Affordable Smaller Turbofans

Abstract

Smaller smarter un-manned air vehicles (UAV’s) and missiles are being proposed and developed to function in a variety of roles previously performed by larger manned aircraft and limited range missiles. The endurance and stealth demanded of such advanced vehicles requires small turbofan engines possessing relatively higher power density combined with increased thermal efficiency. It is shown that these two major performance related criteria are indirectly related to improved affordability for a specific turbofan design configuration. Changing the design configuration by implementing lower component count and flowpath simplification however is an alternate trade-off route to improved affordability, albeit at a slight performance decrement. As an example the classic two spool (2SC) turbofan with concentric shafting for the inner spool is the engine of choice for extended range applications, but can present shaft dynamic and bearing lubrication mechanical design concerns as size is reduced and core rotational speeds approach and exceed 100Krpm. Preliminary design features of two alternate non concentric drive small turbofan flowpath candidates are presented, neither of which can match the full performance capability of the classic two spool concentric drive turbofan, yet they potentially offer several affordability virtues, inasmuch as lower component count, reduced cost, higher reliability, oil less lubrication, and electric driven accessories.