Sans,
Please correct me if I'm wrong, but I understood the Groen (DARPA) machine to be a "tip jet" aircraft. The Carter copter family use a heavily weighted tip to produce extra inertia when spun up, so the aircraft can't hover but it does do impressive jump takeoffs.
One of the proposed uses for the DARPA aircraft was long range SAR which would have required a hover capability, possible only with a fully driven main rotor.
I have seen the videos of the new prototype Carter machine and it is a real cracker though.
Cheers now