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Railguns fire projectiles that don't require chemical energy to be carried onboard, i.e., they don't need a chemical propellant and usually don't have warheads, instead relying on the kinetic energy associated with their much higher velocities (compared to traditional shells). This means the...

Consider that the Bernoulli equation is simultaneously a conservation of momentum and energy equation. It can be derived directly from the more general versions of each. For example, if you start with the 1-D, incompressible Euler equation (e.g. taken along a streamline) \rho u\;du = -dp you...

They already released the preliminary findings. Just like they thought, tons of SIGINT equipment for measuring and triangulating signals. A propeller with a rudder. It's a spy balloon.

https://www.washingtonpost.com/national-security/2023/02/07/china-spy-balloon-intelligence/

Very little of this makes sense. The balloon had something like a 1 ton payload. It had a lot more than just a camera on it for taking photos. Further, you can get photos of silos from satellites and the silos don't really do anything worth watching anyway. They sit there in the middle of...

Years of grift and internal rot due to such pervasive corruption from top to bottom will do that, I guess. But it has clearly surprised the Pentagon, who originally thought this would all be over in a month or two... until the fighting started and Russia exposed itself as a hollowed out shell of...

Meanwhile, the Russians continue to talk to each other over commercial, unencrypted radios in Ukraine.

Just general use. The threshold for actually using a nuclear weapon is that the country basically needs to be facing an existential external threat thanks to mutually assured destruction. The threshold for using a balloon for collecting intelligence is basically zero. The Chinese government...

The distances between silos are usually quite large. Cables like that would probably be vulnerable to sabotage. There's no perfect solution. Any over the air comms are likely encrypted but even the pattern of use could be potentially useful. It's so-called pattern-of-life analysis that...

I didn't contradict that. I contradicted an idea earlier in the thread that balloons aren't a concern because China has nukes, which are scarier. The key idea is the threshold for use of a nuke is almost impossibly high. Not so for a balloon.

I haven't seen whatever he said. I suppose you could put a nuke on a balloon and it would be harder to detect but easier to shoot down than an ICBM. Also, if a country decided to openly nuclearize a balloon, it immediately means they can't use balloons for other more useful purposes because...

A good discussion of why balloons are pertinent: https://www.c4isrnet.com/battlefield-tech/space/2023/02/06/how-stratospheric-balloons-could-complement-space-based-intelligence/ The issue of nukes is irrelevant. It's a totally different mission and threat.

Sure, but it does offer one possible explanation why the government may want to let it overfly the country while gathering a ton of intelligence: so we can track them better in the future.

The length of a plane in the Embraer E-Jet family ranges from about 2 to 3 coach bus lengths, so it seems consistent. But I don't know if that is metric coach buses or US customary coach buses (also known as greyhounds).

That's a pertinent question and I don't know the answer. Normally the military (or CIA or FBI, for that matter) isn't allowed to collect intelligence on US soil. So I'm not sure who makes that call. I would guess probably the FISA Court would have to issue an appropriate warrant.

To be fair, it appears (according to NORAD) that those balloon incursions were only identified after the fact:

Some new details from USNORTHCOM via Aviation Week: Summary: The US military conducted a lot of signals intelligence on the balloon as it traversed the country, needed special permission to do so The AIM-9X was chosen in part to limit damage to the payload, had never been used in this way...

For one, the Chinese government is not a friend to the United States (or the West more broadly). The point of releasing information is to take control of the narrative more broadly. China knows what was on the balloon and what we likely now have to examine. We aren't really telling them...

Neither did we when it first crossed our airspace. Clearly there is more to this than just who can shoot the balloon down first. The thing was tracked all through Alaska, which is home to Joint Base Elmendorf-Richardson, home of the Pacific Air Forces and a squadron of F-22s. We obviously waited...

I don't see why you would assume Canada wouldn't have shot the balloon down had it stayed up there. If the US asked Canada to down it had it returned to Canadian territory, I'd bet they would do it.

According to the Pentagon, the F-22s were at 58,000 feet and the balloon was between 60,000 and 65,000 feet. Source: https://www.nytimes.com/2023/02/04/us/politics/chinese-spy-balloon-shot-down.html I might guess they intentionally fired from below to try to direct the blast frag from the...

Why would that be the plan? Canada is a NATO member, a member of the Five Eyes, and the other half of NORAD. Why would Canada be some safe haven from the US and why would anyone think they'd be any less bothered by an airspace violation by a spy balloon than the US?

