K^2

How about glass fiber? We were able to get it in Russia in the 90s in quantities sufficient to make surf boards. The only reason I recommended carbon fiber is that I'm partially stuck in the, "It needs to be light," mentality and it's only slightly more expensive than glass fiber around here. Glass fiber is just as easy to work with and just as strong. It'll just be a bit heavier. Glass fiber also requires a bit more care and protection while you are cutting and fitting it, but you ought to be working with either of these materials wearing good gloves, covering clothes, eye protection, and at least a dust mask. That's on top of the safety precautions generally associated with epoxy resin. Well ventilated area, preferably outside, is a key requirement for that one. There are a lot of videos on YouTube that demonstrate procedure of building anything out of either of these materials. There is a bit of a finesse required to build something aero/hydrodynamic, but if you don't care too much about what it looks like, it's very easy to just go with papier mache approach of cutting strips, coating them with epoxy resin, and sticking them on to of whatever you're trying to reinforce.

Math so far checks out. Are you basing the chamber/nozzle geometry on something standard, or just eye-balling it? If you want to get ISP above 40s or so, nozzle shape actually becomes important. A few general pointers. Assume it will explode. Base all your safety precautions on that assumption. If it doesn't, great! If it does, you really don't want to be unprepared for it. Metal isn't the best material for a hobby engine chamber/nozzle It's one of two places things are likely to explode, and the less shrapnel you have flying, the better. Consider graphite. It'd be single use, but way safer. If you can't get graphite for the whole thing, narrowest part of nozzle is also the hottest. At your burn temperatures, you'll have hard time not letting it melt. At least make that part from graphite. In a pressure-fed system, your chamber pressure will average to feed pressure. So you need to feed at about 300psi. But a liquid fuel engine tend to work a bit like a whistle, so pressure will fluctuate. Make sure your system can take higher pressure. Due to the above pressure variations, blowback to tanks is likely. If you can come up with a simple one-way valve that can take the abuse, you'll save yourself a lot of sorrow. If blowback carries fuel into oxidizer tank or vice versa, you'll get combustion in tanks, and pressure will spike even higher. Pressure feed tanks take a lot of beating. With that in mind, metal tanks aren't a great idea either. Composite is your best bet, since you aren't doing cryo. A soda bottle wrapped in carbon fiber makes for an impressively good light-weight tank. Failing that, aluminum. If you get a metal tank explosion, aluminum is the least likely to generate deadly shrapnel. Test your tanks by pressurizing them with water first. A ruptured tank filled with water at 600psi will make a scary noise and get you wet. Every liter of air at 600psi is equivalent to 1g of high explosive. Use inert gas to pressurize your fuel tank for actual run. If you fill your methanol tank with air and get any sort of a blowback, you're almost guaranteed a fireball.

Well, we tend to describe things from perspective of, "How does it interact with things we care about." It's been shown that at long ranges DM interacts very weekly both with ordinary matter and with itself. That leaves only two possibilities. It's either compact or weakly interacting. If we are looking at something more fluid-like, it has to be the later. Mind, it can still interact in ways ordinary matter doesn't. It can have an entire spectrum of particles with completely new sets of symmetries and charges. But if it interacts with ordinary matter so weakly as to be undetectable, none of it matters for our physics. If the only appreciable interaction with matter is gravity, then the only parameters we care about are its density and viscosity, which we can study without needing information about actual particle composition. As for annihilation being source of these gamma rays, I find it somewhat dubious. Basic field theory. Annihilation process is equivalent to scattering. If particle X annihilates with antiparticle X' to produce photon, then particle X can scatter a photon. In other words, it would imply that DM interacts with light. And that'd make it anything but weakly interacting.

Yes, the fact that it hangs around galaxies suggests some viscosity. Also, whatever it is, it must at very least interact gravitationally, which also leads to conclusion that there will be at least some viscosity. In fact, it should be possible to derive the density, at least as a function of a few parameters like density and temperature, which we have decent estimates for. I'll see if I can remember enough fluid dynamics to get an estimate.

