IC
right about the tip jets thing, in city local airpports it can be a problem, not talking about the difficulty to develop a such system.. but, indeed, it has an interest on flight satefy by giving the rotor a residual power (nothing to do with a full powered tip jet rotor such as the SO-DJINN).
The hydraulic prerotators seem actually the most efficient, but not usable in-flight.
About Groen bros. i stated that they used ram tip jets, but i was wrong, i have read it on a news but it maybe for one of their "projects". But he system has been tried on other rotorcrafts
the Fairy ROTODYNE was an amazing bird.. a shame it has not been developped further.
cheers