I guess it would be hard to make the shaft turn at the same speed as the rotors. You would almost have to have the shaft mounted to the rotor and then you get into a whole bunch of other engineering problems.

 

yeah, if it was one solid peice with a fixed offset (stroke), then you'd either have only one working chamber all the way around (which wouldn't work... wait, no that chamber wouldn't work either), and if you had it fixed in a centered position in teh rotor, you'd only get a pinwheel; there'd be no compression. so then you also end up with 3 non functional "combustion" chambers...

yes, the design is as beautiful and elegant a solution as i've ever seen.