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Post by brotherofthemoon on Mar 29, 2006 13:44:51 GMT -4
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Post by JayUtah on Mar 29, 2006 14:28:13 GMT -4
We have all numbers including the mass, engine thrust and orbit height of a spacecraft in earth orbit. In order to break earth gravity, space craft needs to reach second cosimc speed of 11.2 m/s.
No. Injection velocities depend on the desired orbit. There is no one-size-fits-all velocity. You inject first into a stable parking orbit around Earth. You can, through various means, measure that orbit with great precision. You want to inject from there into a transfer orbit of known properties. With engines of known capacity and spacecraft of known mass, that works out to a burn of precise duration with the spacecraft pointed in a precise location.
Those manuevers are generally computed by large computers on the ground ahead of time. That is, the spacecraft's current state and desired state are fed to computer programs on mainframes that compute the "pad", or commands to the onboard computer. That pad consists first of an orientation for the spacecraft according to some previously agreed-upon reference: basically a vector in three dimensions, so three double-precision numbers.
The onboard computer renders that into a local vector according to the REFSMMAT, the "reference to stable member matrix", or the local coordinate system of the spacecraft relative to its gyroscopic guidance platform. Any student of linear algebra knows how to do a change of basis, and the AGC can do the necessary computation in a fraction of a second. What this gives you is a set of data that is palatable to the onboard digital autopilot in "attitude-hold" mode.
The digital autopilot -- really just a program running on the AGC -- periodically compares the spacecraft's actual orientation to its desired, preprogrammed orientation. If there is a significant enough difference, the program follows a sequence of jet firings designed to return the ship to its proper orientation. MIT wrote several papers in the 1960s on how to do this in a fuel-optimal fashion. Their work is still the basis for spacecraft attitude control.
So the pad simply feeds the new orientation into the digital autopilot. On its next cycle, the autopilot notices that the spacecraft's orientation is "wrong", and fires the jets to "correct" it.
With the spacecraft now properly oriented for the burn, the next stage of the pad gives the programmed change in velocity required.
Actually some of the others here have given you some slightly inaccurate information. There is certainly a dead-reckoning approach that says, "burn an engine of known thrust for a certain time on a spacecraft of given mass, and you will change your velocity by a certain amount." That's absolutely defensible according to physics. Unfortunately a spacecraft is a variable-mass vehicle, so that requires you to keep track of the rate at which you lose propellant mass during a burn.
It turns out there's a much easier. much safer way of doing it. Instead of trying to compute the acceleration from moment to moment, just measure it. The guidance platform of, well, anything contains accelerometers. The most accurate and popular type for guiding rockets and spacecraft is the PIPA -- a "pulsed integrating pendulous accelerometer".
"Pulsed" meaning it sends out a discrete electronic signal for each increment of acceleration it measures. Apollo PIPAs were precise to 0.1 fps/s. So each time you have 0.1 fps/s of additional acceleration, the PIPA sends out an electrical pulse on one of its output pins.
"Integrating" means that it registers a rate. It's up to some other bit of electronics to integrate those signals over time into a velocity. In Apollo's case, the AGC counted the pulses in each direction (forward and backward) along each of the cardinal axes that were received in its cycle, and from that was able to update velocity vectors during an actual burn.
So you fire the engine and you may count three pulses from your PIPA in 0.1 second. That means that for that 0.1 second you had an acceleration of 0.3 fps/s. (Now there are actually some averaging techniques you use, but I'll simplify.) So you can increment your velocity vector along that axis by 0.03 fps. And in the next 0.1 second you may count 2 additional PIPA pulses, so you're now at an acceleration of 0.5 fps/s. If you get no further pulses from the PIPA during the burn, you can continue to integrate the new velocity using the 0.5 fps/s value. When you shut off the engine, the PIPA registers incremental decreases in acceleration on another pin. We had 5 clicks up, and we should expect 5 clicks down, although they may not happen during one poll interval.
"Pendulous" meaning the mechanical mechanism by which it worked involved cantilevering a known mass and measuring the deflection of the cantilever by piezoelectric or optical means. A certain degree of deflection results in a certain acceleration.
