Thrust in execess of that required for cruise and flight maneuvering is necessary to provide an aircraft with a VTOL capability. The extra thrust may be obtained by enlarged cruise engines with thrust vectoring or by retaining the optimum-size cruise engine possibly with thrust vectoring and adding a lift power plant in the form of lift jets or lift fans. A brief outline of extensive experience with lift jets, thrust vectoring, and lift fans is given and the importance of this background in making it possible to design more advanced engines which will satisfactorily meet practical operational requirements is brought out. Experience in two generations of lift-jet flight testing has shown many important areas where specal features must be incorporated in the design from the beginning to achieve high thrust for a compact volume, light weight, and high integrity in the relatively severe environment in which the lift jet has to operate. Examples are cited. The relative importance of thrust/volume and thrust/weight ratio is shown with reference to VTOL strike and transport aircraft. An integrated concept, using compact lift jets for VTOL strike aircraft and compact low-efflux-velocity lift fans using the same engine as a gas generator, is briefly noted.
