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Post by inquisitivemind on Oct 26, 2010 11:18:44 GMT -4
I'll try to make myself clear: The CSM has a velocity of 6000km/h relatively to the moon; this velocity allows to counter the lunar attraction, because the LCM is too close to the moon to behave like a satellite and be naturally stationary to the moon. So, when the LEM leaves the LCM, it initially has this horizontal velocity, but no vertical velocity. The LEM could initially take a vertical attitude, but it means it would have to give an important push to counter the centrifugal force, and it would keep this horizontal velocity it absolutely must null before reaching the moon. That's why it starts horizontal and counters the horizontal speed with its main reactor. Progressively, its horizontal speed decreases, and so does the centrifugal force; so it begins being attracted by the moon, and its vertical velocity starts increasing. So it must progressively turn, so that a part of the thrust also goes on the vertical axis and allows to counter the lunar attraction. The more the horizontal velocity decreases,and the more the LEM must counter the lunar attraction by becoming more vertical (and also because it must arrive vertical on the moon). All along the trajectory, the computer computes decelerations the LEM should have on the two axes, and compares them with the readings on the accelerometers; it computes a compensation to make the computed accelerations closer to the next readings; this is done regularly at each computational period; even if the computational period is not fast, the motion is regular enough (if done the right way) for the computer to have the time to do corrections. If the LEM was doing brusk rotations, it would not be the case, they would not help to the guidance, and would waste fuel.
No, I don't consider the moves of the LCM relatively to the photograph (that is the LEM), but relatively to the moon.
As soon as the LEM leaves the LCM, it must start maneuvering.
The CSM pilot doesn't have to see all sides of the LM, it absolutely doesn't help the guidance of the LEM.
Well, for someone who makes "naive" assumptions, I'm able to give a lot of explanations. And what do you do for a living?
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Post by tedward on Oct 26, 2010 11:23:14 GMT -4
I can't help but think from those comments that you expect the rocket to be on the go all the while. How does a GPS or other lower orbit satellite stay in orbit around the Earth? It cannot carry fuel for years of thrust? Or have I got the wrong end of the stick?
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Post by inquisitivemind on Oct 26, 2010 11:26:55 GMT -4
Listen, I can't spend all my time on this forum, I have already spent too much. But post your counter-argulments, I promise to respond to them later.
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Post by JayUtah on Oct 26, 2010 11:30:06 GMT -4
Even if there was no physical string the LCM was attached to, the lunar attractions plays the function of this string.No. The tidal force is orders of magnitude smaller than the moment of inertia of the CSM. No, it's not easier, because these aerodynamic forces help to control the planeNo, the ambient air provides a means by which we can control the plane. The control must still come from somewhere. once it was giving a push on a vertical reactor...You're misusing the English word "vertical." It refers to one axis only, whereas the LM -- and all spacecraft -- have RCS activity in all three axes. ..it was starting to turn, and it had to give an opposite push to counter the rotation and stop itThat's exactly how Boeing ailerons work too. I fail to see a material difference. ...it was not so easy, and was requiring a constant control.I don't see what's so hard about it. It's covered at length in chapters 4-8 of Marcel Sidi's excellent book. I've programmed these systems myself. You're simply trying to tell us it was impossible, without saying why. Only the vertical reactors were used to control the attitude. The horizontal reactors were only used to move the LEM horizontally;False. Horizontal velocity was controlled by vectoring the DPS. This was done by using the RCS to effect pitch and roll deflections. If a deflection was held for more than a few seconds, the DPS was gimballed to accommodate it. The horizontal reactors were also necessary on the return to control the altitude of the lem while it was trying to join the LCM so it would remain on the same orbit as the LCM.No, the entire RCS was used to orient the vehicle. With a computational period of 2 seconds, the LEM had better not make too brutal rotations.Please tell us why you think the LM was asked to maneuver at rates that were outside the capacity of its control system. There is no big one step.Yes there is. The DAP applies rotation rates commensurate with