Thanks for the information guys - fascinating stuff. As for contra rotating spools in modern engines, you can sort of see why from the gas flow/reaction point of view, but any bearing between two spools will naturally have elements running at twice the speed of a fixed outer/moving inner arrangement and several times that of spools rotating in the same orientation. In that case I imagine one spool would be running at a percentage of the other and thus the relative speeds are quite small.
Roger.