A Nacelle is an outboard engine housing structure on spacecraft.
Impulse nacelles of a sublight shuttlecraft house the ships Impulse Engines. (TNG: "In Theory", "Descent" display graphic) The warp nacelles in warp-capable shuttles and starships house the Warp Coils, necessary components of warp drive. The warp coils create a subspace displacement field, which "warps" the space around the vessel allowing it to "ride" on a spatial distortion, and travel faster than the speed of light. (ENT: "Cold Front") While not always present on starships, warp nacelles are the most common component of warp flight, dating as far back as Zefram Cochrane's original warp ship, the Phoenix, circa 2063. They were also sometimes known as "power nacelles" in the 23rd century. (Star Trek: First Contact; TOS: "The Doomsday Machine")
Aboard Federation vessels of the 24th century, warp coils are fed by plasma conduits from the warp core reactor assembly. Nacelles are usually separated from the main structure of the ship because of radiation generated by the nacelles; when at optimal levels, the radiation could be deleterious to the safety of ship and crew.
Nacelles are separated from the ship by large pylons, and usually house a Bussard Collector at the fore end, primarily used for collecting hydrogen from space.
A nacelle's interior also includes a small control room, accessible in nominal conditions by a Jefferies tube that permits maintenance and monitoring of the system's operation. (TNG: "Eye of the Beholder") Aboard the prototypical NX Class Enterprise of the 22nd century, a long catwalk spanned the length of each nacelle and, in emergency situations, acted as shelter for the ship's crew against certain types of radiation such as that created by neutronic storms. (ENT: "The Catwalk", "The Crossing")
Most vessels typically have two nacelles. However, vessels can operate with one nacelle disabled, but at reduced warp speeds. (VOY: "Year of Hell", ENT: "Twilight") It is not unprecedented, though, for vessels to have different nacelle configurationss. For example, Federation Freedom Class starships have one, Niagara Class starships have three, Constellation Class and Prometheus Class starships have four nacelles. (TNG: "The Battle", "The Best of Both Worlds, Part II", VOY: "Message in a Bottle")
Two additional nacelles deployed from the saucer section of Prometheus-class starships when they entered Multi-vector assault mode, these were not visible during normal operation. In an alternative future, depicted in TNG: "All Good Things...", a refit version of the USS Enterprise-D was equipped with three nacelles.
Even at sublight speeds serious impacts from weapons or other objects can spell disaster for a ship because of a feedback of energy throughout the vessel. The Enterprise-D was destroyed by such an impact in an alternate timeline when the USS Bozeman collided with one of Enterprise's warp nacelles. (TNG: "Cause and Effect")
Some starship classes, such as the Defiant Class and the Steamrunner Class, possess nacelles that are integrated into the ship's main structure without pylons. (Star Trek: Deep Space Nine; Star Trek: First Contact)
In 2377, the expression "third nacelle" was analogous to "fifth wheel". (VOY: "Inside Man") ==Background]]
The use of the word nacelle in spacecraft design descends from its use for similar housing structures in air and water craft design.
According to the Book: Spaceflight Chronology, a three nacelle configuration gives a third more power to the warp drive, greater acceleration and faster engagement. However, in the first experiments, the third nacelle accenuated discrepancies in the warp field causing warp vibrations. This creates difficulties in steering, and would have resulted in the ship shaking itself apart at higher warp speeds. It was however noted that if the design was constructed perfectly it is possible for it to deliver what it promises. (page 139)
The Star Trek: The Next Generation Technical Manual states that the experiments with single and more than two nacelle designs, conducted in 2269, proved that having two nacelles is the optimal configuration for vessel control and power generation. On Galaxy Class starships, there is an emergency separation system for the nacelles. In the event the ship is damaged and unable to retain a nacelle safely, explosive latches separate the nacelle from the pylon and lift it up at 30 meters per second. If a nacelle is lost during warp flight, it can tear the ship apart, as the loss would cause different areas of the ship to travel at different warp factors. (pages 63, 65 and 66)