the error magnitude. There is a dead-band, a slow-rate error band, and a high-rate error band. Slow-rate error bands are handled by gimballing the DPS if possible. A too unfit initial maneuver would take time to correct; it's better and more precise to make little corrections step by step.No. Only the precision of the final orientation matters. Accomplishing it by means of initially coarse maneuvers is fuel optimal. Well, I can see that you have not well read their documentation; I have.But you clearly don't understand it. Not only am I very familiar with the AGC, I've written programs for it myself. I've also programmed actual flight guidance systems for operational satellites. Please tell us how many operational spacecraft you have personally actually programmed for. I would like an actual number. In it, they say that the task which was doing the guidance computations, SERVICER, was running every two seconds.Servicer was called every two seconds. It did not take two seconds to compute the attitude error and error rate. 0.5 Hz was deemed sufficient for velocity control through testing and simulation. They were also making the computer count hardware pulses with instructions thay call "unprogrammed", but which take CPU time.I assume you refer to counter interrupts. Normally the counter interrupt load was negligible and was considered an acceptable tradeoff for the value of having sensor parameters in-core for computation. Otherwise the CPU would have had to wait for I/O in order to query the sensor. The counter interrupt load was a problem only for Apollo 11 because its radar power supplies were not synchronized. This resulted inadvertently in double the number of counter interrupts. It makes no sense at all, because the CPU has other more vital things to do than counting hardware pulses, especially when it's running with a very slow clock.Your opinion of what the computer should have been doing doesn't really matter. The design of the AGC and the assignment of tasks to it took place in the larger context of system design. Many times engineers discover that what seems counterintuitive and wrong to the layman is actually the best way to do it when you run the numbers. Furthermore, they were also making unit conversions, not just for the astronauts, but even between different devices; talk about wasting computer resources!No. It is important to present information to skilled pilots in the form in which they have come to expect it. Unit conversion occupies negligible resources.
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Post by Jason Thompson on Oct 26, 2010 11:33:11 GMT -4
Even if there was no physical string the LCM was attached to, the lunar attractions plays the function of this string. Except that it has no physical attachment to one point of the orbiting craft, and therefore does not exert a preferential force to keep one side of the spacecraft facing the Moon. Normally? You are describing one specific set of satellites in a geostationary orbit. To say that is 'normal' is to overlook the vast number of satellites that are not in any such orbit, and which even a cursory look into the sky on a clear night would allow you to see. The majority of satellites are imn low Earth orbit, and complete one revolution every ninety minutes or so. And they do not keep one side facing Earth all the time. The Hubble space telescope indeed must not do that or it could never work! And they also cause the plane to lose control when unexpected gusts of wind or atmospheric turbulence buffet it. The LM never has to contend with such unexpected problems. The entire RCS system was used to control the attitude. No, they were used for yaw menauvres, to turn the LM. The RCS system can and did turn the LM in any direction, and give small alterations to speed. It's a question of firing the right thrusters in the right combinations.
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Post by Jason Thompson on Oct 26, 2010 11:34:38 GMT -4
The CSM pilot doesn't have to see all sides of the LM, it absolutely doesn't help the guidance of the LEM. No-one says it does. It is to perform a visual inspection of the craft, to tell the guys in the LM that the landing legs had deployed correctly, and make sure there were no obvious problems with the spacxecraft. Why do you have such trouble grasping this idea?
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Post by echnaton on Oct 26, 2010 11:56:23 GMT -4
I can't help but think from those comments that you expect the rocket to be on the go all the while. I was about to post the same thought. He has a serious and fundamental misunderstanding of physics. It appears too large for any conversation to resolve his complaints until he reconciles his views with reality.
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Post by scooter on Oct 26, 2010 12:15:20 GMT -4
We need to get this guy a copy of "Orbiter".
