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In the Star Trek fictional universe, traditional Federation starship design divided Starfleet ships into with two main sections: The "primary hull" or "saucer section" provides crew quarters and living facilities, science and mission facilities. The primary hull also has weapons arrays, impulse engines, and command facilities such as the main bridge. The primary hull is also known as the "saucer section" due to its resemblance to a saucer. The second part of the ship, the "engineering hull" (also known as the "stardrive" or "battle section") is attached to the primary hull by a neck. The stardrive section holds all warp drive-related components, as well as the navigational deflector array, shield generator and additional weapons. The stardrive section could also contain fuel storage, cargo bays, shuttle bays and other systems. In most cases the saucer section can detach from the stardrive section and function entirely autonomously, sans warp drive. With the introduction of the Galaxy-class ship, the ability to separate and reconnect the two ships on the fly was finally possible. This distinction has become less clear in most 24th century Starfleet ships. This is most clearly illustrated by the USS Defiant.
The primary use of the separation tactic was to accomplish two separate goals simultaneously, such as engaging an enemy on two fronts (for example, the Borg attack on the Federation of 2366-2367, in which the Enterprise-D separated in an attempt to slow the Borg cube's progress toward System 001 -- Earth). As a last resort, most Starfleet vessels that followed the saucer and secondary hulls configuration possessed the ability to separate the two sections as a disaster aversion tactic in the event of catastrophic failure of some sort. In such cases reconnection was only possible at drydock facilities; due to the nature of the separation, a new stardrive (or saucer) section was usually needed, as such a situation typically demanded the destruction of one part of the ship. More recent vessels, such as the Galaxy class possessed the ability to separate the two sections and reconnect them sections on the fly (as the Enterprise-D aptly deomonstrated in 2364, this was possible but extremely risky during warp travel). As originally envisioned, this was to make it possible to have only mission-critical functions and crew brought into a dangerous situation.
Most Federation vessels (either or both sections) were designed to withstand at least one ballistic atmospheric re-entry; whether they would be capable of leaving the planet again was another story entirely.
In the event of a catastrophic failure of some shipboard system, such as the vessel's warp core, a cataclysmic explosion could usually be avoided for a short time while inhabitants are moved into the ship's primary section, and said section is separated and makes an attempt to escape. Though intended as an additional safety measure, such an explosion would likely damage the saucer of the ship so badly that it could no longer maneuver under its own power. Failing that, the saucer would be in for a very long ride, as the typical Federation vessel utilizes only impulse engines on its saucer section. The USS Enterprise (NCC-1701-D) provided a key example of this: a band of Klingon militants managed to obtain the Enterprise's shield harmonics frequency, and were able to fire their disruptors directly through the ship's shielding. Before being destroyed themselves by a photon torpedo from the Enterprise, the militants managed to hit the Federation vessel in such a way that a warp core coolant leak began, resulting in the reactor's overheat and imminent explosion. Fortunately the crew were able to evacuate the stardrive section before the core went critical, and even managed to separate the saucer and move a substantial distance away from the stardrive section before the warp core exploded brilliantly. The Enterprise's impulse engines were damaged and helm controls were knocked offline, resulting in the saucer crashing on the planet Veridian III, around which it was currently orbiting. Few casualties were suffered, but the Enterprise-D suffered extreme damage and would never fly again.
- Saucer section at Memory Alpha, a Star Trek WikiWiki