors could not operate within an atmosphere. ()

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. ( display graphic)

The Galaxy-class starships were equipped with main and saucer impulse drives. In 2367, the drives were managed by the impulse propulsion power system. In that year, when Data hijacked the , he issued a command override on this system giving him complete control of the system from the bridge. () In 2368, on a schematic of the  (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. () 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. ()

In 2371, after the Battle of Veridian III, the Enterprise-D's engine room had been hit, causing a coolant leak, which became a warp core breach, and after the saucer separated from the stardrive section, Deanna Troi engaged the saucer impulse drives. ()

Several ships, including Jem'Hadar fighters, Galileo-type shuttlecraft,  runabouts, the , and class V planetary probes used impulse thrusters for sublight propulsion. (; ; ) Magnetic constrictors were a component in the Danube-class impulse thrusters. ()

Parts of the system 
 Avidyne engine
 Driver coil
 Impulse capacitance cell
 Impulse burst
 Impulse control system 
 Impulse deck
 Impulse pack
 Maneuvering jet 
 Impulse manifold
 Impulse nacelle
 Impulse reactor
 Fusion reactor
 Impulse response filter
 Impulse thruster
 Magnetic constrictor
 Primary drive coil
 Radioactive disposal vent
 Torque buffer

Appendices

See also 
 Hyper-impulse drive
 Sub-impulse thruster

Background information 
Early drafts of the  episode  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  and , 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 motors and internal combustion engines in a modern hybrid car. It would also explain Scott's remark in  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 , "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 (~1.8% the speed of light). One quarter impulse for the shuttle could be estimated at 1,852 kilometers per second. 

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. One quarter impulse for Voyager would be 18,665 km/s. Voyagers one quarter impulse is 10 times faster than that of the shuttle. Therefore, the term "¼ impulse" isn't a fixed speed as much as it is one fourth of the full speed of the impulse engines' maximum.

External links 
 
 

cs:Impulzní pohon
de:Impulsantrieb
fr:Moteur à impulsion
it:Motore a Impulso
ja:インパルス・ドライブ
nl:Impulsaandrijving
pl:Napęd impulsowy
pt:Motor de impulso
Propulsion technology