Expert: Mark Janus - 10/2/2007

Question
Hi,

I've read that only 8% of the thrust for a supersonic aircraft is generated by the engine, but the bulk of it is actually generated by the inlet and diffuser.  Could you please explain why this is so?

Thanks!

Answer
Rajesh,

I'm not sure of where you read that, but...  For an aircraft that goes supersonic (like fighters, and high speed bombers), in general the air going through the engine is subsonic (except for very high speed aircraft which may use a RAMjet or SCRAMjet).  For a typical jet engine configuration, there is an inlet and "subsonic" diffuser leading into the compressor face.  These devices are designed to slow the flow down (from supersonic to subsonic speeds before entering the engine). The engine then accepts this flow (that hopefully has been decelerated efficiently) and then further compresses it to then add thermal energy (in the combustor).  The high pressure/temperature low speed flow is then (re)accelerated to a very high speed which subsequently lowers the pressure to (ideally) a pressure equivalent to the surrounding pressure.  A typical subsonic combustion turbojet may derive some "thrust" from the pressure differentials seen on the inlet/diffuser combination, I believe this will be mitigated by pressure differentials further down the system.  
On the other hand, for a RAMjet or SCRAMjet this is not the case, as some thrust does come from the inlet system.  Actually on the SR71, whose engines operate as both a turbojet (in low speed) and a RAMjet (in high speed operation), I believe around 10 of the engine's thrust comes from the inlet cones.  This is simply due to pressure differentials on those surfaces that are NOT mitigated further downstream.  

Hope this helps a bit, and sorry for the delay in getting back...
mj