Assessment of variable‐cycle propulsion systems for ASTOVL

The requirements imposed upon advanced short take‐off and vertical landing (ASTOVL) aircraft give rise to challenging demands on their propulsion systems. One possible approach is to have a high‐performance turbofan of traditional design and an additional, but separate, fan to provide a major part of the lift during the take‐off and landing manoeuvres. For such a design, there are several quite‐different choices of layout for providing the power to drive the remote fan by means of the core engine. These include shaft‐driven and bleed‐driven options. The choice will depend on the anatomy and required thermodynamic‐performance of the whole system. In this paper, several pertinent alternative engine‐designs are discussed. Four of these, based on a high‐performance low‐bypass‐ratio core engine, are studied in detail and their behaviours compared. Prima facie, the preferred choice is the engine with the shaft‐driven fan. A slightly less acceptable choice is the high‐pressure turbine exit‐bleed driven remote‐fan.