The secondary hull, sometimes referred to as the stardrive section, engineering hull or battle section, was a structure that formed part of a starship. It was usually located to the rear and below a ship's primary hull, and was either directly attached, or separated by a connecting dorsal.
Federation starship design
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In Federation starships, the navigational deflector, engineering, battle bridge, cargo and shuttle bays were generally located in the secondary hull. (TOS reference: Mr Scott's Guide to the Enterprise; TNG references: Star Trek: The Next Generation Technical Manual, Star Trek: The Next Generation Sketchbook: The Movies; TLE novel: The Buried Age)
The secondary hull is encompassed by the aft or secondary warp field lobe generated by the warp nacelles. The concave aft hull undercut found in many Federation starships prevents 'pinwheeling' as it allows for varying degrees of field flow attachment. (Ships of the Line 2011; TNG reference: Star Trek: The Next Generation Technical Manual)
Development history
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22nd century
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The Enterprise following refit to the Columbia-class.
23rd century
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The USS Confederate's secondary hull at warp in 2270.
24th century
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The separated primary and secondary hulls of the USS Enterprise-D in 2364.
Alternate timeline
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In the War of the Prophets timeline, the USS Boreth and the USS Opaka had engineering hulls. (DS9 - Millennium novels: The Fall of Terok Nor, The War of the Prophets)