Slowrotor, if the mission is just to climb fast i have read books that prove that afterburners are as weight efficient as an additional engine (look at the EE Lightning). For long duration supersonic cruise the world had to wait for combustion chamber engineers to figure how to burn all that fuel, and turbine engineers to handle the temperatures (Concorde cheated with 4 engines). My point is that if there already exists a technology to give improved all round performance, why limit the machine to just that mission?
For +250kias i am convinced that X2 represents the next transition in aircraft capability. If you design a multiple rotor machine you are designing in antiquety. Study X2 and figure how you can make that more cost effective for reduced power/weight piston engines would be my starting point.
Actually i quite like Dave's latest interleaver concepts (but don't tell him that). My experience as a design engineer has taught me that the solution always works it's way towards the simplest design. This is because the commercial pressures on a product force the fewest parts to do as much as physically possible. To me one rotor above the other is simpler than X-shafts.
Dave, i think we've established use of the word pressure is acceptable, if not wholly accurate. Incompressible gas is a mathematical convenience, which it seems does not trully apply to the real world. I'm itching to get some CFD time to understand this better...