This chart, from the Mazda book "Rotary Engine" by Kenichi Yamamoto, of bearing 
load shows why rotor weight is the main limitations to RPM.
The rotor CG is making an orbit around the e-shaft center lines so 
the load on the rotor bearing is m*V^2/r.  m is the mass of the rotor,
V is the tangential velocity of the CG of the rotor and r is the radius
from the e-shaft center line to the CG of the rotor.

Paul Lamar
 
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