Plausible. Locally, such a fluid would behave as an ideal gas, because it interacts so weakly, but in bulk of gallactic scale, it would behave as a fluid with very small, yet finite viscosity. If that's true, we could make some conclusions about it. I wonder if anyone ever tried to model something like that.

You've started with something I find hard to believe, and progressed into pure fantasy. All else being the same, F-16 has a number of advantages over Su-35 in pure air-to-air combat which while do not guarantee outcome will statistically weigh heavily in favor of F-16. A group of modernized F-15s will wipe out similar group of modern Su-35s almost certainly without losses. There is no competition there. Again, all Sukhoi aircraft are front line fighters at most. Which means they are not designed to assure air superiority solo. If you want to bring up Syria here, would you please recall what Putin's response to downing of the Su-24 was? Did he order some Su-35s to protect Russian bombers? No, because that'd be useless. He has ordered S-400 AA systems delivered to Syria. This was part of the Soviet doctrine as well. Soviet aviation was never envisioned as operating completely independently from ground forces. Which is very different from US doctrine of aircraft operating remotely by themselves, which is why US spent so much money developing actual superiority fighters.

https://en.wikipedia.org/wiki/Eddy_(fluid_dynamics) Erm, sorry, didn't see the small text. But I'm going to leave the link up just for sake of clarity.

It's kind of like saying that F/A-18 is primarily an interceptor. Yes, air combat capability was a key requirement. But a lot has been sacrificed to make it a better ground attack plane. Neither Su-27 nor Su-35 could remotely compete in an even air combat against same generation of dedicated US fighters. And if that was their main role, they'd be entirely non-competitive on the market. Yet they are. Because they make pretty decent front line fighters.

Hm. I'll have to read up on that. That does put a different spin on things. That's almost exactly what WIMP hypothesis is. That dark matter consists of kind of particles that have a lot of mass, but (almost?) do not interact with normal matter by any means other than gravity. It could make such matter entirely undetectable other than through its gravitational interaction. Basically, picture neutrino, only without even the weak charge, and a bit heavier. It'll pass right through your detector, you, your assistant, walls of the lab, and fly off merilly into space without anyone being the wiser. And with gravitational interaction being the only source of "drag" on this stuff, it wouldn't exactly stick to planets or stars, but would concentrate around galaxies. Still, even such a weird thing would have some sort of "hydrodynamics" to it. I wonder if we could detect eddies in it. They'd have some very interesting gravitational properties. (Edit: The new editor sucks. And apparently, IPB does not provide a way to turn it off. Can we please have a mod installed for that? This is terribly annoying.)

I dunno. The way I've seen it is that distribution of DM roughly matches distributions of stellar bodies in the galaxy. That includes old, dead stars of the galactic halo. In contrast, luminous matter is primarily confined to the disk, where new stars are generated. Obviously, simply having way more dwarfs, neutron stars, and black holes out there would do it, but that doesn't seem to match with evolution of other galaxies. Most of these objects must have been stars at some point, and that would be apparent from "young" galaxies we see far from us. If that's out, then star systems themselves must simply be way heavier. Natural way to have that is to have compact objects in debris circling the star. I could easily be out of date on that, though. If you have a reference for research that suggests that DM distribution is very different from star distribution, I'd be interested in reading it.

This is not true of modern fly-by-wire fighters. On older planes, decision to make them nose-heavy was down to stability. Nose-heavy aircraft will naturally dive if speed is too low, or pitch up if speed is too high. So once you're trimmed, any changes in thrust will simply result in a climb or descent. In contrast, a tail-heavy aircraft is inherently unstable. It has tendency to dive or stall, and the pilot has to constantly fight the controls. On a fly-by-wire jet, this fighting can be shifted over to the computer. The aircraft is designed to be completely unstable and uncontrolable without the computer doing constant fine adjustments to all the control surfaces. This has an entire host of benefits. First, the tail on such an aircraft is lifting, not dragging. So the net drag is lower for the same lift. Maneuverability is higher, since aircraft has natural tendency to pitch over. And finally, for the same reason, the response time between pilot's input and plane actually turning is reduced. You get performance, precision, and maneuverability, all at the cost of needing a computer between the stick and the surfaces. And yeah, canards exist essentially for the same reason, but it was basically just an attempt to get the same performance gains without needing a computer. On modern fighters, however, they serve a completely different purpose. While general aviation canards generate positive lift, fighter canards are designed to generate negative lift, just like a tail of a conventional aircraft. The purpose of this, again, is to make fighter less stable. This actually sacrifices performance for maneuverability, so it's mostly limited to experimental aircraft and a handful of European fighters. And its absence on US fighters is not due to stealth. F-15 is a perfect example of a pre-stealth aircraft that's designed to fly by wire. It is inherently unstable and tail-heavy. Both its wings and tail generate positive lift.

