Thanks for the good look you took at the number
I really like the idea of a 150 seat twin TP-400 powered airliner, but more hard numbers are needed to assess its feasibility.
I don't think we have to go to deep for that one.
I took some data from a nice MIT study of the various relevant efficiencies influencing overall efficiency for different aircraft & propulsion types.
Report by
Raffi Babikian, Stephen P. Lukachko and Ian A. Waitz can be found at: http://web.mit.edu/aeroastro/people/...s/Babikian.pdf, 2003?
As can be seen in the graph Turboprop engines tend to be 15-20% more fuel efficient then turbofans. main driver of this is the very high BPR of over 1:30. The engines used on 737 and A320 (CFM56 / V2500) are improved versions of 20-30 year old designs, the limited BPR is only 1:5. The TP400 with the latest technology build into a blank paper design is probably closer or even over 20%. SFC probably around around .4
As you mentioned Turboprops are also structurally more efficient then jet aircraft. This has to do with their limited flight envelope and dedicated short haul passenger design.
This structural efficiency results in lower weights and lift induced drag. Taxiing efficiency is also much better for high frequency operations. Other efficiencies are of a lower order. These considerations made me conclude a 25 % more fuel efficient operation should be possible with the Propliner compared to e.g. a similar sized 737 or A320.
Question arises why it hasn't been tried earlier. I think fuel was a smaller part of total operating costs. Increased speed (=frequencies) and medium range flexibility were more important. Nobody felt the need to develop a higher BPR engine like open rotor or real big turboprop.