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Everything posted by K^2
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All in all, I think it would be a pretty forgetable movie if it were not for the visuals. But the visuals were there, and they make the movie. If you have not seen Gravity yet, or if you haven't watched it in 3D, watch it in 3D. It makes all the difference. This is probably the only movie I know where 3D isn't a gimmick. It really makes the experience. As for the plot, yeah, it's full of holes. The distance they put between the stations, the crippled Soyuz could have made on maneuvering thrusters. And if the separation was great enough to require main engines, there is no way in hell to make it across without guidance from ground anyways. Basically, almost anything to do with physics of actual flight or navigation was wrong. But they still managed zero-G better than almost any other movie, so big props to them on that. And the errors don't distract from the main themes of the movie. Ah, and I got a chuckle out of Shenzhou basically being a Soyuz with Chinese instead of Russian text on buttons.
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Special Relativity has nothing to do with orbital mechanics near black hole.
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The problem with this explanation is that from ship's perspective it never gets heavier. As viewed by the crew, the ship can manage the same acceleration it always did no matter how fast they are going. Of course, from crew's perspective, you can go as fast as you like. Faster than light by an arbitrary factor. The objects outside still moves at less than c relative to the ship, but distances get smaller, so it's the same thing in the end.
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I'm quite familiar with drag coefficient and typical notation. I was looking for dependence of drag coefficient on velocity in hypersonic range, and this is the first time I see someone mention a linear dependence.
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Classical orbital mechanics is just an approximation. Real trajectories are not closed ellipses even when orbiting something that you can assume to be a perfect sphere. Mercury's orbit, for example, precesses due to the fact that in General Relativity the trajectories slightly deviate from eliptical ones. As you drop your periapsis closer and closer to event horizon, this gets worse. In fact, trajectories near a black hole can get really crazy with loops and everything. Close enough to the black hole, objects can actually spiral in, and if you drop bellow the event horizon, you are never coming out. All trajectories inside the black hole lead into the singularity.
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True enough, and with cosmic censor in place it doesn't really matter. But theory also shows that if we dump enough charge into a black hole with enough angular momentum, the event horizon should vanish, exposing a naked ring singularity. At that point, we no longer have the luxury of saying that what happens below event horizon stays below event horizon, and causality violations become a factor for the rest of the universe. Closed time-like curves become a feature of our space-time and you can smoke your paradoxes in a pipe. Whether such a singularity exists is a question. But since everything suggests that it's allowed, the rest of the physics cannot possibly depend on global causality.
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Cool. I was actually trying to find this earlier. Do you know any references on modeling drag at hypersonic speeds and/or transonic regions? Just the general qualitative forms for each regime would be nice. I can fit the coefficients from some data.
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At two minutes I wanted to cry, so I stopped it. I think this video needs to be shown to every teacher and professor who ever felt like passing a student out of pity.
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Not as different as you'd think. The biggest difference is heating. Not only is there much more kinetic energy in the gas you encounter, but a lot more of that energy ends up deposited in the fuselage. This will mostly affect the leading edges, but it does factor into how you can use control surfaces as well. One of the consequences is that a hypersonic aircraft will not have quite such sharp leading edges as a supersonic one. Think wings and nose of the Space Shuttle compared to something like a Concorde. This helps create a protective normal shock in front of the leading edge as well as spread heat over larger area. A sharp leading edge would start overheating at these speeds. The other obvious effect is change in overall desirable geometry for the aircraft. As I mentioned above, containing your aircraft inside the shock wave is a good idea, and at Mach 5+ the envelope is going to be narrower than at 3+. It's also not worth it trying to generate secondary shocks, generally. So unlike the 3+ (think SR-71 Blackbird here) you won't get many sharp features. But all the other requirements are pretty much the same. You still have to deal with shock waves anywhere an edge enters a supersonic stream, and you still want to ensure that there is a subsonic boundary layer on your lifting surfaces and control surfaces or you won't get much use out of them due to separation. The transition, however, is still only around Mach 1. There are no special transitions at higher numbers, so calling a flow a hypersonic rather than a supersonic one is somewhat arbitrary. The physics is much the same. Edit: For more details on how things are actually done in practice, take a look at Boeing X-51. It addresses most of the questions raised here. It is capable of Mach 5, and it does use control surfaces rather than rely on thrust vectoring alone.