General Relativity is the most precisely tested theory perhaps with exception of Quantum Electrodynamics. If you add to that the fact that both GR and QED are based on the same underlying field theory, the evidence becomes completely overwhelming. We have tests ranging from dynamics of individual electrons to physics of neutron stars fitting the theory to 12 decimal places. Sure, many assumptions could be wrong. Yes, theory might be incomplete and might require corrections further down the line. But discrepancy between ordinary and dark matter being due to faulty theory is a statistical impossibility at this point. Which is why scientists are perfectly happy imagining all sorts of crazy new particle fields as possible source of dark matter. As crazy as any of these sound, it's not nearly as crazy as an idea that our theory of gravity is that far off.

Experience shows that humans don't check these things very well either. It's a combination of factors, from pilot not wanting to risk own life on a chance it's not the right target, to our general trust in instruments. After all, modern fighter is practically flown by the computer. All we have to do is make drones make fewer mistakes than our pilots, and that bar isn't as high as it seems. During operations in Iraq, we've had incidents of friendly fire where pilot checked with ground control and got authorization to destroy targets that turned out to be friendlies. Pilot was uncertain, saw no immediate threat, requested confirmation, received it, and still fired on allied vehicles. It's nothing a drone couldn't have done. Except, drone would never fire due to lack of experience or out of fear. Human element must be maintained. And there will certainly be situations where you want a human in the plane, but there are also a lot of situations where you can completely rely on drones. If you want to set up a guaranteed no-fly zones to anything without an IFF, drones are a good way to do this. If you want to then carry out air-strikes from within such a zone, you can use remote-controlled UAVs.

Gravity isn't the limiting factor. We can't build and launch a mirror of sufficiently large diameter to make a dramatic difference against Hubble, and for an interferometer, even with a solid frame and microgravity, you'll need optical tracking and adjustment that can work just as well in LEO. The only thing Saturn's distance gives you is reduction of sunlight, but you can get that in L2 just as easily and for way less. L2 interferometers have been proposed and even briefly planned. Then budget got cut. Go thank your senator. But that's kind of a moot point. An interferometer would allow us to take an actual look at exoplanets. Perhaps even do spectrography to detect atmospheric gases. Finding a more Earth-like exoplanet would be interesting. Finding one with gaseous oxygen lines in its spectrum, that would be the biggest discovery mankind has ever made bar none. Looking for Oort cloud objects, though? Not that helpful. If MACHO holds, the dominant objects in the cloud would be large enough for Hubble to find. You just have to point it at one. And having a bigger telescope with better sensitivity and resolution won't make it easier. Just the opposite. A 1ly sphere around the Sun has surface area of well over 10ly². Our Sun is about 5 light seconds across. Plastered across that 1ly sphere, you could fit approximately 550,000,000,000,000 Suns. And we are looking for mass that accounds for, maybe five Suns. Telescope won't help you. Spotting these objects by direct observation is statistically impossible. What we can do, and what we are looking for, are indirect indications of these objects there. If there is that much junk out there, and if there are just a few massive enough bodies floating about, they are going to knock things loose. And we should be seeing these things come visit. And we do. But we aren't seeing quantities or sizes we expect. Of course, our expectations are based on a lot of assumptions. There might not be any large objects out there. Just a ton of little ones. That would be very strange and indicate very serious flaw in our understanding of Sol's formation. Or there could be one really big object, which managed to clear out a chunk of the cloud already and get rid of any competition. That's completely plausible, but that's the sort of thing we should be able to find from periods of comets, etc. People are trying with no success so far. We are looking. We are finding zilch. We can't really be looking any harder with tech we have, and probably won't be able to even with tech we can invision. Though, an interferometer might help us detect such a heavy cloud around other stars. These things are easier to see from without than from within. At any rate, there are indirect indications we expected and did not find. All it means that we still have questions about it.

