Comment by hinkley

The estimates are that a fully functional RDE will be up to 25% more efficient than a standard bell/deflagration design.

Efficiency means less heat per kg of mass to orbit. And with the rocket equation, that’s a lot less engine per kg of payload.

Efficiency means either less heat, or intensely more incentive to solve the heat problem even with unobtanium. You could make the damned things out of molybdenum, which has 8 times the thermal conductivity of titanium, and a higher melting point.