[mkn]

I've always been skeptical of these liquid-air cycles, but this scheme strikes me as especially problematic in that an ejector ramjet would be mechanically simpler and have the same top speed. For the curious, an ejector-ramjet (or ram-rocket or air-augmented rocket or any of a number of other names) is just a ramjet with a small rocket motor behind the center body of the diffuser. The exhaust plume entrains the inlet air, boosting the combustion chamber pressure by cooling the rocket exhaust plume. (It also acts as a fairly, err, robust flameholder.) If you're wed to LH2 as a coolant, just bleed it in along the walls of the rocket combustion chamber, and it will mix with the main mass flow and combust. Et voila! The cooling system is the fuel injection system for the ramjet mode.

Now you just spend all the compressor and heat-exchanger machinery weight that you saved by not going with the Skylon engine by having dedicated thrusters elsewhere on the vehicle for the pure rocket mode.

As someone somewhere else said, Skylon strikes me as a characteristically British solution to the problem, just due to it's sheer over-reliance on plumbing.