Inquisitivemind...when the LM and CSM separated, the CM was directly underneath the LM. This lowering of the altitude in this portion of it's orbit caused it to slowly accellerate ahead of the LM, without using thrusters (RCS) at all. Orbital mechanics...low orbit is faster, high orbit is slower. Makes perfect sense, as the LM will soon be decellerating pretty hard for the landing, and the CSM will have drifted ahead of it a couple/few miles, and will be "out of the way". After the LM went in to land, the CSM firred it's SPS briefly to raise the low point of it's orbit and "re-circularize it's orbit while the LM was on the Moon.
I'm not an aeronautical engineer or orbital mechanics wizard, but it is possible to learn the basics. It's fascinating stuff, and was cleverly used by the Apollo mission designers to conserve fuel usage.
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Post by gwiz on Oct 26, 2010 12:21:50 GMT -4
Inquisitivemind...when the LM and CSM separated, the CM was directly underneath the LM. This lowering of the altitude in this portion of it's orbit caused it to slowly accellerate ahead of the LM, without using thrusters (RCS) at all. Orbital mechanics...low orbit is faster, high orbit is slower. Makes perfect sense, as the LM will soon be decellerating pretty hard for the landing, and the CSM will have drifted ahead of it a couple/few miles, and will be "out of the way". I tried to explain that to him back on page 2, but he obviosly didn't want to understand. I think he is confusing the LM/CSM separation, the descent orbit initiation and the final landing manoeuvres, three manoeuvres well spaced in time. but with his general cluelessness about orbital dynamics, it's hard to be sure.
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Post by Mr Gorsky on Oct 26, 2010 12:36:50 GMT -4
Goodness me ... I am pretty much a layman in all of this "rocket science" stuff (as evidenced undoubtedly by my posts over the years, but inquisitivemind's arguments are even giving me a headache. I can only imagine how much brick wall head tennis is being played by those who really do understand all this.
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Post by JayUtah on Oct 26, 2010 12:37:07 GMT -4
I'll try to make myself clear:No, please cite your references. I'm referring to the published descent orbit of the CSM and LM, which had identical periods but opposing periapses. You're simply telling me I'm wrong. I'm not interested in your handwaving, naive derivation of your notion of a descent orbit. I'm asking you to work with the published orbit. Tell me what you think is wrong with it. So, when the LEM leaves the LCM, it initially has this horizontal velocity, but no vertical velocity.When the LM undocks from the CSM it's in an orbit. The LEM could initially take a vertical attitude, but it means it would have to give an important push to counter the centrifugal force...No. I've had to read your descriptions several times to try to understand what you're saying. You really don't seem to have any working knowledge of orbital mechanics, and this is leading you to think of "horizontal" and "vertical" velocities as if this were a pure ballistics problem. It isn't. If the LM applied DPS thrust "vertically" (i.e., in the plane of the orbit but perpendicular to its orbital velocity vector) it will just change the shape of the orbit. It won't land. That's why it starts horizontal and counters the horizontal speed with its main reactor.Rather, it applies retrograde thrust along the velocity vector in order to progressively lower the periapsis. Progressively, its horizontal speed decreases, and so does the centrifugal force; so it begins being attracted by the moon...No, it's not quite that simple. As orbital velocity decreases, altitude decreases. But that results in an increase in orbital velocity, per the mathematics of orbital motion. All along the trajectory, the computer computes decelerations the LEM should have on the two axes...No. The descent is broken up into discrete phases or stages, with a different underlying mathematical model embodied in each program. Initially the model is an orbital mechanics model with thrust parameters. Near the end it is a ballistic descent model with independent horizontal control. Theoretically the problem transitions smoothly from orbital to ground-centric ballistic. In practice you implement discrete transitions between flight regimes. If the LEM was doing brusk rotations, it would not be the case, they would not help to the guidance, and would waste fuel.No. Even in a slow maneuver you'd still have overshoot and residuals. These would require fuel to correct. No, I don't consider the moves of the LCM relatively to the photograph (that is the LEM), but relatively to the moon.How is that valid? The pictures were taken at vastly different times with a different lunar background. As soon as the LEM leaves the LCM, it must start maneuvering.No. As soon as it leaves the CSM it is merely in a coincident orbit. It