Intrepid class at warp
Variable-geometry pylons are a technology introduced to Federation starships around 2370.
One of the unique aspects of the new technology is that the warp nacelles have the ability to be raised into position for warp speeds, then lowered into a more streamline position when at slower-than-light speeds. The Intrepid class starship was equipped with this technology, most famous of which is the USS Voyager. (VOY: "Caretaker")
The pylons were raised anytime a warp field was generated, even if the ship was not traveling at warp speeds. (VOY: "Learning Curve")
- Variable-geometry pylons were intended to correct the subspace damage spoken of in TNG:Force of Nature. The concept of the variable-geometry pylons preventing damage to space was not explained in an episode, but was conceived and included as part of the premise for Star Trek: Voyager. No other known class of starship incorporates this technology.