Post by JayUtah on Nov 4, 2010 0:43:18 GMT -4
One member of this forum has a wrong misconception...
No. Everyone but you agrees that the space-fixed attitude state is the expected behavior. You are the only one here who believes that the natural tendency of a spacecraft is to maintain some particular orientation with respect to the planetary coordinate system. Instead of trying to understand the patient explanations of these people, you try to tell them they're all somehow deluded and you're the only correct one.
He thinks that keeping the LCM horizontal relatively to the moon would need an attitude control.
Yes, I do. And I provided you the specific names of the devices on the CSM and LM that provided that attitude control. You could easily go research whether those systems actually existed and did the jobs we say they did. But you chose to ignore it.
But this member doesn't believe me; he thinks he detains the knowledge...
Yes, I do. And my knowledge comes from actual practical experience in spacecraft guidance and control, while yours apparently comes only from cursory web searches. You infer that spacecraft rotate to match the gravity vector, based on your flawed understanding of gravity. I observe spacecraft to retain a space-fixed attitude state. Which is generally more robust: an inference or an observation?
...I'm just an "ignorant layman" who knows nothing about space navigation.
Yes. But it goes beyond that. You have professed to be an aerospace engineer. In other words, you're trying to say that you're an expert in these matters. That's a substantial claim, and it requires some substantial qualification. You seem to think your "explanations" provide that. But you don't seem to realize when your explanations are wrong. And you don't listen to anyone else when they try to show you what's wrong with them.
You've convinced yourself that you know about space. The problem is that you haven't allowed your knowledge to be tested in a rigorous and meaningful way. This is what happens when all you do is Google. You need to go to school or take a profession that provides consequences for one's failure to understand. In formal terms, you have only propositional knowledge. You have no practical knowledge. Reading a book on how to play golf does not make you an expert golfer.
I have told him that, if he makes an object turns with a string, he can see that the attitude of the object naturally follows its orbits by the property of the centrifugal force.
No, twirling an object on a string is not physically the same as an orbit. You have ignored the other force imparted by the string on the object, that is not present in orbits. You have wrongly assumed that an orbit is like an object on a string in every respect. In this you have committed a common error that many beginning students also commit.
As it was already explained to you, the string couples to the object in a way that gravity does not. Upon a rigid cohesive body, gravity effectively acts only at the center of gravity. Where the center of gravity and center of mass coincide, which is typically the case within practical tolerances, gravity can exert no moment or torque. Only when the center of gravity is far enough away from the center of mass to create a non-negligible moment arm does any sort of moment arise. Hence for rotation, which occurs around the center of mass, gravity is irrelevant.
The string couples in a way that applies its centripetal force, not to the center of gravity or mass of the object, but only to selected points along its surface. This is utterly unlike gravity. Inertia continues to apply at the center of mass. This creates a moment arm between the center of mass and the string's attachment points. The centripetal tension on the string applies across the moment arm to create a torque that tends to keep that moment arm aligned with the string. After a few steady swings, the body's angular momentum harmonizes with the rotational period and the system becomes reasonably stable.
Do you understand now why string is not like gravity?
The orientation that the orbiting object takes relatively to the body it orbits around depends on its distribution of mass.
No. If you read the elementary dynamics treatise I linked to, it will discuss the concept of centroids, centers of mass, and centers of gravity. This is something every engineer is required to learn. You clearly did not.
When the LCM is oriented this way, the right part is in an inner orbit, and the left part is in an outer orbit...
Yes, you're trying to introduce the concept of a gravity gradient. I addressed that early on in our discussion, but you completely ignored what I said. Since then, several other people have given you the same correct explanation of how gravity gradients work. You completely ignore them too -- you simply say everyone but you is wrong.
Further, your misconception of the gravity-gradient principle is so egregious you don't even pick up on the basic fact that it aligns the major axis of the body with the gravity vector. Even if we allow the lunar gravity gradient the weeks it would take to have an effect on the CSM, with the DAP turned off, it will orient the CSM in a nose-up or nose-down attitude, not along the horizontal.
I have found an article for this reluctant member proving that a satellite has a natural orientation which is always the same relatively to the earth:
No. "Flat spin" has a specific meaning in English, and it's not even close to the strained meaning you're trying to attach to it. It has an even more specific meaning in aerospace dynamics. And if you were an aerospace engineer as you claim, you'd have a hard time not knowing that.
This means that, if no attitude control was exerted on the satellite, then it would always show its flat side to the earth...