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Black hole, not a neutron star. Neutron star barely puts a dent in space-time. You need a good and proper twist, and for that you need a rotating black hole. But it doesn't mean causality isn't practically useful. Nothing you are likely to interact with in your life time is going to have a world-line that passes through a region of any more than slightly stretched space-time in any time frame that matters. So in every practical sense, causality works here, on Earth. And almost anywhere else that matters. Yes, it's a local property, but so long as space-times you are dealing with are more or less uniform, a local property is as good as a global one. Now, if we figure out how to build Alcubierre Drive, that will be a different question. Ask me again when we do.
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Nose of the aircraft generates a shock wave. Behind the shock wave, the fluid, in this case air, is going to move at a different speed relative to the aircraft. If you look at the shape of the hypersonic aircraft, their wings are designed to be contained inside the shock wave cone. The flow there is still supersonic, since we are dealing with an oblique shock, but considerably slower. P.S. I'm oversimplifying a lot of things. But this is the general idea behind making hypersonic flight more manageable. You still have to deal with supersonic flows near your lifting and control surfaces, and the solution is to make sure that the boundary layer is subsonic, so that you don't have separation. Then you still can generate lift and use control surfaces normally.
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Causality isn't safe either way. We know that there are situations when it breaks down. In particular, near rotating black holes. So Sgr A* at the center of our galaxy already provides a causality breakdown.
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Faster, actually. Gravitational time dilation on Earth's surface is stronger than the combined time dilation from gravity and GPS satellite's velocity. This is true for any satellite in circular orbit above approximately 9,500 km from Earth's center. Bellow that altitude, satellites move faster and experience stronger gravitational effect, and so the net time dilation is stronger than on the Earth's surface. So time on Earth's surface flows faster than on ISS, for example, but slower than on a GPS satellite. It does. Causality is a strictly local feature. Global causality may be violated.
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Because c is exactly the same relative to any observer. Weren't you paying attention? Light moves at the same speed relative to center of the galaxy as it does relative to the Sun and as it does relative to the Earth. Any frame of reference you chose, you can measure the speed of light and have it right.
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Air is a lifting gas at 100km altitude on Venus
K^2 replied to Agent86's topic in Science & Spaceflight
Absolute wind speeds aren't a problem. Yes, shear would be bad, but I don't think it's going to be that significant. From Wikipedia. That's not so bad. The fact that flows are primarily zonal means that most of the atmosphere is going to be moving together, resulting in strong winds with little shear. And when the winds themselves are going to be in 100m/s range, this suggests that relative wind at any part of the station is going to be manageable. -
Air is a lifting gas at 100km altitude on Venus
K^2 replied to Agent86's topic in Science & Spaceflight
100km? Why? That's practically vacuum even on Venus. 55km sounds more like home. Half an atmosphere of pressure and 25°C temperature. Just make habitats a little oxygen rich and you don't have to build any pressure vessels or worry about heating/cooling. And since most of the atmo is CO2 outside, you're right, ordinary air is going to work as a lifting gas. Could work. meve12, you'd want to use composites for this, not metals. There are still many questions about logistics, however. Launching from 55km on Venus is not going to be easy. And many resources would still have to be lifted from the ground, which would require something like a bathyscaphe attached to a zeppelin. Still, nothing really impossible about that. -
Cassini Gets New Views of Titan's Land of Lakes
K^2 replied to czokletmuss's topic in Science & Spaceflight
Yeah, so all interactions with it are on the order of 1-2 kJ/mol. So maybe as little as 1/10th of water. At temperatures less than a third of terrestrial, that's quite normal. Keeping in mind that we have thermophiles at up to 400K with water as solvent, 90K for methane seems like an extreme range of a warm environment. So long as all of your organics is going to be non-polar and dipole-dipole interactions replace hydrogen bonds this is just the right temperature range. You are still thinking in terms of interactions you want at 300K+. For 90K, methane is just what you want. For starters, I'm not sure why you'd want to break C-H bonds. The building material we are considering is acetylene, which already has just one hydrogen bound to a carbon. So at worst, we need to break C-C bonds, which is considerably easier, especially if you start with a triple bond there. But ok, I'll bite. Lets talk about the C-H bond. You insist that it's necessary to be able to break it for biosynthesis. 435kJ/mol is 4.5eV per bond. At 300K you are going to get 0.013 eV per C-H bond from thermal excitation. At 90K you'll have 0.0039eV. So to break the C-H bond at 90K, you have to supply 4.496 eV of energy, and at 300K you have to supply 4.487 eV. Are you seeing a lot of difference? Because I don't. Let me put it another way. Temperature at which average energy will exceed 4.5eV per bond is going to be over 100,000K. So it make absolutely no difference if you compare that to 90K or 300K. You have to do something creative with it if you want to break the bond. So it's your turn. Why don't you go into details of how this C-H bond is broken at 300K and try to explain what it is that prevents it from happening at 90K. -