maneuvers only when it wants to alter that orbit, such as to present itself for inspection, to initiate the descent orbit, or to change attitude or to translate with the RCS. In English we use the French word "maneuver" in a flight dynamics context to mean applying control inputs. It does not mean passive flight in English. The CSM pilot doesn't have to see all sides of the LM, it absolutely doesn't help the guidance of the LEM.The inspection of the LM has nothing to do with LM guidance. The CSM pilot must see all sides of the LM to inspect it for damage prior to beginning the landing, while it's still in orbit. You have used pictures of the LM in coelliptical or coincident orbit with the CSM and you apparently think it's a picture of the LM in powered descent. Well, for someone who makes "naive" assumptions, I'm able to give a lot of explanations.Your "explanations" are wrong at nearly every step. And you stubbornly refuse to listen to the people trying to correct you. Your ability to write a lot of words does not mean you know what you're talking about. You've apparently thought you could pass yourself off as an aeronautical engineer, hoping to prevail in debate by bluster and bluff. Sadly while that may work at YouTube, it does not work here. You clearly have very little understanding of what actually is involved in flying a spaceship. And what do you do for a living?I build and fly commercial spacecraft. Unlike you, I really an am aerospace engineer.
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Post by JayUtah on Oct 26, 2010 12:50:57 GMT -4
I can only imagine how much brick wall head tennis is being played by those who really do understand all this. You have no idea. As I said elsewhere, people who grossly misunderstand something and aren't aware of that misunderstanding habitually don't elucidate their underlying assumptions and beliefs. They don't think they have to, because they assume you're either completely ignorant or are working under identical assumptions. So the hardest part of any debate like this is trying to figure out what's going on in the proponent's head -- what wrong concept(s) he's working under. For example, he seems to believe that orbital tides will keep a spacecraft oriented along a gravity vector. Not so. Those forces exist, but are simply not strong enough to overcome the CSM moment of inertia in the short term. He completely misunderstands both the nature and the magnitude of tidal forces. And he completely misunderstands how P63, P64, and P65 work. More importantly, he misunderstands why there are those three programs instead of just one program to "smoothly" alter the LM thrust vector. This is the difference between people who may have studied a thing or two about guidance, and people who actually have to program it.
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Post by LunarOrbit on Oct 26, 2010 12:52:38 GMT -4
The CSM has a velocity of 6000km/h relatively to the moon; this velocity allows to counter the lunar attraction, because the LCM is too close to the moon to behave like a satellite and be naturally stationary to the moon. So, when the LEM leaves the LCM, it initially has this horizontal velocity, but no vertical velocity. I don't think anyone is disputing this. But you seem to believe that the LM had to begin it's descent immediately after it separated from the CSM, but it didn't. The LM didn't begin it's descent until 84 minutes later. During the time between separation and descent, the CSM and LM flew together in tandem so that the astronauts could inspect both spacecraft. If either spacecraft had been damaged the landing would be aborted. Again, we don't dispute that the LM had velocity that had to be nullified before landing. But the initial orientation of the LM and CSM is not relevant. The LM can changed it's orientation prior to beginning it's descent. The orientation of a spacecraft only matters when the engine is firing.
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Post by JayUtah on Oct 26, 2010 13:10:37 GMT -4
[He has a serious and fundamental misunderstanding of physics. Agreed. He may have been able to convince a few people at YouTube that he's an aeronautical engineer, but the standards in the real world are much, much higher. Perhaps based on confidence obtained at YouTube, he believes he can similarly bluff his way elsewhere.
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Post by theteacher on Oct 26, 2010 14:04:24 GMT -4
So the hardest part of any debate like this is trying to figure out what's going on in the proponent's head -- what wrong concept(s) he's working under. Yes :-) You don't actually know, if he has a concept of tidal forces, or - if he has - how it is structured. It might as well be, that he applies the analogy of the string as gravity in a wrong way by driving the analogy too far, thus it might have been wiser to investigate that to begin with instead of assuming it as a fact in the first place imho.
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