|
Post by PhantomWolf on Oct 26, 2010 23:50:13 GMT -4
The Space Shuttles fly around the ISS after undocking to take photographs. I wonder if inquisitivemind thinks that is impossible too? I wouldn't go there, the fact that the Shuttle does a blackflip and orbits upside-down and backwards with just enough rotation to do a single revolution every orbit would probably blow his mind as it is.
|
|
|
Post by nomuse on Oct 27, 2010 0:16:53 GMT -4
It cannot calculate a 90 degrees pitch precisely in advance, it would be very imprecise. In order to make rotations, it gives an order, checks with the measured result, computes a new order by comparing with the measured result, and so on, step by step. But it can't do it precisely if the rotation is too fast, and it had a very slow computer. How fast (and how powerful) do you think THIS computer is: www.youtube.com/watch?v=8YlYSyWiBs8That's doing a fully autonomous flight with multiple waypoints and even returning to a preset location after the transmitter is switched off.
|
|
|
Post by kallewirsch on Oct 27, 2010 5:17:54 GMT -4
I also remember to heave heared, that the LM was 'over' the CSM because it is easier to inspect it with a black background, then to try to make out details with the brightly lit surface of the moon in the background
Why? If you know the roll-rate given a specific thruster pulse, the whole operation just goes like this: give the thruster pulse, start counting 1, 2, 3, 4 give a thruster pulse in the opposite direction. And yes, since they knew there engines, they could precompute how long it will take for a specific orientation.
And since the whole point of the manouver was to present the LM to the CSM crew member, I am not even sure that the whole manouver was done under computer control. The whole point was to give the CSM pilot a good look at the LM as a whole and not to get a specific attitude. The captain just pushed his steering stick and the LM started rolling. The captain pushed it again in the opposite direction and the LM stopped rolling. Thats it, there isn't much more to it. No big deal at all. A commercial airliner pilot is not required to inspect his airplane during preflight checkout from always exactly the same viewpoints, measured in millimeters and angles. If he just looks into his gear system such that he can see everything this is enough. Same here: The point was to see, if the LM has survived earth ascent, disposition and docking during the lunar coasting without any major visible damage and that the landing gear struts were in a locked position. For that you don't need a precisely 90° roll, 85° or 105° would be fine too.
And please IM: get you usage of the acronyms LM and CSM right. It just makes you look silly and clueless if you use the wrong acronyms.
|
|
|
Post by Mr Gorsky on Oct 27, 2010 6:29:45 GMT -4
You know, I can explain all kinds of things. It doesn't mean those explanations will be correct. Surely that's more important than just being able to explain. Absolutely ... my teenage children are now calling me out on all of those things I "explained" to them when they were younger.
|
|
|
Post by chew on Oct 27, 2010 10:40:21 GMT -4
And please IM: get you usage of the acronyms LM and CSM right. It just makes you look silly and clueless if you use the wrong acronyms. And stop confusing "centrifugal force" with "centripetal force" in your attempts to describe orbital motion.
|
|
Bob B.
Bob the Excel Guru?
Posts: 3,072
|
Post by Bob B. on Oct 27, 2010 10:50:30 GMT -4
The Space Shuttles fly around the ISS after undocking to take photographs. I wonder if inquisitivemind thinks that is impossible too? I'm not exactly sure what maneuvers are performed to do that, but flying a circle around another vehicle is pretty easy - just thrust up or down. Let's say we have two vehicles in identical circular orbits, with vehicle #2 trailing close behind vehicle #1. If #2 thrusts down, only the eccentricity of the orbit changes - the semi-major axis and period remain the same. Vehicle #2 starts to descend, and in doing so, spends up. #2 will drop below and directly under #1. After reaching perigee, #2 starts to rise and slow down, but its still going faster than #1. #2 will move to a position directly ahead of #1. #2 continues to rise but is now moving slower than #1. #2 will move to a position directly above #1. After reaching apogee, #2 starts to fall and speed up, but it's still going slower than #1. #2 will move back to its starting position directly behind #1. Vehicle #2 has made a complete circle around #1 over the course of one complete orbit. Similarly, #1 could make a loop around #2, but it would have to thrust up - moving successively above, behind, below and back in front of #2. Edited to clarify: By thrusting down I mean the acceleration is downward - the thrusters exhaust upward.