Cassini Gets New Views of Titan's Land of Lakes
K^2 replied to czokletmuss's topic in Science & Spaceflight
I'd be shocked if there weren't. You just need a molecule that interacts with itself on one end a bit stronger than it does with methane, and you need it not to freeze solid at 90K. We aren't talking about subsurface oceans. We are talking about the seas and lakes of methane on the surface. And the nutrient for potential life there would be acetylene, which is quite abundant and renewable thanks to solar radiation. -
Cassini Gets New Views of Titan's Land of Lakes
K^2 replied to czokletmuss's topic in Science & Spaceflight
The same thing that makes water a better solvent at typical Earth temperatures, the hydrogen bonds, also makes it unusable at low temperatures. Methane is a better choice for cryogenic life. The thing about organic chemistry is that it's all about organic compounds at typical temperatures. When you drop down to 90K, you have to make sure that your intermolecular bonds are much weaker. That means you have to get away from any polar molecule and use a non-polar solvent. Since you obviously slept through all of your other chemistry courses, including physical chemistry and quantum chemistry, I'll remind you that the main requirement for bio-chemistry is ability to change reaction rates with very small changes in temperatures or concentrations. That means, your intermolecular forces need to be dominated by a bond that's comparable to energy at relevant temperature. At 300K the hydrogen bonds are it, and that makes water an ideal solvent. At 90K you are looking at much weaker bonding. Something like dipole-dipole interactions are likely to replace hydrogen bonds. And that means that a non-polar solvent, such as methane, is going to be quite sufficient. -
Cassini Gets New Views of Titan's Land of Lakes
K^2 replied to czokletmuss's topic in Science & Spaceflight
Well, Squad have been talking about a second gas giant. Something that is Saturn-like. I would bet that they will include a moon similar to Titan when this new planet is added. I wouldn't discount metanogens either. It would require a completely different chemistry to use methane as solvent, but low temperatures mean that you would need something other than hydrogen bonds to keep your complex molecules together, and having a non-polar solvent does allow you to do that. So we might see life in methane oceans that does not rely on water at all. -
If you look at how the plants do it, they do it in very similar ways. The primary difference is that their chemical factories are miniature. We're getting there with both nanotech and biotech. Worry not.
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Cassini Gets New Views of Titan's Land of Lakes
K^2 replied to czokletmuss's topic in Science & Spaceflight
We don't really have any indication of life on Europa. All we know is that there might be liquid water there, which is a tiny fraction of requirements for life. And even if we thought it to be likely enough to look for, finding it there would be really hard. If it's there, it's under a thick layer of ice. We have hard time digging that deep here, on Earth. Doing it on a remote moon with nothing but robots to take care of any emergent problems? A manned mission to Mars is easy in comparison. If we knew for sure that there is life there... But as it stands, I'd keep Europa on the back burner for now. Titan, on the other hand is relatively easy to land a probe on, and if there is life there, of which there is some indication, a robotic lab comparable to what we've sent to Mars already will be able to confirm it. So if we are planning a rover mission to the outer system, I don't think anything other than Titan should even be considered. That's the most interesting target out there. Furthermore, confirmation of life on Titan would be an even bigger deal than that on Europa. If two completely different forms of life exist in Solar system, the rest of the universe is filled with it. And if we can expand the search from Earth-like planets to things more like Titan, systems without some kind of life are going to turn out to be a minority in the galaxy. That would be the biggest discovery bar none. So forget Europa. All it has going for it is hype. Titan is where science is. -
I've been looking for a Penny-Farthing, because it looks like a hoot to ride on. The cheapest I've found so far goes for about $900. That's for two wheels and a frame. In contrast, I can buy an aluminum frame, 21-speed bicycle with full suspension, and an absurd number of moving parts for under $100. Sometimes, going retro costs way, way more. Nibb's post actually covers the reasons rather well. I just wanted to put it into a perspective with something simple.
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Japanese 'space cannon' to be fired into asteroid
K^2 replied to kiwi1960's topic in Science & Spaceflight
I was excited about the mission when I read the thread title, merely interested in it after I read what it's actually about, and now I'm filled with nothing but regret over what could have been after reading your post. Thank you very much. -
Japanese 'space cannon' to be fired into asteroid
K^2 replied to kiwi1960's topic in Science & Spaceflight
So they are going to blast an asteroid with high power explosives to see if some oil comes out? I don't think Japan is adopting the right aspects of American culture.