|
|
Post by lonto on Nov 4, 2010 13:56:56 GMT -4
No, that's ridiculous. It's the other way around, the Sun always shows the same face to Earth because it revolves around the Earth, with the string effect. At first, this sounds like a load of BS, but it would explain why we never see the dark side of the sun. |
|
Bob B.
Bob the Excel Guru?
Posts: 3,072
|
Post by Bob B. on Nov 4, 2010 13:59:30 GMT -4
Please, if you are right, show me a serious scientific article which backs you up. If you are right, you should be able to find one! It is hard to find an article that explains such a fundamental thing. However, I did find the following from the book Space Mission Analysis and Design: www.braeunig.us/misc/scan1.pdfPlease refer to paragraph 11.1.2 Selection of Spacecraft Control Type. The subsection titled Passive Control Techniques describes gravity-gradient control and backs up everything the members of this forum have been telling you about alignment to the gravity gradient. The subsection titled Spin Control Techniques states the following: In other words, you're wrong and everybody else is right. The section goes on to explain how a spacecraft is stable when spinning about the axis having the largest moment of inertia and how energy dissipation will cause a spacecraft to head toward this state, just as ka9q so clearly explained here. This book clearly vindicates what others have been trying to explain to you. |
|
|
|
Post by drewid on Nov 4, 2010 14:10:48 GMT -4
No, that's ridiculous. It's the other way around, the Sun always shows the same face to Earth because it revolves around the Earth, with the string effect. At first, this sounds like a load of BS, but it would explain why we never see the dark side of the sun. Wakka wakka |
|
|
|
Post by JayUtah on Nov 4, 2010 15:29:57 GMT -4
The subsection titled Spin Control Techniques states... ...a very well-known fact and often-used technique. My work on the Delta III launch vehicle included the spin-up decouplers for gyrostabilized payloads. I'm sure you, I, and two or three others here can speak about various control and stability schemes with considerable authority. The engineering and project planning literature is full of them, and for good reason. The problem I see is in Inquisitivemind's understanding of what would be the case were no such schemes employed. I don't see any dispute over the proposition that a spacecraft can be controlled into any of a number of attitude states and reference frames. The dispute is over the natural tendency of the spacecraft: I.m believes that, left to itself and absent any kind of control, the spacecraft would naturally turn to keep its "flat" side toward the Earth at all times instead of remaining Newtonianly in its original angular condition. Gravity-gradient control is the method implied by his first post to this thread, and the method I surmised he was alluding to in the original thread. It just doesn't work every well. It doesn't work very well because the magnitude of the torque is so very small for all but the most egregiously eccentric structures. Over a long period of time, absent any other effect, the gravity gradient will prevail. But not over the course of one or two revs in an orbit. Spin stabilization is used by about half of large outbound payloads during the apogee boost phase. It is infrequently used for on-station stabilization for reasons the "flat spin" article elucidates. Reaction wheels work better and provide more flexible mission profiles. |
|
|
|
Post by PhantomWolf on Nov 4, 2010 15:59:01 GMT -4
After Gemini they should know how objects in space move... They learned a huge lesson in that as early as Explorer 1. Point of order. NASA can't have learned the leason that early, since they didn't exist until July 1958, Explorer launched in Jan of 1958, 6 months eariler. However I think most of us get your point.  |
|
|
|
Post by gillianren on Nov 4, 2010 16:06:24 GMT -4
Ignorant English major question, here. Do all satellites have a flat side?
|
|
|
|
Post by PhantomWolf on Nov 4, 2010 16:28:58 GMT -4
No.
|
|
|
|
Post by gillianren on Nov 4, 2010 16:38:36 GMT -4
I didn't think so.
