2 hours? What's the point? It could be 0 minutes with a 0% explosion rate and the total travel time would probably be longer.
Fuel efficiency is the answer for "why would you do this?" That's really important for rockets, since to launch each gram of fuel you have to have some more fuel which you also have to lift.
The point is reduced fuel use and faster response times (either emergencies or warfare).
Maintaining a detonation (as opposed to deflagration) is quite a technical challenge. If nothing else, it pushes mechanical engineering and material science forward.
At supersonic speeds, the efficiency of an engine like this can be very high, and at high altitudes, travel is very efficient. People don’t seem to get that there is potential for more efficient transport at supersonic speeds/ hypersonic speeds. The ultimate example of this a satellite. How many times does it circle the world every 45 min, all on that initial fuel it took to get it up there? It seems wasteful, but it’s actually the most efficient form of transportation in existence.
Of course, hypersonic transport would not be as efficient as a satellite, but still, at 100k feet, for example, you can go 5000km/h while only consuming the same amount of fuel that you would use to go 300km/h at sea level. The potential for efficiency gains are huge, and the fuel cost of getting to altitude is mostly returned in the descent.
Of course, the technical challenges are also huge, so I don’t expect to see hypersonic airliners any time soon, but when we do get them, they could easily be 6-7x as efficient as existing transports. And it would be nice to have long haul flights be 3x as fast or faster.
People mistakenly assume that this kind of travel only has applications for the military or the elites, and, like air travel in general, that will probably be initially true. But after the rich and the government foots the bill in lives and research to make it safe and economical, it should hold huge gains for society at large.
Science and engineering is one of the only cases where trickle down economics actually kind of works. In knowledge, a rising tide really does lift all boats.
> the fuel cost of getting to altitude is mostly returned in the descent
In energetic terms this is clearly true, but that leaves you at the end of the descent with very high speed / lots of kinetic energy. Great for missiles, because you dump the energy into the target. But I'm struggling to see how it's useful for any other application?
I would assume that any efficient hypersonic airliner would need to use variable geometry or powered manipulation of airflows to attain a reasonable efficiency in the transition zones of its flight envelope, so while you obviously would not get all of the acceleration energy back, it would transition into efficient lower velocity flight and reduction in engine thrust needed during the deceleration phase. But if we really knew how to do all of this we would already be flying transatlantic in an hour.
For the same reasons that theoretically possible doesn’t mean necessarily practical, theoretically difficult doesn’t mean practically impossible. We just won’t know until we significantly advance the state of the art. We might find out that atmospheric flight is a waste of time and that exoatmopheric ballistic trajectories are where it’s at. Or we might find out that zeppelins were the way all along.
We are still a long way off from having engines that can operate any where near theoretical efficiencies except within very narrow parameters and altitudes. Until we sort that we will have difficulty traversing a wide speed envelope without waste.
What about noise? So far all we have gotten is companies trying to build planes that still are very loud and produce audible booms all the way along their travel path. Pitched to the empathetic heros that are billionaires. And to sway public opinion they fly test planes at slightly over mach one in specific weather and altitude setting that avoids having the sonic booms reach the ground. While marketing and selling the idea of not mach 1.1 but mach 2-3 supersonics. Which is AFAIK completely impossible without the boom issue, even if made less bad - but arguably still bad - by fancy wing shapes. So all I see is selfish people trying to build something at expense of everyone else, including the environment.
Flying it the high limits of the atmosphere reduces sonic boom intensity due to both the extremely low density of the air and the distance from the ground. Ultimately, sonic boom is a sign of wasted energy, so if (big if) we can make aircraft that conform to the required aerodynamic ideals for efficient hypersonic flight, they will be relatively quiet.
Just like with regular air travel, though, I would expect a few decades of practical application for specialized service before it was commodified to be really useful for large amounts of passengers.
what about noise is you're supersonic noise and sound is about a mile back
I think the concern is for the frothing masses of earthbound unfortunates, not the passengers hurtling gloriously towards destiny miles above the troublesome surface of the planet.
Presumably, you have to be travelling quite a distance before hypersonic/high-altitude flight becomes more efficient than current commercial airliners.
Certainly not for short hops, but if regular aviation is any indication, it’s more efficient even if you don’t even reach cruising altitude before beginning to descend, so I’d imagine any flight that currently is around 4 hours or more would potentially benefit. (Given full technology maturity) that said, we are a long, long way from having efficient engines for that wide of a speed envelope.