It's likely faux outrage and the target audience is their own people and any sympathetic people internationally. They know we have the balloon and can easily tell if it really is a weather balloon (unlikely), but that won't get reported everywhere. It's all theater.

I suspect the official statement is only a half truth. Yes the risk of impacting people on the ground is higher, but I'd guess a bigger reason is that recovery would be more difficult. Even in a sparsely populated area, shooting it down over land would mean some local yahoos are going to come...

F-16s can't fly high enough. F-22s have the highest service ceiling of US fighters.

According to this (generally a good source) it has a laser proximity fuse and can also be guided remotely by the F-22 based on radar. https://www.thedrive.com/the-war-zone/f-22-shoots-down-chinese-spy-balloon-off-carolinas-with-missile

Why does the title refer to it as a "weather" balloon?

Because it's more than just heat seeking.

It will still be laminar over some small distance near the entrance. The question is how small.

https://www.amazon.com/dp/048664071X/ I'm actually shocked it isn't half that price.

Should I assume you've already explored references like Chandrasekhar?

Appropriate references are going to vary by the background of the student.

I've never heard it called orbital velocity before. I'll preface this by saying I'm not super well-versed in turbulence theory. If you look at a turbulent power spectrum, it is highly biased toward low frequencies representing the largest eddies. This implies that the rms of the fluctuations...

Failing that, get your hands on a textbook on the topic. They can be expensive but I am sure the industrious folks around here can find a way to get them. For the most part, a lot of thought goes into making textbooks suitable for teaching/guiding someone through a topic in a logical way.

Well that's not really accurate. Diesel engines are still "being researched" and yet have been in practice use for over a century.

That's only true in a viscous flow. Bernoulli's equation does not apply in that situation without modification for viscous losses. In a truly ideal flow, you do not need a pressure gradient to sustain a flow. Applying a pressure gradient to an ideal flow results in acceleration. In a real...

It turns out viscosity plays no role in hydrostatics, a huge role in very, very slow flows, and an increasingly small role as you get faster (or more precisely, as ##Re## increases).

Laminar versus turbulent flow has nothing to do with the original question here. Viscosity is an intrinsic property of the fluid whether the flow is laminar or turbulent. Why is this an either-or thing? You also have to be careful in how you think about molecules moving vertically to transfer...

Why are we paging me? I don't know jack about life as a CFD jockey. I'm a wind tunnel guy! o0) I think the best way to sum up the differences are as follows: Experimentalists have wind tunnels (or similar facilities) that "solve" the Navier-Stokes equations in real time over the full scale of a...

...to the last I grapple with thee; from hell's heart I stab at thee; for hate's sake I spit my last breath at thee

Is it even an artillery shell if it can no longer perform the basic duties of an artillery shell? On the other hand, you could make a shell of approximately the same size with, say, a ramjet integrated into it and keep the same supply chain and logistics processes in place and use the same guns...

AA missiles have a very different propulsion mechanism, a very different deployment vehicle, a very different target, and a very different logistical chain than artillery shells. Grenades also shower a target in frag but that is also not relevant here. AA missiles are not as long and slender as...

Sure, but that doesn't include up and to the sides. If you have a long, rod-shaped projectile, it would be difficult to shape a charge that does anything other than send shrapnel out horizontally when detonated at ground level, when what you really want is an air burst that sends a cone of...

He's not entirely of base here. A higher muzzle velocity, all else being equal, will generally result in a longer range. The M777 with standard dumb ammunition has roughly equivalent (or slightly less) range than shells fired by the M1. Even the rocket-assisted M549 shells only get slightly...

To expand on what @cjl said (with math, for anyone who stumbles across this who isn't the OP and since that is more or less necessary for topics like these)... 1. For a converging-diverging nozzle, the flow is quasi-1D, so you can generally treat it as a function of only one spatial variable...

There's a whole lot of misinformation (or, in this case, disinformation) out there. I get it, since it's a highly technical topic, but as someone familiar with the field, it is frustrating.

Most of the really power hungry US tunnels were built near TVA dams to provide abundant power, but there are definitely some that can only run at night. The important point is that this is a predictable, well-publicized issue so it's a bit surprising.

Allegedly the shiny new wind tunnel that China has been bragging about can't run reliably because the local power grid can't support it. 🤣

Those are under development but are not what ARRW or OpFires are.