While it may not be economical, most cool ideas aren't exactly economical either. The American SST program was an awesome design, and had there not been all those tree-hugging hippies making a stink about how the plane would make too much noise and pollute the ozone layer (the effects were drastically exaggerated); it probably would have flown. As I understand it, most of the problems with the 2707-300 had been worked out by this point.
Looking at the design, it clearly uses exhaust-gasses to drive the aft-fan turbine; does it use exhaust-gasses to drive the core turbine, or does it use hydrogen expansion to drive the turbine? Looking at some of these diagrams, it is unclear.
J-D The drawings for Scimitar are apparently intentionally unclear, however it appears that the hydrogen pre-burner exhaust is also powering the hub turbine for the fan as well as providing some fuel for the rocket motor and afterburner sections. The main turbine appears to be an expansion turbine since it is not associated with a combustion area of the engine.
J-D, That may be true, but that central "combustor" with the convergent divergent nozzle is so close to one in geometry that there isn't much difference. I suppose the real question is whether or not the head end of that combustor is closed and able to develop high chamber pressures or not.
Following this thread with some interest from the antipodes. A country with red earth and hopping animals... That'd be one of the places that would benefit if this engine and the A2 design ever became a reality. Can you imagine how life would change if it was possible to commute say London - Sydney in 4 hours? I am by no means rich but I'd fork out $10,000 to fly just once on the thing - Mach 5 - that's a mile a second, people - X15 speed. Imagine the economic multiplier effects to business, industry etc. I would have thought $12bn would have been a relatively small development cost to pay for such paradigm changing technology - about the price of the A380? From what you all seem to be saying though - the Sabre/Scimitar design appears to be a long pursued pipe-dream. Are there any engineers following this thread who can offer a reasoned assessment of whether this thing will ever fly? Excuse my naivety, but personally I hope I live long enough to see the world's population able to commute at speeds internationally as they do domestically at the moment. Work the week in New York, and Friday night, commute home to Aussie for the weekend...