In the US, a rotating detonation rocket engine takes flight

Steve Mould has a "rotating flame" video which also helps visualize this: https://www.youtube.com/watch?v=SqhXQUzVMlQ

My first thought when I read this: "I bet Scott Manley has a video about this."

> His channel has a certain flavor

It's kind of an anti-tomato flavor.

That makes no sense at all. How do you think a turbine engine works?

To be a bit more clear, the detonation is racing around a circular track in one plane, but the net thrust is in the direction perpendicular to that plane. Fuel and oxidiser are continuously pumped in all along the track, dropping to ~0 after a detonation and rising afterwards until the detonation front arrives again. Since you don't have to pump into a continual high pressure deflagration like in a conventional rocket engine this looks like it should be easier in terms of pump power.

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.

From my understanding, it's more that the explosion phase of a detonation is more fuel-efficient than the burning phase, but can't last very long under normal circumstances because the flame front is moving so fast-- This is a trick to continue fueling the explosion to sustain it over an indefinite time period.

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.

You've got the speed at which a detonation progresses within the explosive material and the speed at which the shock wave travels through the air. Neither is going to change based on the bomb's arrangement. But if you detonate a string of bombs so that each goes as the shock front from the first reaches it you can get, through constructive and destructive interference, a shockwave going mostly in one direction. But that's mostly not related to how this engine works.

What are ‘large primes’ here?

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.