UTMC

A craft that circumnavigates the earth near the surface, in the upper atmosphere or thereabouts? Such a craft would probably have to expend more energy on overcoming the enormous centrifugal force (yes, I know, but how else will you describe the effective force that applies??) than on propulsion. In low earth orbit, the shuttle takes a positively pedestrian 90 minutes to go around. Go faster than the shuttle and the orbital period actually increases, as you move further out the faster you go, all the way to geostationary. Go faster than that, and the earth starts overtaking you.

Hm...now I feel the need for math, dammit.

The earth's mean radius at the equator is 6,378.1 km, add 80 km for altitude that's still in the atmosphere, of sorts, 6,458 km, 6,458,000 m.
That gives a circumference of 40,576 km. To cover that in ten minutes, you need to go 243,460 km/h, or 67,627 m/s.
I can't be bothered working out the angular velocity (I always hated radians), but the internet provides, doesn't it.
According to that, g-forces aboard that pulse-jet would be 72g, subtract 1g for earth gravity pulling the other way, that poor pilot would weigh 71 times what he weighed at breakfast. If that pulse-jet was indeed manned, it was manned only once, and only for a very brief period of time.

I'm not saying it's impossible, going around the globe in an hour at that altitude would only cause about 2g, subtracting earth gravity, that pilot would just be sitting upside down quite happily (unless they build the plane to fly that way around to begin with, I dunno what sitting upside down for an hour does to you, but I suspect it's not nice).
Since the 1950s, here and there, some people have proposed arming ICBMs with tungsten rods as kinetic kill vehicles, for instant retribution without having to wait the hours and hours it takes a bomber or even a cruise missile to get to a distant target, but without having to go nuclear. Had anyone come up with an engine that could shove a plane around the world in an hour, it wouldn't make sense to keep the rapid responses this enables a secret. The point is that your enemies fear you, so that they don't do shit to begin with. Having a dagger up your sleeve may be useful in war, but constant domination of the world stage is a different matter. Currently, to keep up that domination, the USA is spending billions and billions every year for all those carrier groups constantly keeping watch on distant corners of the globe. So that, if necessary, the US can bomb anyone who acts up within a day or so. If a super plane that fast existed, all of that, more even, could be achieved without a single boat in a single ocean, just from an airbase somewhere in the middle of the US. Just to get the republicans to cut defense spending, or just so that the Air Force can get one over on the Navy, someone would have revealed that such amazing capabilities exist.

I recall reading about pulsejets and super secret super secret squirrel planes like that since the 1990s. Surely, they played around with some truly top-secret stuff, like drones and stuff, but had they actually come up with a game changer like that, it would probably have gotten out by now. Anything that goes around the earth must, by definition, at some point cross the airspace of someone else. Not moving in space but where there's actually some atmosphere for that jet to breathe, they would have been picked up by radar and the clever sensors that pick up sonic booms. Not to mention that, traveling at more than twice the speed of that rock what hit Russia, they would have been about twice as visible. No stealth technology can hide air glowing white hot from that sort of friction. If there was, indeed, a ridiculously fast plane, even one not quite as fast as that, and had that plane flown more than once, more people would have noticed, wouldn't they. These people would have been military, most likely, watching airspace and all that. Complaints would have been made, diplomats would eventually have gotten involved. Diplomats who would have been briefed that, not only must they deny any US involvement in that ghost plane, they must do their darndest to make sure information about that plane doesn't leak to the public.
And then Wikileaks would have unmasked all of that.

I suspect most of the stuff that's been going on at Nellis and other places like that in the last two decade or so has been testing of fairly mundane weapon systems, upgrades and new tricks for existing weapons and planes and developing drones. I further suspect that the vast majority of "evidence" for super planes being tested can actually be explained in quite mundane ways, and the few that are actually down to secret hardware will really not be that much different from effects cause by un-secret machines.

Sisyphus wrote:Recently they've developed molecular structures that can bend light, hinting at the ability to make something invisible. Nobody saw that coming ten years ago, either.
Still, I'll believe it when I see it. Or not. Whatever.

