Wasn't sure from text that Sikorsky XV2 was a tip jet, 21st Century, but i can't see a means for torque reaction either...
Actually, what i was interested in is why tip jet suffers from such low fuel efficiency. A ram jet is a non-starter, because it would be operating nowhere near fast enough. However, a turbo charger or radial compressor is limited by tip speed for similar reasons to a rotor blade (compressibility). Such a compressor will generate ~2 bar in unideal conditions and ~4 bar in ideal conditions. This means that if the nozzle was designed to convert all pressure to velocity (convergent-divergent if necessary) then even a tip burning design should be reasonably efficient. Similarly if the design was ducting compressor bleed it should also be efficient (ignoring duct losses). The nozzle area is relatively small, but travelling at high speed so you would still get a good mass flow rate.
The question may be unfair on this forum, but there are some smart engineers out there who might chip in.
Mart