One of the problem with the hypersonic scram jet is that it does not work well at speeds lower than about Mach 4 or so, and a turbojet pretty much runs out of steam at about Mach 3, and even then it gets big and heavy and complex, as evidenced by the size and weight of the bypass turbofans in the SR-71 (bottom pic), and it looks like DARPA is has put the pulse detonation engines (PDE) at the head of a list to bridge this gap in its Vulcan program. (paid subscription required) (top picture)Similar in concept, though much different in execution to the pulse jet that powered the V-1 "Buzzbomb", it is increasingly being looked at as a way to bridge the gap between turbojet and scramjet.
Additionally, it is being examined as a possible replacement for both combustors in turbine engines, and for afterburners behind turbine engines.
One thing that I'm not clear on here though is why you need the turbine engine in the first place with a PDE, as they do operate at 0 airspeed, as evidenced by the (very noisy) flight of a Long-EZ powered by a such an engine about 11 months ago.
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