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?

Yes, but they use mass for damping like solid shafts. Weight is
nowhere near the issue it is for aircraft.

Dan, U.S. Air Force, retired