Hypersonic speeds are defined as Mach 5 (3,805 mph at sea level) and above. Only a few specialised craft have ever achieved them, and those rarely for any duration. Outside of military aircraft, the only commercial planes to ever exceed Mach 1 were Concorde and its Russian brethren, the Tupolev Tu-144. Most aircraft stay well below Mach 1 for a number of good reasons, fuel efficiency among them.
So why will there never be a hypersonic passenger jet? Well, for a number of reasons, both technical and practical:
- Flying Limitations
Let's have a quick run through them. Heat is an issue both for materials and design. We think of the air around us as empty and light, but it's actually fairly dense and at speed it causes a lot of friction, which generates heat. Part of the reason planes fly so high is because the atmosphere is thinner, meaning less friction (Concorde flew at 55,000+ feet, most commercial planes fly at 30-35,000 feet). This puts the materials used under a heavy cycle of freezing and thawing as the plane speeds up and slows down (the outside temperature at altitude can be as low as -55°C). It also means you have to cope with expansion, Concorde grew by nearly a foot in length over the course of a flight. Due to the materials of the nose, Concorde was limited to Mach 2.02 as it couldn't cope with the higher temperatures faster speeds would generate and the whole plane had to be painted white to help reflect heat.
The SR-71 Blackbird, capable of a mere Mach 3, used to leak fuel on the ground because it was designed to expand so much during flight that panels and fittings had to be loose. We're looking at some big leaps in materials development to cope with hypersonic speeds on a regular basis.
Speed has other implications too, such as fuel consumption. Travelling at 80mph takes about 17% more fuel than travelling at 70, for example (figures vary from 10% to 25%). Punching through air, even the thin atmosphere way up high, incurs a huge amount of drag and the faster you go, the more it costs to attain and maintain that speed. That means carrying a lot of fuel, which adds even more weight and increases the size of the craft (creating more drag) while reducing the number of passengers (Concorde only carried 100, proposed high speed designs suggest 300 -- a 747 seats north of 400, an A380 over 500).
The current designs for hypersonic transport suggest using hydrogen and burning it with oxygen from the atmosphere. That keeps weight down as you don't need to carry as much fuel. The problem is producing hydrogen is expensive:
At present, to create enough hydrogen to fly 10 hypersonic planes from the UK to Australia every day would use up to 20% of the UK's national grid, according to one calculation.
Let's say you get the high speed engines to run on hydrogen, but you also need to power the aircraft when it's moving too slow to scoop oxygen out of the atmosphere. There are some designs that can handle both, but no one has even a prototype near ready yet, despite years of development and millions already spent (the Waverider was dropped from a B-52 don't forget, so it didn't take off, it bypassed this whole stage). There's major development needed in this area before anything can get into the air.
So much innovation means high costs. Not just to develop, but also to test and prove it's reliable and safe, this is technology even few space agencies have used before, let alone civilian regulators. You're talking new materials, new manufacturing processes, new engines, new fuel production techniques as well as new testing equipment and strategies. That all adds massively to the price. Concorde, a mere supersonic craft, cost £1.3 billion in 1976, adjusted for inflation that's £8.6 billion. Sure they were pushing the boundaries, but not like hypersonic would. So take that figure and multiple by ten, that'd be my guess, giving you an estimated cost of £86 billion. That's about half what the entire Apollo space program cost, for comparison.
Concorde was operated commercially by BA for a number of years, but only after having bought them at a vastly knock-down price, not what any commercial company would have charged them, which would have meant a much longer time to return investment.
Regardless, that would be fine if you could see a return on your investment, but there's no indication that there's a demand for this sort of high speed travel, which would be limited, certainly initially, to first class travellers in the same way Concorde was. There would always be people willing to pay for speed, but even business travellers tend to look for bargains, taking multiple legs to save money. Not to mention that the most commercial route for Concorde was London to New York, but a hypersonic plane couldn't get to altitude fast enough to do that route, ruling it out and taking out a vast amount of demand. Other routes would open as China, India and some of the other developing nations take a bigger role in the world, but would people be prepared to save a few hours? Let's not forget that in Concorde's heyday, the internet was barely around, now telecommuting and video conferences are growing, enabling instantaneous travel.
Lastly we have flying limitations. Concorde was intended to fly to LA, but the US banned over-land flight due to the sonic booms it created. Assuming the same overflight rules would apply (even extreme altitude doesn't stop it) then you limit the routes an aircraft can take significantly, having to skirt all land masses not only makes routes longer, but, as with Concorde, once at speed, these planes won't be able to change course.
So there you go, a few reasons why all the talk of hypersonic passenger jets is rubbish. We're more likely to see the likes of sub-orbital spaceplanes, designed to climb to the upper atmosphere and then skip out and fall back into the atmosphere to generate speed and save fuel. These will be more rocket than plane and, as such, are much closer to the current developments by commercial rocket enterprises. But even they're a long way off.