Regret bump: I'd lost access to this site for several months...
D'uh, there is much discussion about that tapered wave-guide notion...
Problem is how you do the math: If it provides thrust, we'll all learn to do the math to suit. This is known, IIRC, as paradigm change...
If, per 'standard' version, it does NOT provide thrust, we'll toast the Old Math and drown our accelerated interplanetary dreams with same tankard of terrestial plonk...