"Dan Horton" wrote

There is nothing that eliminates a long shaft from the design of a
PSRU. Nonbelievers might be advised to consider ship propulsion; long
shafts, low cylinder counts, propellers operating in uneven flow, often
via a gearbox. Sound familiar?

I think you will find that they do it on ships, with pure weight. A big,
heavy, solid steel shaft. Very heavy! That is how they get the stiffness.

Also, the shaft turns very slowly, so there are many pulses per revolution;
more than you will get with a 4 or 6 cylinder, 4 cycle airplane engine, in
most cases.

I agree with the rest of your post; dig into the engineering text books.
--
Jim in NC