At the 2011 biennial Paris Air Show (June 20-26), the European Aeronautic Defence and Space Company (EADS) announced a hybrid-engined transport airliner concept, called the zero-emission high-speed transport (ZEHST), that could be used to ferry people (about 60 passengers) at a cruising speed of Mach 4 and at altitudes of 105,000 ft (32,000 km). They predict that the travel time between Los Angeles and Tokyo could be cut down to a mere 2 hours and 20 minutes. The concept is similar to the Aerojet’s TriJet concept described earlier in that they plan to use three different engines in a combined cycle, including rocket-augmented ramjets. But unlike the TriJet, the EADS’ ZEHST uses a ramjet that will not transition into scramjet mode and will remain below the hypersonic milestone of Mach 5. To qualify for a zero-emission badge, EADS plans to fly the ZEHST on bio-fuel derived from seaweed. (I highly doubt that will work out!! There are many bio-fuel initiatives, including by the US Air Force, but the costs work against bio-fuel being an economically viable option in the near future. Large scale bio-aviation fuel production would also mean converting farmlands from food production to fuel production, which could mean rise in global food prices. Check out this article from the US Air Force, in which, the Undersecretary of the Air Force, Erin Conaton speaks out against bio-fuels.)
EADS showed a concept design with delta wings, twin vertical fins, twin underwing ramjets and rear-mounted cryogenic rockets and turbofans. It also featured helium, hydrogen and oxygen tanks running the rear two-thirds of the fuselage. Using the turbofans the aircraft would depart from a regular runway, then accelerate into the supersonic regime with rocket engines before the ramjets took over. It would then enter a glide descent until it reached a height at which subsonic engine control was again required, at which point the turbofans would re-ignite. Loiter, approach and touchdown phases would be conventional.
EADS said the study, which has attracted Japanese interest, is sponsored by the French civil aviation administration DGAC, and added that it formed part of its Flightpath 2050 initiative to explore potential future aviation concepts.
EADS predicts a prototype aircraft only after 2020. I think that if they are serious about it, they could certainly have a demonstrator in less than a decade.