Star Trek: 10 Secrets Of USS Voyager You Need To Know
4. Two-ish Warp Cores
Two episodes of Star Trek: Voyager, "Day of Honor" and "Renaissance Man", specifically dealt with aliens trying to capture Voyager's warp core. In both episodes the core was ejected and the crew forced to recover the valuable tech lest they be stranded in the Delta Quadrant even more than they already were.
However, according the cutaway graphic of Voyager prominently displayed at the back of the bridge, the ship was actually designed with two warp cores. While the primary warp core was quite obviously located in engineering, according to the graphic, a secondary warp core was located directly behind Voyager's navigational deflector dish. On top of this, the filming miniature of Voyager also featured two warp core ejection hatches on the belly of the ship.
So why didn't the Voyager crew just let the aliens have their core and install the spare?
The ship's designer, Rick Sternbach, has speculated that the secondary core may not have been operational, but was a spare assembly used for parts that otherwise couldn't be replicated. In reality, the answer is simply that if Janeway had just ordered B'Elanna to install the spare warp core and get under way, both "Day of Honor" and "Renaissance Man" would've been short episodes.