The secondary hull (also known as the engineering hull, the engineering section or the stardrive section) was a major component of the overall hull structure of many starship designs.
The secondary hull usually was the location for the vessel main engineering, which had the vessels main power generating unit, such as a warp core, and other engineering support systems. The warp nacelles were typically connected to the secondary hull.
Federation designs usually had the secondary hull below and behind their primary hull. In these designs it was typical for the ship's navigational deflector to be located at the fore of the secondary hull and for the warp nacelles to be connected to the hull. Some designs had the ability to separate their primary hull from the secondary hull in case of emergencies.