I’ve personally worked in rotating detonation engine (RDE) research, so I’d like to add some background on the field as well as my perspective on the novelty of the results in this paper.

But first, I’ll link some cool videos of RDEs firing on Youtube that you might enjoy.

Purdue University firing a RDE using LOX/Natural Gas (successful firing at ~45 second mark):
https://www.youtube.com/watch?v=pHcxI-8GtZg

UT Arlington video from 2009 of RDE running H2/O2 (it looks like a successful firing occurs at 3 minutes, 22 seconds).
https://www.youtube.com/watch?v=QbJgOk2rDbI

I agree with another comment here that it can be hard to tell when an advancement, such as the one claimed by the authors of the paper, really first occurs. However, the work of the article linked is certainly not the first time that a rotating detonation engine running H2/O2 has been demonstrated successfully or for an extended period of time (read: dozens of minutes). Some short tests using H2/O2 are shown in the UT Arlington video above. A wealth of research on RDEs successfully running H2/O2 can be found on Google Scholar going more than 7 years back.

AFRL East (specifically the Turbines and Combustion Branch at Wright-Patterson AFB) has been running and publishing results of rotating detonation engines using oxidizer/fuel combos of atmospheric air and standard liquid hydrocarbon aviation fuels for several years now. AFRL West (in this case referring to Edwards AFB, which was involved in the research in the linked article) is interested in RDE applications where there is no atmosphere to use as an oxidizer supply, so an oxidizer would need to be held in a tank. AFRL East is instead more interested in terrestrial applications, so RDE research work there has naturally focused on using air as the oxidizer.

There are several aspects of using liquid fuels and regular air that create additional challenge compared to using gaseous fuel/oxidizer combos (including H2/O2), not the least of which is ensuring proper atomization and mixing of the liquid fuel in the vary narrow window of time that it can be fed into the detonation annulus before a detonation front returns to consume them.

For a 6-inch diameter RDE, the detonation wave circles the annulus around 3000 times per second, depending on the fuel and oxidizer used and their equivalence ratio (the ratio of fuel and oxidizer in the annular detonation chamber). This creates an incredibly loud sound that is roughly 3 kHz, which is directly related to the 3000 times/sec the wave circles the annulus. Sometimes, two or more (in the paper they mention five) separate waves (distinct detonation fronts) will emerge, usually distributed equi-radially. This means that a detonation wave will pass a given point in the annulus five times as frequently, so the noise heard by an observer is then 5*3kHz = 15kHz.

When using aviation fuels in air-breathing RDEs, it is typically necessary to preheat the liquid fuel before mechanical atomization via injector, which is easier in H2/02 configurations. Hydrogen and oxyen are stored in tanks as liquid and may be quite cold, but between raising their temperature before injection using waste heat [0] and their higher volatility, getting them to atomize and mix is considerably simpler.

At my former employer, when we tested new parts for our research RDE rig that were fresh from the machine shop, we would run H2/O2 or H2/N2O as an easier cheat test to see if there were any glaring manufacturing issues before moving on to the more involved air and liquid aviation fuels tests.

So yes, I’ve personally seen larger RDEs running on H2/O2 fire literally hundreds of separate times. I can think of five or so other institutions doing RDE research that have already accomplished (and published on) more than what I see in this article. I love the RDE research space, but the linked article has me scratching my head at their claim to be first.

[0] https://en.wikipedia.org/wiki/Regenerative_cooling_(rocket)