Bunsen
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Posts posted by Bunsen
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Short version: Ain't gonna happen.
Long version: Physicists like to play with impossible ideas sometimes. It's sort of the scientific version of telling stories about elves and wizards, and it's easy to do on paper. You can write down equations that describe magnetic monopoles, then see what consequences their existence would have. You can write down equations that describe a universe with no mass in it, and see what would happen. You can examine the consequences of changing the speed of light to ten miles per hour, or making Planck's constant a billion times bigger, and all sorts of crazy sh*t comes out of the equations. It's fun, it stretches the mental musculature, and it has no implications whatsoever concerning the actual existence of any of those phenomena. There are no magnetic monopoles, the universe does contain mass, the speed of light is not ten miles per hour, et cetera.
One of the more amusing fictions turns out to be the existence of matter with negative mass. You can write down Einstein's field equations for a metric that incorporates negative mass, and find that crazy sh*t happens -- specifically, certain arrangements of it cause pockets of spacetime to do very strange things, like drag matter along at speeds greater than c relative to matter outside that odd little pocket, which then breaks causality. Miguel Alcubierre did that, and published a paper about the curious consequences of that particular nonexistent universe.
Attention whores like Harold White seized upon this and started trumpeting "Warp drive can be real! Give me money and I'll have Captain Kirk banging green chicks by this time next year! Just don't look to closely at the part where none of it works without a galaxy's worth of negative mass packed into a cubic meter, which you have to conjure into existence and then make disappear at will." More recently, further media attention was attracted when they figured out that if you make the nonexistent stuff magically appear and disappear really fast, then you can get by with a lot less of it.
White et al. are very careful to downplay the fact that the entire concept is built upon fiction. In their papers, they will very occasionally mention "exotic matter," which is their preferred euphemism for things that don't exist, but will otherwise treat it as granted that you can drive down to Costco and buy the stuff by the pallet. In the recent paper, they never even get as far as admitting that mass-energy (negative or otherwise) tends not to appear and disappear, despite the proposed scheme depending on exactly that.
Most physicists, sadly, tend to enable them due to a tendency to be exceedingly precise about what we can prove mathematically, and what is mere empirical fact. When asked about the Alcubierre scheme and its negative mass, they say things like "Well, negative mass has never been observed, but there's nothing in the equations that explicitly prohibits it." This is true, but possibly misleading -- there's nothing in the equations that prohibits the existence of magnetic monopoles, or Planck's constant from being a billion times bigger than it is, but those situations are quite solidly not the case. Journalists, naturally, translate that quote as "Warp drive is real! Physicist says negative mass can be made," which is utterly wrong. And as agencies like NASA are populated with people who know sh*tloads about turbopumps, electronics, metallurgy, and hypersonic aerodynamics, but are usually no more familiar with general relativity than the journalists, this occasionally results in research funding for the Unicorn Fart Warp Drive.
To be clear, my beef with White is not that he's continuing to examine the implications of metrics with strong local negative curvature. That's fine and dandy, and wild hypothesizing is even worth funding sometimes. The problem is that he's selling it (quite literally, I've heard his funding pitch disguised as a conference talk) as practical technology, which is a flat-out lie.
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That doesn't particularly make sense, or are you talking about jerry-rigging a commercial GPS device onto a rocket?
That's exactly what they're doing. I can't speak to the situation in Denmark, but in the US it's a matter of some difficulty to get an unlocked GPS receiver, involving federal agencies and Paperwork. I'm working on a cubesat project for an Air Force competition (so the part where you have to convince them that you have a legitimate purpose for an unlocked receiver was easy), and it still took several months to get our GPS module unlocked.
I can certainly see how, even if it were possible to convince the authorities to let them have an unlocked GPS, the time and effort wouldn't be worth it for low-altitude test launches with a significant risk of losing the payload.
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Time to show off my first creation in my new minimalist line. It's small, it's compact, it rolls like an SUV at a high speed tight turn, it's functional, and it has a tiny part count.
Fair warning: that gets worse on pretty much every other planet or moon you'll want to drive the thing on. The lower the gravity, the slower you have to go to avoid doing a tumbleweed impression. (The reason for that is a standard Physics 101 homework problem, but it's actually instructive to think about so I won't give it away.) You want a wide wheelbase and low center of mass to go anywhere. That's perfectly achievable with low mass and part count, but compactness tends to go to hell.