Yep, true, that they did. And that's the next big thing, invisible tanks and perhaps invisible soldiers. So you can do in broad daylight what right now you can only do at night.
But none of that is in any great conflict with known laws of nature, as the practicability of a pulsejet would be.
Note that, while battlefield invisibility is probably the mother, father and creepy uncle of the holy grails of military technology, and while that is a technology that would perhaps be more useful to keep secret, so that you can put a whole new meaning to "covert ops", we still got to read about it in WIRED and other such publications.

As regards fast airplanes, the top of the heap appears to be what the aptly named "Reaction Engines" have developed, a jet engine that runs on liquid hydrogen. Effectively the sort of rocket engine we all know from the Ariane rocket, but that breathes air. Took them about thirty years to develop, they reckon they can go mach 6 with that puppy. Normal turbojet engines would explode before getting close to that, what with the pressure inside the engine climbing not with thrust but with the speed at which the air goes in.
Mach 6 is about the speed the X-15 managed, really just a rocket, but with stubby wings, so technically an airplane.
The people at Reaction Engines are working on an SSTO satellite launcher, a literal spaceplane, but prompted by Brussels bureaucrats who'd like to take their secretaries on a weekend trip to Australia, have also done the math on a plane that could go at mach 6 from Brussels to Australia. Over the fucking North Pole, so it doesn't have to slow down for going over inhabited land. With their engine, that's doable.
There's another reason to doubt that a game-changing ultra-fast plane that can circle the earth in single digit hours exists. If it did, Boeing or one of Boeing's subsidiaries would have to be involved, at least if it's a US project. Right now, Singapore Airlines uses an Airbus for the longest direct-flight on the planet, the 18 hour haul from Singapore to Newark. They'd like to grab all that business for themselves, with the 777 and the Dreamliner, and they have seen once before what happens when you make air travel a few times faster and one plane can suddenly do the job of many. It's what happened when the 707 replaced the Super Constellation and other propeller aircraft, such a super-fast plane with actual long-distance capabilities (unlike Concorde) could do the same again. Sure, fuel would be a question, which is why the idea of using hydrogen like with the Reaction Engines reaction engine appeals. But anything that could do the whole planet in around an hour? That would have to be utlra-efficient fuel wise, as you can hardly alight in something the size of an oil-tanker, as it would have to be with traditional hydrocarbon energy sources. Even if such technology can only work supersonic, it would still have to be a lot less thirsty per mile than existing planes. Running costs are the a, b, c and all the other letters of the alphabet for the airlines, it's why even BOAC would rather buy Boeing than Vickers.
Being able to outrun the competition on the trans-pacific and other trans-oceanic routes, being able to haul many more paying passengers by just doing the run in much less time, passengers who'd even pay a bit more for the short flight, thus being able to operate cost efficiently, all of that would be what such a super secret secret squirrel piece of airplane technology would have to be able to do. No matter how many stamps of Top Secret the Air Force, Pentagon, CIA or anyone else were to put on that, Boeing owns way to many senators to let that opportunity to grab a vast chunk of the airline business pretty much without any competition slip through their hands.

