Originally Posted by
Agent153Orange
I personally think hypersonic transports (Mach 5+) will never be worthwhile: this is because of (1) exorbitant development and production costs even compared to supersonics (Mach 2-3),
I think that's the biggie: building a prototype suborbital transport would probably be cheaper than flying it enough times to demonstrate that it's safe to use for regular passenger flights. Otherwise you'd probably have to develop it to launch things into space and then when it's proven there start simplifying the design to use for suborbital passenger transport.
But note that a suborbital flight would get you anyhwere on the planet in less than an hour, as it would peak at much higher than Mach 5, and I suspect it would probably still be cheaper than building an aircraft that could sustain Mach 5 in the atmosphere. The downside, as one SF writer put it, is that half the time a passenger can't get to the toilet (because you're accelerating or decelerating) and the other half they can't use it (because you're weightless).
(2) ginormous fuel consumption required at such speeds;
It's not that bad. The space shuttle burns about 10kg of fuel per kg it puts into orbit, so if the mass of a passenger and associated structure could be kept to half a ton, a suborbital London to Sydney trip could be maybe four tons of fuel per passenger. LOX/LH2 appears to cost around a dollar a kilo, so at $4,000 that's about the cost of a Concorde flight to New York, isn't it? And if you were burning thousands of tons a day, I'm sure you could get prices down.
The problem is getting the maintenance and turnaround time down to the point where the fuel is the major cost rather than maintenance (AFAIR the space shuttle's LOX/LH2 makes up less than 0.1% of the cost of a flight). Which goes back to flying the test vehicles an awful lot of times to thoroughly debug the system, which goes back to requiring vast billions of development funding with a fairly dubious market at the end of that.