The alternate approach, of course, is to put lander legs on the top so it can right itself after you've tumbled and slid to the bottom of the huge hill that it inevitably happens near the top of.
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In theory, if you go fast enough before the engines cut out, it should be possible?
Approximately nope. The engines cut out before you reach 70km, and your orbit always passes through the point at which you last had thrust (gravity assists excepted). That means whatever trajectory you're on still passes through some point that's inside the atmosphere. The only way around that would be to get going so fast you either get an assist from the Mun or Minmus, or escape Kerbin orbit.
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#EDIT: A better form of control comes to me. I know there's "action groups" or something you cna put together in the VAB. Could I use that to toggle on engines easier? Can someone explain that system to me?
That's exactly what you want to use. Open the Action Groups menu, click on one of the groups (you probably want to use the numbers for custom actions). Click on a part (alt-click to select more parts at the same time). The part shows up in a list, and you can now select actions for that part that comprise the action group. "Toggle engine" is one of the options for engines, naturally. I don't remember if it lets you independently select members of a symmetry group, though, so you might have to place those vernier engines individually.
So if you select the rightmost engine on your rocket and bind its "toggle engine" action to group Custom01, you can turn it on and off as quickly as you like by hitting the "1" key. There's no need to point engines up, either -- you can just turn certain engines off momentarily.
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Here's my own Mun fuel refinery rover, Mun Unit Zappa
Hah! I truly loled.
I do have a question I cannot resolve via the wiki, though. Are there any kind of hinge and mechanical actuator? When I get home tonight, I might want to put something together that can... unfold... for lack of a better term.Not in stock parts, except for the lander legs and solar panels that you can't attach things to. There are some mod part packs for robotics. I haven't messed with them yet or I'd recommend one. If you're crazy enough and want to stay strictly stock, you might be able to rig something with sorta-interlocking girder structures as hinges and release it with decouplers. It will probably explode. I want pictures if you try it.
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I like 100-110 km for stations in Kerbin orbit. Not too hard to get there, but there's enough room below it that you can hang out in a 70 or 80km orbit and catch up if you launch too late.
If it's structurally simple, then I'll put an ASAS module on it and point it due north or due south. It makes aligning the approaching ship for docking really easy; no fiddling around trying to see where the depot is pointing and match it, just point to 0 or 180 on the nav ball. Don't forget the solar panels or an RTG for that probe core, 'cuz the ASAS quits if the probe's battery dies. You eventually will forget those; this is just guaranteeing an "I told ya so!" moment for later.
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The difference between actual distance above the ground and indicated altitude is always so much fun during landing. Try the in-cockpit view; there's a radar altimeter in there that tells you how far above the hard stuff you are (but it only gives readings once that's less than 3000 m). The 2-Kerbal lander can has very spartan instrumentation, but that radar altimeter, nav ball, and vertical speed indicator are enough if you practice a bit and aren't too picky about exactly where you set down.
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Actually, the funny thing is you are the one wrong. You can hook up a solar panel to a light bulb and power the solar panel with the light bulbs light. It works. However, the energy going through lessens each time and it eventually goes out, which is pretty much what the person said to begin with. So I do not see how they are wrong. Te person never said infinite, they said 20 minutes.
Edit: course to clarify , this can not be used to make infinite energy, all your doing is converting the electric to light, then back to electric then back to light and so on and so fourth. It eventually weakens each time you convert.
No, it does not work. The time for that energy to cycle through the system is on the order of a few microseconds (depends mostly on the junction capacitance of the photovoltaic panel and effective impedance of the light source), and it decays by, conservatively, around 90% each cycle. Even if you lit the light with an external energy source at first, after you disconnect that the energy content would exponentially decrease with a time constant about a million times too short for your eyes to perceive. If you used an incandescent light, you'd see the filament glow for a moment just because it takes a bit of time to cool off. Spatzimaus and Sirrobert are entirely correct.
The moral of today's story: Pay attention in science class, kids, and never trust youtube.
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Sweet crispy deep-fried Jesus on a stick, man! My eyeballs are lagging just looking at that thing. I assure you it's possible to get a Mun lander to orbit with far less complexity and lag, y'know, if you're into that sort of thing. (Try longer-burning stages and fewer of 'em. Kerbal engines are heavy, so lots of stages creates lots of dead weight. Just getting to orbit is fairly easy on two stages. I can usually get the third to handle trans-Munar injection, Munar capture, and most of descent, with the fourth providing landing, ascent, and return.)