If you can measure actual acceleration, and you have a computer with dedicated electronics to integrate that acceleration over time to give you a measured change in velocity, you simply burn the engine until the desired amount of change in delta-v is achieved. That way you don't have to worry about whether the engine thrust fluctuates or how changing mass properties should affect acceleration.
So the pad says when to start the burn, and estimates when the burn will stop. But the actual guidance is measured, not computed. It's very accurate. A typical guidance platform for aerospace in the 1960s could measure angles to thousandths of a a degree and accelerations to 0.1 fps. And these were used on airplanes, missiles, and unmanned spacecraft.
We also need side thrusters to make fix the bearing towards the target.
No. The RCS thrusters primarily control attitude -- which direction the ship is pointing. In general, you point it a certain direction and it stays pointed until something acts to change it.
RCS can be used for translational control during docking maneuvers, and it is often used for small changes since its thrust can be more easily controlled than those of larger engines. This lets you do precisely controlled burns.
But once you orient the spacecraft for the main engine burn and then actually do the burn, you don't need to keep correcting things constantly. It doesn't actually matter which direction your spaceship points while it's coasting, at least for guidance purposes.
We have a copmuter on the board that using stored positions of five starts, calculates if the ship is "on the target".
No. The onboard computer does not constantly refer to star charts. The star references tell you only about orientation, not the progress along your path. And the star fixes are used only to correct any drift in the guidance platform.
The computer maintains what we call a "state vector". That's a three-dimensional point in space that represents your position, and a three-dimensional vector in space that represents your velocity.
Engine burns and other explicit, non-inertial accelerations affect the velocity in the state vector and thus the position, which is integrated from the velocity. Inertial changes to the state vector are computed every few seconds.
So the claim is that Houston Control didn't navigate the ship but the astronauts using the computer, were calculating their position and bearing and were adjusting their course using the thrusters. Am I correct so far ?
No, not really. You seem to think that computers have to furiously compute updated positions and apply constant control actions to keep the ship on course. Nothing could be farther from the truth.
Spacecraft generally coast.
For a typical Apollo mission you have the following major discrete maneuvers:
POI - parking orbit injection, accomplished by the S-IVB. TLI - translunar injection, accomplished by the S-IVB, which is then jettisoned. MCC-1, 2 - midcourse corrections (two opportunities), accomplished if necessary either by the SPS or the RCS. For hybrid missions, at least one of these maneuvers is mandatory LOI-1 lunar orbit insertion, accomplished by the SPS. LOI-2 circularization of lunar orbit, accomplished by the SPS. TEI - transearth injection, leaving lunar orbit to come home, accomplished by the SPS MCC-3, 4 -- like MCC-1,2 but for the trip home.
Aside from these the spacecraft merely coasts. Unlike Hollywood tells it, space flight does not require constant burning of engines or steering.
The intertial computations conducted periodically aboard the spacecraft were fairly crude. More detailed computations were done on the ground, and the revised state vector was periodically transmitted up to the spacecraft.
But this was essentially bookkeeping. Real-time computation of progress is not strictly necessary. It's nice to know where you are from moment to moment, but it is not crucial to the flight of the spacecraft that its state vector be kept updated at a very tight granularity. Generally errors were allowed to accumulate if they were small enough, and corrections were applied to the next scheduled burn.
For example, if the spacecraft were approaching the moon too quickly by just a few feet per second, that would be accounted for by adding a few feet per second to the LOI-1 manuever above and beyond that which would have been necessary in a nominal profile. But if a larger error were detected earlier, mission planners might opt for a correction at MCC-2, probably suitable for an RCS burn. A 5-fps burn at MCC-2, for example, might correct an error that would have accumulated to 50 fps at LOI-1, and an analysis of consumables would dictate which was preferable. (The RCS and SPS fuel supplies are separate, so whichever system has the best fuel surplus would address it.)
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Post by scooter on Mar 29, 2006 14:59:49 GMT -4
As you can see, you have come to the right place if you are really interested in learning how the Apollo program worked. These folks are scary smart on the details. It's rocket science, and there are people here that do it for a living.
