Think about it in terms of an old-fashioned gunpowder line fuse: If you lay it out in a ring and have some kind of mechanism to continuously place down new gunpowder on the ring in front of the flame, you can keep it going until you run out of fuel.

Rocket engine efficiency, like all heat engine efficiency[1], governed by the difference in temperature between hot and cold in the cycle. Because at any given point in a detonation engine the combustion engine only comes in pulses it can reach temperatures that would otherwise melt the rocket engine. That's true of regular rocket engines too, which use active cooling, but seems more true of a detonation engine. And I would guess that the exhaust, already being supersonic, needs less of a chock on the de Laval nozzle to ensure laminar flow.

[1] Strictly speaking this only applies to combustion rocket engines. An ion engine, for instance, is a rocket engine but not a heat engine.

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.