If you're just into beating the crap out of your CPU and flying spectacular monstrosities, though, carry on and have fun.
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You can see the ISS's orbital decay and re-boosts here. The ISS was built in a low orbit to make it accessible with a minimum of fuel (or rather to maximize the payload capacity of vehicles with a fixed amount of fuel), but that also means that it's in relatively thick atmosphere for a satellite. Without those periodic burns, it would deorbit within a few years.
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Love the wobbling jet vanes around 12:35 in that video. Maybe you should delay the launch until 0.21 comes out with the improved ASAS?
Seriously though, good luck. May your rockets fly true and your downlink never fade.
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The choice between patched conics (n=1) and multi-body gravitation (n>1) was discussed at length when Kerbin began to acquire moons and neighboring planets. The major factor in the decision was not actually the computational load of in-game physics, but orbit forecasting in the map view.
They didn't have maneuver nodes yet, but they knew they wanted something like that. Think about what would have to happen with n>1 when you drag that maneuver node's delta-v around: At every frame, it would have to propagate your hypothetical orbit through a time-varying multi-body potential with enough accuracy to predict exactly where you'll encounter Eeloo several months later. Doing that sort of calculation at the pace of in-game physics, even at significant time warp, isn't that big a deal. Doing an entire orbit's worth 30 times a second while the crazy user screws around with the planned burn, though, that's hard.
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Taking a hint from our Eve landings, we could choose a high altitude landing site to save on fuel. Maxwell Montes is 11 km high, and 63° north of the equator.
At this latitude, assuming the wind just blows around the planet all the the same direction in all places at 250 km/h, the balloon would make it's way back to it's starting point in only 72 hours. But I think it's actually slower at more northern latitudes, so it might be that the wind takes up to 150 hours to make a circuit around the planet no matter the latitude, the same time it would take at the equator.
With wind speeds like that, I think you'd be better off with a nuclear-powered winged aircraft flying circles (or just keeping station) above the landing site. That comfortable 50km altitude is right in the middle of Sulfuric Acid City, though, so have fun making that engine last more than a few minutes. You might be able to get something to last a while above the clouds; the pressures there aren't out of line with high-altitude aircraft or balloons here on Earth. The low temperatures would make nuclear thermal propulsion pretty efficient.
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Improvements to ASAS? Can't come soon enough!
Shouldn't be all that mathematically difficult either, to make the amount of thruster force depending on some function of F(degrees_to_target) + F(current_rotation_speed) on each axis with say, a dead zone of some fraction of a degree/radian around the target orientation.
That's what it's already doing (PID control based on angular momentum, I believe), but with no dead zone. The problem, as I understand it, is that flexing of the craft severely screws with that algorithm, and you can't change its target attitude without shutting it off. I was just discussing how the vibration problem happens in a thread about improving ASAS, and it sounds like Harv et al. are working on a solution for that now, as well as letting your manual control inputs move the set point. I hope my physicsing helped a bit; it would be pretty awesome to be able to claim that I contributed to KSP.
I will also be happy to have a reason for putting solar panels on all those ships that didn't really need them.
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8) Despite your best efforts you will begin to identify with your Kerbals. But creating your own "Arlington Cemetery" complete with popsicle stick tombstones and eternal flame in the neighbor's rose garden will get you a visit from the local Constabulary. (Hint: hold midnight services.)
Tip: Resist the temptation to apply the "Moar Booster!" principle to the eternal flame.
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It's very sensitive to your entry trajectory and desired results, and there's no simple formula to tell you what periapsis altitude to target. Fortunately, some helpful person wrote a calculator that handles this stuff for you: http://alterbaron.github.io/ksp_aerocalc/
It can be fooled into giving you unreasonable answers, though, so check that its numbers make sense by comparing them to the atmosphere numbers on the wiki, etc.
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"Half of JPL is playing that game right now,†said Douglas Ellison...
Remember Curiosity, folks. It's the last productive thing JPL will ever accomplish.
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I love this book, and have to thank Derek Lowe of In the Pipeline for making me aware of it. Some of the things they were into make Kerbals look like the definition of sanity and sobriety by comparison.