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Post by twinstead on Mar 29, 2006 15:12:21 GMT -4
As you can see, you have come to the right place if you are really interested in learning how the Apollo program worked. These folks are scary smart on the details. It's rocket science, and there are people here that do it for a living. Yea, I was totally amazed when I first started learning about Apollo in this kind of detail just how much documentation and data was amassed during the missions. Few people realize that.
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Post by nomuse on Mar 29, 2006 15:54:44 GMT -4
As a side note, the buzz-bombs that crossed the Channel out of occupied France and landed in London were basically navigated by a clockwork timer and a magnetic compass.
One of my favorite bits of V1 trivia is that after the craft was on the wooden launch cradle and pointing at England the crew would crawl over it with wooden mallets -- aligning the metal skin to local magnetic field so it wouldn't throw off the compass!
(Read about this some years ago, so I may have gotten it wrong).
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Bob B.
Bob the Excel Guru?
Posts: 3,072
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Post by Bob B. on Mar 29, 2006 16:29:47 GMT -4
As a side note, the buzz-bombs that crossed the Channel out of occupied France and landed in London were basically navigated by a clockwork timer and a magnetic compass. I thought the V-1 had a small propeller that spun as it moved through the air. A counter kept track of the number of revolutions and when the proper number was reached, thus indicating the bomb had traveled the prescribed distance, the fuel to the engine was cut off and the bomb dropped on the target.
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Al Johnston
"Cheer up!" they said, "It could be worse!" So I did, and it was.
Posts: 1,453
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Post by Al Johnston on Mar 29, 2006 16:51:36 GMT -4
According to Wikipedia it did. Although there was nothing as sophisticated as a counter involved: a washer on a screw thread making a circuit did the trick. Primitive, but good enough for the job in hand. Another interesting point in the article is how the Germans chose to believe reports provided by British counterintelligence over the radio transmission data from the missiles themselves: potentially saving lives as the V-1s fell short.
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Post by nomuse on Mar 29, 2006 18:32:43 GMT -4
Yup. It's scary how accurate the things actually were. Only reason the Houses of Parliment et al are still standing is that the Brits were as eerily efficient in capturing the spies the Germans were using as forward observers. Unfortunately, that meant some people in the poorer districts of London took the hits instead.
I remember vaguely about the prop thing. Did mean they were relying on accurate information about wind speed.
Aha. Just read the Wiki article. So when that prop closes the contact, a guillotine chops the elevator cables and the V1 dives...co-incidentally stopping the motor because of fuel flow problems? Hehe. I love details like that. I'm glad the Wiki mentions one of my other fun details....the RAF tactic of "tipping" the doodle-bugs by sliding one wing under the bombs....
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Post by snakeriverrufus on Mar 29, 2006 22:02:42 GMT -4
but simply to pass through them and make round trip to the moon orbit, circle the moon few ties and come back . When they circle the moon they should be able to take nice photo of one of the moon bases and or vehicles. Ahhh,you choose not to believe in the most documented achievement in human history but would accept a photo taken today? Said photo would have to be taken by the same agency that you believe faked all earlier landings but now you will believe. does that make sense to anybody else?
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Post by nasamoonedamerica on Mar 29, 2006 23:38:50 GMT -4
I would like to thank brotherofthemoon for pdf documentation about various space programs. Also Thanks to JayUtah about extensive answer.
I have to be honest. I have a lot of reading to do before I continue this discussion. Even when it is one issue at a time. I have a degree in electrical engineering with Computer science major and I hope I'll have enough training to read through the MANNED SPACEFLIGHT PDF DOCUMENTS.
Thanks to all for all your help. I' really have to read more next time before I engage into argument with NASA engineers (at least some of you are close to that). Still Rene's book sounds convincing and at this moment I really don't know whom to believe. That is why I'll have to compare his claims with the official documentation and try to decide with my own brain who is telling the truth and who is not.
THANKS FOR WELCOMING ME ON THE BOARD. When I'm ready I'll start new thread with a discussion.
