It seems in 1950's some countries investigated production of helicopters powered by ramjet engines on rotor tips - examples include American Hiller YH-32 Hornet or Polish JK-1 Trzmiel. The enthusiasm was short-lived - high fuel requirements, a significant load to the power transmission system, noise and high visibility on battlefield were significant disadvantages that ended the research programs which I know of.
Still, that's over half a century ago, and the technology has progressed quite a bit since then so maybe some technical problems of that epoch are now past? - The ramjets are small, lightweight and quite simple in construction, so despite the disadvantages the technology doesn't seem like a total dead-end. Was this line of helicopter propulsion investigated further? Could someone provide a progress report - if there is anything to report? Or is this research branch still dead, with no attempts to revive it?