|
|
|
|
Post by PhantomWolf on Nov 4, 2010 16:39:56 GMT -4
Think about Sputnik 1
|
|
|
|
Post by PhantomWolf on Nov 4, 2010 17:24:09 GMT -4
I find it admirable that when he came back after the first drubbing, he did so by attempting a clearer explanation of what he means, including diagrams and moving GIFs. That kind of action differentiates him from common HPs who simply turn to conspiracy mongering. I think IM is sincere in his beliefs, even if he is utterly wrong. I used to assume much the same about orbits as IM believes. When the Hubble went up and I couldn't understand how it could work if it acted under my assumptions. The difference is that a little study clarified my knowledge so I understood more about gravity and orbits. Now the world of orbits and ellipses is interesting and not some vague misinterpretation from the under-explanations given in the popular media. Eariler he was saying that it was what he was taught at school, which actually wouldn't surprise me, although it might not be exactly what the teachers thought they were, or if they did, then it's likely they were teaching a "dumbed down version." It's a little like electron orbits. Everyone knows that they orbit about the atom's nucleus like planets in a solar system, right? After all that's what we got taught at school, and it's in all the basic textbooks. I can imagine someone arguing that this model is the correct one and not understanding why people are talking about spherical or dumbbell shaped probability shells and phase planes instead. Without the further learning and explainations, such ideas would seem very wrong to what the early level teaching gave, so questions like "Do you think my high school chemistry teacher was an idiot for teaching me that electrons orbited about like planets?" would make a lot of sense. I suspect that I.m's physics learning is not that far above the first few years of high school, supplimented with what he's found on Google. |
|
|
|
Post by theteacher on Nov 4, 2010 17:40:55 GMT -4
It's a little like electron orbits. Everyone knows that they orbit about the atom's nucleus like planets in a solar system, right? After all that's what we got taught at school, and it's in all the basic textbooks. I can imagine someone arguing that this model is the correct one and not understanding why people are talking about spherical or dumbbell shaped probability shells and phase planes instead. Without the further learning and explainations, such ideas would seem very wrong to what the early level teaching gave, so questions like "Do you think my high school chemistry teacher was an idiot for teaching me that electrons orbited about like planets?" would make a lot of sense. Excellent example. |
|
|
|
Post by supermeerkat on Nov 4, 2010 20:31:48 GMT -4
I liked his animations; most HBs don't go to the effort of producing such elaborate things.
|
|
|
|
Post by kallewirsch on Nov 5, 2010 5:48:47 GMT -4
I liked his animations; most HBs don't go to the effort of producing such elaborate things. Yep. Me too As for his canon experiment. He should just try this one out. Just throw a pencil, but be carefull not to introduce any rotation in the throw. His first canon graphic will describe exactly what the pencil will do (maintain its attitude, since the air resistance isn't strong enough to make the pencil turn). I wonder what he did in the past. As an airospace engineer he should more closely watch what is going on, when throwing objects. |
|
|
|
Post by kallewirsch on Nov 5, 2010 6:01:31 GMT -4
Ha also seems to believe, that from the moment of decoupling the LM from the CSM, the decent engines are working all the time until touchdown. Which of course, as we all know, is wrong.
My eye opener in orbital mechanics was a simple computer simulation, which I wrote just for fun. It had 2 masses and used Newtons Laws to iterate acceleration, velocity and position during time. So I saw just 2 points moving according to the laws of gravitation on the screen. It is fascinating to see how orbits develop all by themselfs and how the initial velocity I gave to the space craft made a big difference in orbit shape. After introducing the concept of a booster engine to one of the points I was able to change orbits and there you discover all by yourself in which way prograde and retrograd burns change the orbits geometry. All of this experiments lead by themselfs to a qualitive good understanding how orbital mechanics work and if one sees the forces in action it suddenly makes sense when someone says: In order to catchup with a spacecraft in front of you you have to reduce your orbital velocity and not to speed up. Such a remarkable simple simulation (any computer amateur can write one in less then an hour) will show what is then going on and that in fact this is right, even if it seems counterintuitive at the first glance. Thats because decreasing orbit velocity is just half of the story. That decreased velocity will influence the orbit geometry in this in turn will .....
I strongly believe, that everybody starting to study orbital mechanics, should play around with such a simple simulation. Then the question of descending from orbit in a vertical path would not come up in the first place.
|
|
|
|
Post by PUshift on Nov 5, 2010 11:50:34 GMT -4
 I.M. doesn´t need any orbital trajectory at all. If he believes the LM had powered its engine downwards to decrease altitude he could just head towards the center of moon and stop in front of the surface  |
|
