That's a cost concern, not size and efficiency concerns that I was pointing out. Yes, turbines are expensive and are likely to remain so, even converted APUs. They run at very high RPM and so strength and balance are critical, and the turbine section has to be able to withstand high temperatures. They're not as simple as they look, either; they have complicated air-cooling systems. Only about 25% of the air is used in combustion; the rest is used for cooling.
The main reason that people often even care about efficiency is due to its effect on cost. We often don't care about efficiency for its own sake, or for the sake of design elegance. Cost is often a overriding factor. Piston engines are cheaper to operate than turbines.
Is there some intermediate choice between reciprocating piston engines and turbines -- ie. intermediate on both cost and efficiency?
Is it the Wankel rotary engine? Or is there anything else available?
Not necessarily, but the constraints you give yourself have costs elsewhere. In the case of the SF-1, the look was a big part of the whole package and there’s a lot of drag hiding in that design in order for it to look like an F-22 that you ordered from Wish.
I'd think that the pseudo-ducts on SF-1 would exert that duct drag which Malish said increases significantly beyond 100mph.
The pseudo-ducts on SF-1 don't produce any propulsive thrust like the ducts on PJ-2 Dreamer which have fans inside them.
And yet SF-1's pseudo-ducts have bigger cross-sectional area than the ducts on PJ-2.
But why is it that nobody cares about duct drag for turbofans? Is it just because turbines are producing so much extra power that it doesn't matter?
-in auer case=iff we have big thrust, we can ignore the drag !
(in contrary,for economic "electric" fly we need LOW drag...)
So turbofan ducts will have similar drag problems as ducted fans, but it's just that we don't care because turbofans have plenty of power to spare?
But I again wonder about ionizing airflow, because ionized airflow is naturally more laminar/low-drag than regular air (diatomic gas), and furthermore the air flowing inside a duct should be easier to ionize compared to air flowing around an open aerobody. Also, if you add structures like inlet guide vanes, then it provides even more opportunity for contact & ionization of airflow.
