Star Trek: 10 Things You Didn't Know About Warp Drive
6. Those Ample Nacelles
Most vessels stick to a standard two-nacelle design. These nacelles, taking their name from similar propulsion components on air and water craft, created a subspace distortion field around the ship, allowing vessels to ride through the distortion at extreme velocities. They were usually separated from the rest of the ship by large pylons, an idea stemming from Matt Jefferies, the designer of the original Enterprise.
He felt that, when in use, the nacelles would be flooded with dangerous levels of radiation and that no crewmember should be near them. This rule was relaxed by the seventh season of the Next Generation, as a control room was built into them. Later ships, like the Defiant and Steamrunner class, had the nacelles built into the main hull.
These nacelles were still a weak point on their ships. Damage could be catastrophic, such as the alternate timeline that saw the Enterprise-D explode following a collision with the USS Bozeman. By the 31st century, Starfleet had begun to design ships with detached nacelles. While this was said to make the Warp Drive on these vessels more reliable, it also served to protect the ships from direct damage due to impact with the nacelles.
Several ships, including the Freedom Class, Hermes Class, Saladin Class and, in the alternate timeline, Kelvin Class were able to operate at warp with only one nacelle.