I think your reasoning is, even in very simple static terms, dead wrong. Let's look at scaling a walker. If you double linear dimensions, mass goes up 8x. Cross-sectional areas of bones, etc. go up only 4x, so pressure (remember, pressure is a per-area quantity) on them doubles. Now, halve gravity. You halve the pressure, and you're back to the level of structural stress the original walker experiences. Thus, in half gravity, the walker can be 2x as tall, 8x the mass, still 4x the weight. I don't see how maximum total force of gravity (IOW, weight) has anything to do with it.

Yes, I know it's escape velocity, but given the limited range of densities feasible for solid planets... I'm not talking about making somewhere like Saturn.

"That is then the maximum amount of gravitational force a flyer can cope with." That makes no sense to me. I don't think it's nearly as simple as a limit on gross weight. "the ability of an object to keep itself standing is proportional to the area." But even a walker isn't standing, it's walking, it experiences dynamic loads which are greater than static loads. And I'm talking about flying, and wondering at what size the dynamic loads will break it.

I can follow what the phrase means literally, but I don't know its implications. That's why I asked this question -- in the hopes someone would figure things out and explain their conclusions about the resulting world.

And... I don't normally think about burning as having a "rate" or "speed", because I don't really know what that means. What are the implications of fires burning faster but not hotter? My emphasis isn't on what a technological species can do but on whether there's a significant threat to pre-technological life.

And acetylene isn't the most useful example. It's not something you find naturally in significant quantities in Earth's atmosphere AFAIK, presumably because it's highly flammable already. My big concern is wood.

"Within weeks" and "all three" sound overly restrictive. I think it can be done with only two (US, DE) given that the real Cold War started without SU having the A-bomb yet, and given the late-war Allied advantage IRL, the Axis getting the bomb more than a few weeks ahead of the Allies could still lead to a standoff, I expect. But the general concept is sound. In particular, this kind of scenario offers the easy "out" for JP: the war can still have turned against them, but DE can then say to US, "If you invade Japan, we drop the A-bomb on Britain," forcing an end to the Pacific War.

@user2352714 "Imagine if in the IRL Cold War there was a third superpower who would benefit immensely if the US and USSR nuked each other. " I guess I've been operating on the assumption that they realized / believed the "nuclear winter" outcome -- that is, a large-scale nuclear war between two major powers would wipe out all civilization on Earth even if it wasn't directly targeted. (That's talking about the 20th century -- my story's "present", due to spaceflight-related tech developments, has reached the stage where nuclear war is no longer MAD, and there's now a big risk of a hot WW3...)

"A stoichiometric mixture of Acetelyne into Air at sealevel pressure only burn(deflagrates). The exact same mixture at 2 bar(or higher) of pressure detonates" IOW, you're using 2x the partial pressure of acetylene gas as well, so is that a fair comparison?