I think he's going from the outdated information. The measurement stations that were responsible for the infamous "hockey stick" graph were very poorly placed and poorly monitored, resulting in very unreliable data. Unfortunately, the general trend has since been confirmed with satellite data. It's not quite as bad as the "hockey stick" graph suggested, but it's bad enough. Satellite data also confirms very strong correlation with CO2 levels, which is also alarming. What we don't know for sure is how much of the effect is actually due to CO2 levels. There is a lot of very bad theoretical work that's been published over the years, and I am yet to see a model that definitively answers the question. The more thorough and detailed studies tend to have error band that covers everything from "we contribute a little" to "we are responsible for all of it". And we really need to know which one it is. If climate is directly controlled by CO2 levels, and we are responsible for enough CO2 output to directly control it, then we "just" need to cut all of the CO2 output and we'll be fine. If it's already a runaway effect we can't stop, whether it's human-caused or not, we also need to know it now. Wasting resources on trying to limit CO2 output if it's out of our control by now is stupid. If that is the case, we should be investing all of our resources into finding ways to deal with a changing climate, including improving our cities. That will cost money we can't waste on a hopeless "green" effort. The problem with climate alarmists isn't that they are wrong. They might very well not be. It's that they are so focused on a sole possible cause and a sole possible solution to climate change. If we let alarmists dictate policy and we don't actually have control over climate change, we are thoroughly screwing our civilization.

Agreed. Dark matter must either be uniformly distributed as a yet undetected field, or be predominantly found far from stars. (Hence the whole WIMP or MACHO thing.) Anything else would be reasonably easy to detect. Even if Sol was somehow unique, we would have seen clumps of dark matter near other stars during our exoplanet search efforts. Although, this does remind me of that "Alien Megastructures" thread. If MACHO hypothesis is valid, we might simply be looking at an exception to the rule, where the huge amount of non-luminous matter is located close to the host star rather than far from it. A close pass of another star could have caused this if there was enough debris orbiting the star to begin with at great distance. For all we know, that's all there is to it. Of course, it's still puzzling that we are having trouble detecting it. Could Sol be "special" anyways? Could something have stripped majority of our halo mass, making us make such incorrect predictions about other star systems? I do recall some stellar evolution simulations showing way more stuff orbiting a Sol-sized star than what we see out there.

To get a "3D" thrust vectoring, you need at least two engines with independent two degrees of freedom on their vectoring. The only US production jets with twins have only pitch vectoring. Hence the Russian claim to uniqueness of their jets. What you need to understand, however, is that Sukhoi specializes on ground attack, light bombers, and CAS aircraft. These are not air-superiority fighters. As indicated above, you don't need a lot of maneuverability for modern BVR combat. It's all about electronics and stealth with these. Ground attack missions, in contrast, frequently require low altitude low speed flights where maneuverability is the difference between getting shot out of the sky from the ground and not. Sukhoi aircraft should best be compared to US F/A and CAS aircraft, like F/A-18 and A-10. Of course, even that style of air support is getting phased out with all sorts of "smart" munition that can be deployed from "safe" altitude, which is why US hasn't really built proper replacements for these aircraft. In contrast, Russians have historically relied a lot more on hardware than software, so they still do CAS the old fashioned way. It hasn't bitten them back yet, but they haven't really been dealing with any sort of a serious ground opposition recently. The fact that they've been sticking mostly to cassette bombs in Syria might be an indication that it just isn't safe to fly your ground attack jets low even if you have the most maneuverable ones in the world.

[quote name='llanthas']Hard to describe without graphs, but picture how 2 spinning bicycle wheels are able to stabilize the much-heavier rider above them once they get moving.[/QUOTE] Angular momentum of bicycle wheels is much too small to contribute significantly to bicycle stability. I know it is an example used frequently, but it is a poor one. But yeah, reaction wheels have to store a lot of angular momentum to do their job. So precession torque resulting from trying to flip one will be quite great.

