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In the fictional Star Trek universe, many Starfleet vessels are designed with two main sections. While the "primary hull" (also called the "saucer section", due to its distinctive circular shape) provides living quarters, scientific and research laboratories, impulse engines (necessary for sub-light travel), and the vessel's primary shuttle bay, the "engineering hull", also called the "stardrive" or "battle section," holds all warp drive-related components (most notably the warp-core and warp nacelles), as well as the main deflector array, heavy weapons (notably photon torpedoes), matter/anti-matter containment pods, main engineering facilities, industrial fabrication units, and extensive cargo bays.
The warp core is the key component of any stardrive section — this is from whence the name derives. The warp core is a highly advanced device capable of projecting a warp field around a ship, thus separating it in a way from the physical universe and allowing it to travel at speeds far faster than light. The core is often found in the center of the stardrive section and runs the entire height of the ship. The substantial increase in efficiency with this design aside, though, the size and positioning of the core (to say nothing of energy emissions) allows an enemy to target it easily. Since it is so volatile, even slight damage may result in a disastrous breach which may require the separation of the stardrive and saucer sections in order to save the crew.
Connected to the warp core are several warp nacelles, often two, which are oblong in shape and approximately half the length of the ship. These nacelles distribute the warp field generated by the core around the ship, thus facilitating the entire warp process. Due to the wide variety of configurations available, the nacelles may appear literally anywhere: above or below the ship, off to the sides, or (as is the case with Galaxy and Sovereign-class starships, suspended above the main stardrive structure by sturdy arms. The nacelles are also very delicate instruments, and require good shielding to protect them from enemy fire — a damaged nacelle may not be capable of projecting a warp field, or worse, may project it sporadically, which may result in damage or destruction of the entire vessel.
In the Galaxy-class, very few systems are present aft of the warp core, as the stardrive section flattens out somewhat and leaves little room for vital components. However, a ship may contain a photon torpedo launcher, phasers, a secondary shuttle bay, and possibly impulse engines aft of the core, based on its design.
Forward of the core, one finds the medical bay, a crucial component to any vessel with large numbers of people. A typical medical lab contains all the standard functions of a hospital, albeit somewhat compacted due to limited space. Since the medical bay is located in the same section of the ship as the warp core, it must be evacuated in the event of a catastrophic core breach.
On the lower stern of the stardrive section, a deflector array is typically placed. The specific function of the deflector array is not specified, although it has been used for everything from a weapon to a method of escaping dangerous gravitational fields. The array's primary function is as a particle emitter, meaning that it may be reconfigured for a number of different purposes.
In the remaining space, there are some living quarters, computer systems, and possibly even a holodeck. A large portion of the stardrive section's stern is taken up by a battle bridge, a secondary command center where the stardrive section could be controlled from.
Generally, the stardrive section connects to the saucer by way of a strong neck, in which the battle bridge and deflector array are located and atop which reside magnetic clamps for holding the two sections of the ship together.
The idea of separating the saucer and stardrive sections and reconnecting them on the fly is a recent innovation first used on the Galaxy-class starships. In dangerous battle situations the idea is that non-essential personnel may be evacuated to the saucer section, which would separate and leave the battle field. The heavily armed stardrive section will remain behind to engage the armed opponent. A dedicated turbolift connects the primary and battle bridges when the vessel is whole, allowing rapid transport to ensure speed and efficiency in disconnecting the sections and tidying up a situation.
When the Galaxy-class entered operational service, however, this was rarely done in battle situations. The reason was that armed conflicts were fluid and rapid situations in which there was no chance to separate the two sections. Also, on the Galaxy-class the saucer section is capable only of impulse power, which typically does not provide enough speed to give the saucer a chance to withdraw quickly from a battle. As such, when the Enterprise's saucer was separated, both sections tended to remain at the battle.
During the lifetime of the Enterprise-D, separation of the stardrive section was seen only four times on screen:
|Incidents of Enterprise-D Saucer Separation|
|"Encounter at Farpoint"|
|"The Arsenal of Freedom"|
|"The Best of Both Worlds Part II"|
|Star Trek Generations|
The manoeuvre was also performed in at least two non-canon novels, Foreign Foes and Rogue Saucer. The stardrive section of the Enterprise-D was separated a number of times during maintenance layovers. This proved to be far more common than in battle situations. In one such instance, the battle bridge module was replaced.
Starfleet did not entirely abandon the concept of separable starships. Around 2373, the Sovereign-class was designed for on the fly separation (like the Galaxy-class); however, this ability has not yet been observed. The ability was mentioned in the non-canon novel Ship of the Line: The USS Prometheus, an experimental warship, featured "multivector attack mode," whereby the ship separated into three sections that could engage targets independently (Star Trek: Voyager: "Message in a Bottle"). Each section — designated Alpha (command module), Beta (dorsal engineering module), and Gamma (ventral engineering module) — also possessed independent warp capability, implying multiple warp cores. It's unclear how many additional Prometheus-class vessels entered service following construction of the prototype vessel.