Fusion Warhead

Fusion warheads are powered up Atomic Warheads, that use nuclear fussion instead of fission. This has both positive and negative side effects, that force the missiles to be sufficiently different from normal nuclear warheads to be considered a diferent type of warhead altogether.
Nuclear Fussion warheads detonate with more power than their fission equivalents, but are much more troublesome to build, and tend to be more bulky. They are almost never encountered in LSK and conventional missiles, but more advanced countries sometimes arm their battleships with enough fussion warhead SK missiles for a single, potentially devastating salvoes.
On the other hand, there are really rare and it is actually doubtful if it really is a cost efficient weapon, which makes many fleet refrain from using it for cost sake.