The term penetration fighter has been used to describe a long-range fighter aircraft designed to penetrate enemy air defences and attack defensive interceptors. The concept is similar to the escort fighter, but differs primarily in that the aircraft would not operate in close concert with bombers. Both types are sub-classes of the strategic fighter. The same general mission is also carried out by intruders, but these are generally night fighters or light bombers that do not have the air combat performance of this concept.