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Post by PeterB on Mar 29, 2006 23:42:31 GMT -4
nasamoonedamerica I appreciate your honesty and willingness to learn, and I think a few other people here would say the same thing. It isn't easy to say what you've said. Feel free to come back and ask more questions if you're interested. Otherwise, I can highly recommend a visit to the Apollo Lunar Surface Journal for in depth information about the astronauts on the Moon: www.hq.nasa.gov/alsj/
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Post by JayUtah on Mar 29, 2006 23:52:32 GMT -4
Also Thanks to JayUtah about extensive answer.My pleasure. I have to be honest. I have a lot of reading to do before I continue this discussion.Maybe my bibliography section will help. www.clavius.org/bibidx.htmlI hope I'll have enough training to read through the MANNED SPACEFLIGHT PDF DOCUMENTS.Ask. We're actually very nice people. We help each other all the time with that sort of thing. ...before I engage into argument with NASA engineers (at least some of you are close to that).Well I'm not a NASA engineer. That's actually one of the funny things about the hoax theories. They all seem to base themselves on the tacit premise that only NASA would know the difference. It is possible to be an engineer working with actual space hardware without having anything to do with NASA. Up until very recently I had no direct ties with NASA. However, my company will announce fairly soon a major procurement by NASA's Goddard Space Flight Center. But I had absolutely nothing to do with landing it, nor will I likely have anything to do with building it aside from my normal duties. Still Rene's book sounds convincing and at this moment I really don't know whom to believe.It's meant to be convincing. Unfortunately it's not necessarily meant to be true. Re-read it with the notion in mind that Rene has had no training or experience in any of the topics he discusses.
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Post by sts60 on Mar 30, 2006 1:18:34 GMT -4
I have to be honest. I have a lot of reading to do before I continue this discussion. Even when it is one issue at a time.
Good for you. That's not meant to be patronizing; it's nice to have an Apollo-doubter (for want of a better term) who doesn't just come in insisting they have all the answers and spouting dozens of claims at a time, ignoring all counterarguments.
I have a degree in electrical engineering with Computer science major and I hope I'll have enough training to read through the MANNED SPACEFLIGHT PDF DOCUMENTS.
That certainly can't hurt, but as an MSEE myself I can tell you it's only part of the story. For example, does your comp sci concentration include work in embedded systems? If so, that will help you understand why the AGC didn't need the bloated memory of today's PCs to do its job.
Thanks to all for all your help. I' really have to read more next time before I engage into argument with NASA engineers (at least some of you are close to that).
I'm also happy to help, especially for someone who doesn't come in hear insisting that anyone who disagrees is an idiot/dupe/NASA stooge. I've worked on NASA-funded projects (and DoD-funded, and purely commercial) but I'm not a NASA engineer either.
Still Rene's book sounds convincing and at this moment I really don't know whom to believe.
HBs and CTs love to wave all sorts of impressive-sounding claims around, but in general when you start to examine their claims one at a time they turn out to be bogus. So you go on to the next one, and that's bogus (or irrelevant). And so on. But, like you said, let's take one thing at a time and see what happens.
That is why I'll have to compare his claims with the official documentation and try to decide with my own brain who is telling the truth and who is not.
Not only "official documentation", but in many cases standard engineering practice, or fairly straightforward physics.
THANKS FOR WELCOMING ME ON THE BOARD. When I'm ready I'll start new thread with a discussion.
Welcome again, and looking forward to an in-depth discussion.
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Post by dwight on Mar 30, 2006 2:38:02 GMT -4
I have to say nasamoonedamerica, your response is the most honest and courageous things I have read on this board for a long time. (Not putting down anyone else by that). I am not an engineer as such, but I do work in TV and have extensive experience with satellite uplinks and the like, so I guess the TV, USB side of Apollo is my speciality. Unfortunately I dont have the eloquence of Jay and the others when it comes to expressing how things go in the TV realm, but I do think I have a good grasp (Oh Lord I honestly _hope_ I know what I am talking about) with regards to the videos of Apollo.
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Post by Obviousman on Mar 30, 2006 5:27:19 GMT -4
The onboard computer renders that into a local vector according to the REFSMMAT, the "reference to stable member matrix", or the local coordinate system of the spacecraft relative to its gyroscopic guidance platform. Sorry to interject, but it reminds me of something: Who's that? Sheet man, that's Captain Refsmmat, the ideal Flight Controller! He's the best man we ever had in the Trench!
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