But what sounds really appealing is SYD-LAX in 4 hours or so
Would that really requre Mach 5 ?
I can really not see an according market for hypersonic travel. I think it is quite safe to assume, that cost will increase at least linearly with Mach, so would it really make sense to go that fast/high with all the implications it means?
The decision to only go for Mach 2 made the Concorde reality, the plan to go higher killed the American SST. Technology may be different today, cost probably still is the same.
I can see a market for supersonic travel, on high demand routes at barely 3 times the speed of today (which would require less than Mach 2.5, still manageable with "conventional" technology). What difference does it make, to take 1.5 hours instead of 2.5 hours NYC-LON, if you anyway need another hour to get from the runway to your meeting? Would anybody pay the extra for such a small gain? Does it make sense to accelerate/climb to Mach 5 and the according altitude for a 2 hrs flight? Would scramjets be really that more efficient?
Hypersonic probably is not the answer to civil air transportation issues. But anyway it is highly interesting technology.