One of NASA’s charter roles is to identify and pursue the development of advanced engine concepts that have the potential to revolutionize propulsion technology. An example such advanced concept is the supersonic through flow fan (SSTF) – a candidate advanced concept for supersonic cruise applications. In this concept, the fan compresses the inlet air without diffusing it down to subsonic axial velocity as is done in conventional supersonic engines (turbomachinery cannot efficiently handle supersonic inflow). This avoids needing a large, heavy, complex supersonic inlet. Hence, theoretically, a fuel savings would accrue – provided the concomitant decrease in fan efficiency is not too large.  The early comparison studies between a hypothetical conventional engine (a mixed flow turbofan, MFTF) and the SSTF were encouraging enough to initiate a limited experimental research program to characterize the SSTF fan performance.  However, the experimental fan efficiency results and parallel CFD analyses were less optimistic about the fan efficiency than originally guessed during the early studies.  Consequently, updated studies performed with the revised fan efficiency values proved that the installed thrust specific fuel consumption (TSFC) was 4 percent worse than the baseline MFTF. This penalty overcame the savings in inlet efficiency, weight, and drag. Hence, the airplane takeoff gross weight and fuel load would actually be worse for the SSTF engine.  Hence,  the pursuit of the SSTF concept was terminated and the funding redirected towards more rewarding concepts.  

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