Lomapaseo – quite right, we do (and should) pick up stuff around the edge of the discussion, however I was addressing what I considered to be the main question. And yes – it is fun.
Lyman, you said “Keeping the gas going in the "right direction" is everything to do with propulsion”.
I’m not disagreeing with you – I was suggesting that the reason the compressor is there is for fundamental cycle effy reasons and not primarily to set the flow direction (it’s perhaps more of a philosophical point I am making here).
You also said : The starting cycle is a simple way of understanding the need for compression. Without compression, there is no work. How can compression become irrelevant to the process at any time?
I’m certainly not saying compression is irrelevant to the process. I’ve established that it is fundamental to getting work out of a machine at a good thermal efficiency. Work is required in the start phase just as in other operational phases (steady-state, accels and decels alike)
But starting is a means to an end, it is all about getting the engine to a stable, self sustaining operation to deliver the required thrust. Starting needs compression, sure, and it needs external cranking power (or windmilling). And yes, compression sets the flow direction, though you have to be especially careful to control the fuel flow (and variable geometry if you have any) otherwise the compressor might get upset (stall). Same comment applies for ‘normal’ (engine-started) operation where reverse flow may occur under some circumstances.
‘Compression is our friend’ – certainly, it keeps me in a job. Compressors however are a nuisance!