Yes, the possibility does exist ... but it's exceptionally remote, and with the circumstances for keeping that secret I described, a bit more remote even.
Actually, some people do sit down and dream up wacky conspiracy theories. They just don't see it that way. They get a crazy idea, like that the US may have developed a super secret super secret squirrel plane. Rather than look at the evidence to make up their mind that such a plane exists, or doesn't, they collect "evidence", clues they decide must prove their theory, regardless of any alternative explanations. They already "know" the truth, right, so it all fits nicely into their closed worldview, anyone who doesn't see it is a sheep or in on the conspiracy! That sort of thing is a common thread with the moon-hoax nutjobs. Yes, we did land on the moon. It was a decade long effort, at the cutting edge of science and technology, conducted in the public spotlight where the technology and the math behind that theory could be examined by anyone. In the 1950s, as rocket technology began to take off, folks like von Braun worked out that, indeed, you could build an enormous rocket that could carry a big payload as far lunar orbit. That payload could be big enough to include a lander, life support, and stuff for getting home again. It was mostly a matter of working out the engineering details. Challenging work, truly.
Also, it should be possible to build an airplane that goes Mach 6 or a lot faster - all you'd need is a rocket engine. You wouldn't be able to go very far, because air resistance will have you running out of fuel in quite a hurry, conservation of energy, you can't argue with that sort of thing. The Saturn 5 needed it's entire first stage, 5 million pounds of the 6.2 million pounds the whole thing weighed, just to get to 67 km altitude and about mach 6. Other rockets don't even bother going faster than mach 2 on their way up - it's a waste of fuel if you're only headed for LOE.
Conventional air-breathing jet engines, or even highly unusual air-breathing jet engines, they can only handle so much pressure before any material known to man will rip apart. Ramjet engines are a different matter, and have been well researched, but they don't work well above Mach 6, either, because things just get too hot and pressure gets too high for air to stay, you know, air, chemistry starts acting different under those conditions.
Even the Reaction Engines engine, which supercools incoming air to almost-but-not-quite liquid state and then pumps it into a rocket-engine style combustion chamber is limited to Mach 6 or so. According to the research company that developed it and would have quite a bit of incentive for overstating the engine's capability, to goad investors.
Who knows, someone may find a way past all of this, someone may have already done so, but it's unlikely they'd be able to keep it a secret, and it's unlikely with what is known about the physics of the matter. Unlike the rockets and computers of the Apollo program, which were only a lot bigger and a lot more advanced than their predecessors, but not fundamentally different, going that fast in the atmosphere requires a whole different set of tricks. As for building the much more advanced and much bigger versions of previously existing technology, it took NASA a massive effort for almost a decade to get there, a similarly large effort, large compared to other efforts in that field of engineering if not quite as large as the Apollo program, would probably be required for building airplanes a lot faster than mach 6. Going around the world on one tank of fuel is something which only Burt Rutan seems to be able to do so far, both planes that did it that I'm aware off were designed by him, and were fairly slow even by regular airplane standards.
The moon landing is one of the reasons for many conspiracy theories. Those who agree that man actually walked on the moon can't believe that we haven't figured out how to do certain other things.

A "glider" that needs an ICBM (Minotaur launcher is a de-militarized Peacekeeper missile) to get going. An ICBM built to haul 10 warheads, that's about the biggest size they make or made solid fuel rockets, isn't it. Quite a ways from a jet, pulse or otherwise, but gimmicks like this one would be one possible explanation for some of the stuff the tinfoil hat crowd get so excited about. But, honestly, this is one technology you cannot keep a secret. Nope, can't be done. In the fifties, maybe, but not when the whole surface of the planet is constantly being monitored by satellites which make up the early warning systems of Russia and probably also China. These things can spot a big rocket engine going off, it's infrared signature, it's how a "launch warning" is generated for the nuclear forces to respond to. That is one of the reasons why the US notifies the Russians each time they want to launch something into space, why everyone is notifying everyone else before they launch something. A missile launched from the continental US, where there still are minuteman silos, would be especially notification worthy. When such notification failed in 1995, and a sounding rocket launched from Norway reached the sort of altitude US SLBMs also reach, the Russian nuclear forces were put on alert. If they found out about a "secret" rocket launch for a glider like that, they would still go very fucking apeshit, so one way or the other of keeping it secret sounds unlikely.

However, going that fucking fast ... you all know about the Shkval and the Barracuda, supercavitation torpedoes, right? These things can go several hundred mph under water by forcing apart the water in front of them, creating a cavity around their body, thus avoiding friction.
I recall hearing about the Russians having worked out a way to do that ... with airplanes. Not using a blunt nose like the Barracuda, but by somehow turning the air into a plasma at the nose of the plane and then using an electrical field to guide it around the plane's body. How the wings would work then was either not explained or I don't recall it. BUT one of the problems of hypersonic flight appears to be that, at those sorts of speeds above mach 6, the friction forces air molecules to dissociate and turn into a plasma. Hence comes that extra bright glow, doesn't it. A tinfoil hat idea, truly, but an intriguing one. If I had to design a super secret hypersonic spyplane for the reptilian overlords that run the UN, that's how I'd do it.