Eventually, all projects involving ZELL aircraft were abandoned, largely due to logistical concerns, as well as the increasing efficiency of guided missiles having rendered the adoption of such aircraft to be less critical in the eyes of strategic planners. Furthermore, the desire to field combat aircraft that lacked any dependence upon relatively vulnerable landing strips had motivated the development of several aircraft capable of either vertical takeoff/landing (VTOL) or short takeoff/landing (STOL) flight profiles; such fighters included production aircraft such as British Hawker Siddeley Harrier and the Soviet Yak-38, as well as experimental prototypes such as the American McDonnell Douglas F-15 STOL/MTD.