Err, think about it. What single LPT turbine blade could aerodynamically convert all the gas path energy exiting from the HPT into torque powering a 115" diameter fan and a three stage booster? What would the airfoil look like? How big would it be? What material would it be made of to withstand the heat? What would be the efficiency compared to a 4 or 5 stage LPT? Why don't we have single stage LPTs on high bypass engines after all these years? Let me know what you come up with.
Turbine D is offline
Oops, I was not thinking about a large Fan engine since the OP only introduced a small engine multi-stage LPT as the subject.
I still don't know what the limiting dimension is regarding my question. Obviously it hasn't been overcome by the manufacturers, I just want to know what part is key. I would of course accept that an assumption of a current HPT exit temperature and pressure should be presumed.