No. And the satellite in the illustration has no "flat side." It's a dual-spinner, with the typical cylindrical rotor. The axis of the cylinder is nadir-oriented -- normal to the orbital plane. The annulus of the cylinder is covered with solar panels.
But the reluctant member denied that this was what the article was meaning...
No. I didn't just "deny" it. I gave a complete explanation of what the article was talking about. I gave you the name of the phenomenon so you could research it. And another regular poster expounded on my explanation. You completely ignored all of that.
...and went on snubbing me as being ignorant.
On this subject you are demonstrably ignorant. There's simply no other way to say it.
Well, if he is true, then, when an obus is fired by a cannon...
Irrelevant analogy. You now bring aerodynamics into the mix. You simply don't know how or why things work in the physical world.
And, if the big satellites always have an attitude control, there also exists small satellites, called "nanosatellites", which don't always have an attitude control.
Whether there's attitude control on a nanosat depends on the mission, not on the chassis. A nanosat can indeed have attitude control, typically in the form of cold-gas jets and strapdown gyros because that's all that will fit into the chassis. Nanosats typically work in constellations, where the attitude of any particular spacecraft is less important than the spatial relationship among them.
These satellites absolutely don't behave like this member imagines...
Tell my clients that. Guess what class of satellites I have typically built?
Now, if you continue to think that your conception is correct and that I'm only an ignorant idiot who thinks wrong, then show me a serious scientific article which confirms what you say.
Serious scientific articles don't go all the way back to first principles. Newton's first law is as basic as it gets, and that's the principle that establishes my expectation and predicts my observation. You seem to think the resolution of your misunderstanding must be found only in some highly technical journal because it's an obscure, esoteric piece of knowledge. Not so; your mistake is a fundamental one.
This process is continuously made, and the control is smooth enough not to need a fast computational period (which is 2 seconds, according to the NASA documentation).
No. You wrongly presume that Servicer handles the attitude control. In fact the DAP does, and it operates according to a 10 ms clock, every 10 cycles of which is a full attitude control pass. The RCS timings go down to 0.625 ms and are interrupt-driven.
When you can go back to your original thread and see what I've written regarding the AGC and its function, and can offer a meaningful commentary and rebuttal, then you can presume to lecture me and others about how guidance works. Until then, you clearly don't know what you're talking about.
I could give an analogy with an helicopter.
Which would be yet another irrelevant analogy.
Your arguments fail to convince because you think in terms of analogues that have only limited fidelity to the problem you're interested in. If one system behaves superficially similar to another system, you wrongly try to generalize all the behavior of your analogue system to the subject system.
Again in formal terms, your understanding is comparative where it needs to be analytical. Rather that compare one thing to another, you need to learn to decompose problems into constituent causes and effects and understand the mechanisms of each and of their contribution to the whole. This is what real engineers are able to do and why we can tell you aren't one.
So, in conclusion, the trajectory in which the lem starts horizontal is the most economical trajectory, even if other trajectories are possible.
For the third or fourth time: I'm well acquainted with the LM landing programs. I've demonstrated my understanding by addressing your claims. You steadfastly ignore practically everything I say.
...why would the lcm take a vertical orientation when the LEM leaves the LCM?
Answered a dozen times already. You wrongly believe the orientation of a spacecraft must always relate to its orbital velocity vector. You wrongly believe that because one picture shows the LM in a particular orientation, that's the orientation it assumed for the powered descent.
In short, you know nothing about space flight.
And they also have to save enough fuel for the return to the LCM!
Are you completely clueless? The descent and landing use an entirely different propellant and engine than ascent or abort. You don't even know one of the most basic facts of the LM.
In these conditions, it is absolutely obvious that they have to put all the chances on their side by not wasting fuel...
You assume they're wasting fuel only because you can't properly interpret the photographs of the lunar module, because you have a completely wrong concept of what a fuel-optimal descent from orbit entails (likely owing to your total ignorance of orbital mechanics), and because you steadfastly ignore all the other details that accompany a real, practical manned space mission such as IMU calibration.
You're trying to take NASA to task because the only solution you can think of to your gross oversimplification and misstatement of the problem is, in your mind, impractical and wasteful.
DOES IT MAKE SENSE, SERIOUSLY, DOES IT MAKE SENSE???
No, nothing you say makes sense. But that's because you have no useful understanding of space flight or the equipment we use to achieve it. And you've somehow managed to convince yourself -- despite a demonstrable lack of even basic comprehension -- that you and only you are the master of these techniques.
I agree with my colleagues: the arrogance you display is beyond all belief.
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