There are a handful of useful low thrust maneuvers that can be described analytically. Good example is raising circular orbit. Everything else is hairy numerics.

Simplest way is to time how long it takes to roll down an incline. If you know how to figure final velocity, average velocity will be exactly half. So it is super easy, but not very precise. If you want a precise measurement, torsion pendulum is the way to go. Also not super complex, but requires more time to set up and math is harder.

[quote name='Veeltch']In his Interstellar Quest Scott Manley mentioned that when going near the speed of light you can send messages into the past. How does that work? Does the radio wave get in front of the warping ship and reaches twice the speed of light, or something?[/QUOTE] That doesn't sound right. Do you have a link and time-stamp to where he says that?

[quote name='WedgeAntilles']What was a significant cause of global warming? Environmentalists, by way of eliminating global dimming. The above isn't a "maybe" either. Global dimming is a known phenomenon that has a significant impact on global temperatures.[/QUOTE] That's why Venus is such a chill place. All the dimming. Yup. Except, of course, that's not how thermodynamics works. Dimming can have short-term effect by reducing amount of radiation deposited to ground directly. Unfortunately, there is a slower process riding on top of that. If you've ever flown an airplane and payed attention to outside temperature reports, you might have noticed that it's a hell of a lot colder up there. The reason for that is that the atmospheric circulation with higher pressure at the bottom and lower at the top works like a refrigerator. When you shift heat exchange to higher altitude, which is what really happens in "global dimming", that works like turning the AC around and sitting on the hot side of it. It takes a [i]while[/i] to kick in, but by far the worst thing we could possibly do is make upper atmosphere opaque. Ground temperatures will eventually reach the same differential we observe now, resulting in global temperatures going up by tens of degrees. Might sound extreme, but there's a reason why led melts on the surface of Venus. CO2 effect is far, far subtler, because it does [i]not[/i] block incoming radiation, allowing radiation equilibrium to establish at surface level, and it only blocks a small fraction of IR going up. And many of the models greatly exaggerate the impact of CO2, because they do not take into account altitude distributions and convection, but until we actually have better models to account for all the factors, it's hard to say how much predictions are off by. One thing's for certain, though. If you think that humans are or ever have been causing the planet to get [i]cooler[/i], you are delusional. There is no scientific basis for it. Not a [i]single[/i] scientist, even a die-hard opponent of anthropogenic climate change, would ever tell you otherwise. We have a pretty wide pool of models and predictions, and they all vary from "Humans make no impact," to "Humans are the only reason we aren't in ice age yet." The only people who think pollution can make planet colder are the ones who never took a real science class in their lives.

What secondary reactions? The products of most of these are extremely stable, otherwise there would be neutrons flying about to begin with, and because recoil's all in gamma and/or beta, product nuclei don't end up buzzing about at several MeV. So if your fuel is just at the barrier for fusion, products are far, far from the barrier to continue fusion. E.g. proton-boron fusion, which yields slow alpha particles and gamma. The only way to get any of this to fuse further is to get some proton-proton reactions going. And yes, proton-boron fusion is technically a fusion-fission reaction. H + [sup]11[/sup]B → [sup]12[/sup]C[sup]*[/sup] → [sup]4[/sup]He + [sup]8[/sup]Be[sup]*[/sup] → 3 [sup]4[/sup]He. The excited states decay via alpha emission rather than gamma emission. But it's still considered an aneutronic fusion reaction.

[quote name='SargeRho']>Nuclear >Lightweight >Doesn't kill the crew Choose 2.[/QUOTE] That's why OP mentioned aneutronic fusion. It's one of possible ways to get all three. Nuclear isomers are another. Probably not something we're going to be doing in the nearest decades, of course, but no reason not to talk about possible applications for the future.

[quote name='xuxiwen']so confuse?????[/QUOTE] That doesn't tell any of us which parts you don't understand. Can you be more specific? Do you understand what notation [Pb[sup]2+[/sup]] means, for example? Do you know what dissociation means? If none of this rings a bell, you probably need to go back to either your textbook or get help from teacher/professor/TA for your class. If you can follow some of this, we might be able to help with the rest. But not without knowing where you get stuck.