I'm not sure about the strict legality of the online copy, but it's long out of print and nobody seems to have asserted copyright over it in quite a number of years.
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I guess what I bought was 0.14, but I paid my $7 when all you got was a promise that there would someday be a paid version. That and the warm, fuzzy feeling of helping to keep the development running.
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And what about this: when I calculate both transfers (Kerb Orb vs Min Orb) here: http://ksp.olex.biz/ it's 1015 delta v from 150km Kerbin orbit and 664 delta v from 47000km Kerbin orbit (equivalent to starting from Minmus). It looks like it's more efficient to go from Minmus. Plus I'm starting there with full tanks.
If you don't care about the effort of getting things to Minmus orbit in the first place, then you're right. Maybe you've got a kethane refinery running up there or something, I dunno. But if lifting things from Kerbin is part of the cost, going from LKO to 47000km takes somewhere around 900m/s of delta-v and then there's a little more for the capture burn.
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Building in Minmus orbit is probably not helpful. You have to haul all the same hardware to Kerbin orbit, then make an intermediate stop at Minmus (including capture and Minmus-escape burns). Also, you're doing your interplanetary transfer burn from further up Kerbin's gravity well, which means old Herr Oberth is not being your friend.
The one benefit I could see is that you could get a boost from Minmus's orbital velocity; sort of a slingshot that you pause in the middle by capturing into Minmus orbit. I don't think that would be enough help to outweigh the delta-v wasted on capture/escape burns, let alone the Oberth effect penalty. You also have Minmus's orbital inclination to contend with, which probably isn't going to be lined up with your desired interplanetary transfer orbit, so it will cost a little more delta-v to correct that.
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So all in all, PID will consume 39% more fuel for a given deviation in attitude and 27% more fuel for a given deviation in angular velocity. This is not all together insignificant, but either one is far, far better than what ASAS is doing right now.
Okay, now that we've scienced the living hell out of the idealized version of the problem (I love this thread, BTW -- ain't many games out there that inspire forum posts with differential equations), let's confront the reasons that the current ASAS behavior sucks so profoundly:
1. The PID gains are not optimized for each craft. This should be solvable with some computation of the moments of inertia and available control torques. Thrust vectoring is a complication, because it makes the control torque change with throttle setting. That still might be within reach of a well-programmed plugin.
2. The system is not a rigid body. Rockets bend. Rapid changes in control inputs excite vibrational modes of the craft, which cause the ASAS pod's measured state (IIRC, this is measured at the command pod) to deviate from the actual dynamics. That deviation tends to include phase lag, and when that gets between 1/4 and 3/4 of a cycle, control inputs intended to counteract motion amplify it instead, and things undergo rapid, unplanned deintegration. Or at least you waste craptons of RCS propellant.
The second one is, I think, the big reason we hate ASAS. A half-cycle lag for the fundamental bending mode is basically guaranteed when you have a command pod at the top of a long rocket and a vectoring engine at the bottom, or RCS clusters in their most efficient positions at the ends of the rocket. I see two obvious solutions, both of which kinda suck: Either smooth out the changes in control inputs enough that they don't significantly excite any vibrational modes (there goes any thought of bang-bang efficiency, and now your control is smeared out over a timescale longer than the rocket's fundamental vibrational period), or smooth out your state measurements enough to filter the vibrations out of the measurement (and again, you get sluggish response). The practical solution is to build stiffer rockets and make that fundamental frequency high enough not to cause problems, but that's only feasible for fairly small rockets, and small rockets bring shame to Jeb and the entire space program.
That seems like a pretty universal problem for control theory, though, so people have probably already thought up some incredibly clever ways to confront it. I hope.
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I think you ought to know I'm feeling very depressed.
"And wow! Hey! What's this thing coming towards me very fast? Very very fast. So big and flat and round, it needs a big wide sounding word like... ow... ound... round... ground! That's it! That's a good name - ground! I wonder if it will be friends with me."
Actually, the whale's entire thought process is pretty much spot on.
Warp drives
in Science & Spaceflight
Posted
The problem is not with the quantity of energy. Simply burning an entire star in a few seconds is trivial in this context, because it actually sorta happens. Not that supernovae are easy to harness, but they do exist. Negative mass does not, and only the most unreasonably charitable definition of "valid" would encompass a scheme that depends on it.