|
|
|
Post by JayUtah on Oct 27, 2010 10:54:42 GMT -4
If you know the roll-rate given a specific thruster pulse...But you don't, because that depends on the spacecraft moment of inertia in that axis at the time of the firing, which you know only approximately from time to time because of fuel depletion, fuel slosh and crew movement. Closed-loop control is simpler and more accurate. That said, for programs that implement outer-loop control, such as P64, FINDCDUW computes a lag angle. FINDCDUW is the program that translates high-level steering commands into specific DAP set points. For ATT HOLD lag angles are not computed. One of the functions of a lag angle is to provide a temporary hysteresis during a rotation in progress. In this function it represents an angular offset from the desired attitude, at which the reverse impulse will begin. And yes, it is computed from best-guesses for angular acceleration. It doesn't have to be spot-on because it's only a refinement on the basic algorithm that minimizes rate errors and overshoot. In the Boeing roll-channel example, the lag angle in the roll channel was given as 5º, at which point from the desired roll angle the roll controller begins to roll out according to a ramped gain. The roll angle set-point in that example was 20º, which Boeing's nominal bank angle, so when the roll angle reaches 15º, the ramp-down begins. That lag angle value will have been determined empirically to approximate the roll angle through which the airplane will have moved while reverse aileron is applied. That's to say, "I'm going to apply reverse aileron when my roll angle gets to within 5º of the desired angle, because by the time the ailerons apply their full effect I will have rolled another 5º." Every control system overshoots. Which is to say, if your thermostat is set to 70 F, the heater will begin its shutdown when the thermometer reaches 70 F, but latency in the thermometer and the residual venting will push the temperature to 71 F. You overshot. A smart thermostat would introduce a lag temperature offset of 1 degree F which is added to the error -- actual temperature 69 F minus set-point temperature 70 F is -1 F, plus the lag of 1 F, equals a zero error, which begins the shutdown sequence. The error lag in a control system is a first-order approximation to account for likely overshoot -- a dumb-as-dirt differential controller. In ATT HOLD mode, as opposed to outer-loop control, which requires AUTO mode, overshoot is considered acceptable. Because of the magnitude-scaled HSLOPE desired error rate, the system will converge. Outer-loop controllers such as P64 are allowed to command a maneuver rate, from fractions of a degree per second up to 10º/s. Again, the DAP is simple and stupid. P64 is meant to be brilliant. But if you simply dumped a new attitude set point into the DAP, it would zip there at the default rate WL, which might be too slow or fast according to the wishes of outer-loop control (which is a policy agent). Hence P64 is allowed to say, "Move from your present pitch attitude to a pitch angle of 30º at a rate of 9º/sec (smartly, because we want to reacquire radar lock ASAP)." The lag angle here also prevents rate overshoot by making the initial attitude error look artificially small. And since the whole point of the manouver was to present the LM to the CSM crew member, I am not even sure that the whole manouver was done under computer control.Correct. Servicer is running, so the state vector is being updated as you move around. But MSFN is going to track you more accurately anyway and read up your final s.v. prior to PDI. So at this point you can do whatever maneuvers you need as long as you don't fire up the DPS or SPS and wreck MSFN's gross orbital fix. But yes, there is no outer-loop control for the separation and checkout procedure. To fly the LM "manually" in coast/orbital flight: Guidance Select to PGNCS DAP Mode to ATT HOLD then grab the hand controllers and go to town. You can select minimum-impulse RCS with V76E on the DSKY if you're fuel-conscious. And patient. The point was to see, if the LM has survived [...] For that you don't need a precisely 90° roll, 85° or 105° would be fine too.True, but I gather from his context that he's talking about the P64 pitchover. That's a 30-40 degree change in pitch, but the final pitch has a narrow tolerance (half a degree or so). Thankfully P64 starts, warms up (i.e., acquires at least two s.v. updates via Servicer), and then commands its pitchover under FINDCDUW filtration, so we're good. get you usage of the acronyms LM and CSM right. It just makes you look silly and clueless if you use the wrong acronyms.I'm assuming some of those are the acronyms for the French names of the spacecraft.
|
|
|
Post by inquisitivemind on Oct 27, 2010 11:01:35 GMT -4
I say that the LCM (or CSM, call it like you like) naturally remains horizontal, and by this I mean tangential to the moon surface. What you are saying is that the CSM keeps the same orientation in an absolute system independent from the moon, which would cause its attitude to change relatively to the moon as it orbits it. I have looked for an article which would explain you how it works, but it seems very difficult to find the article you are looking for on internet. But I have found this: celestrak.com/columns/v04n09/"keeping the antennas pointed at the earth and preventing the satellite from going into a flat spin (which is the natural tendency)." This means that if no attitude control was exerted on the satellite, then it would show always the same attitude relatively to the earth (the wrong one). If the satellite was keeping the same attitude relatively to an absolute system independent from the earth, and was constantly changing its attitude relatively to the earth as it orbits it, then it would have no "natural tendency". The side it would show to the earth would constantly change. So, may be you are saying that this article is wrong too and "naive"? Furthermore, if what you are saying was true, that the CSM always keeps the same absolute attitude independent from the moon, and thence its attitude was constantly changing when it orbits the moon, and that it would require a "control" to keep it parallel to the moon, then we should see the horizon line progressively go down on the sequence of photos in Apollo 11 in which we can see the earth rise over the moon! The truth is that no control is required to keep the CSM parallel to the moon as it orbits it, for it is its natural orientation when it orbits it. Of course it can change its orientation with its lateral reactors, but it requires a little effort. The CSM didn't have to change it natural horizontal attiude, because this attitude was just what the LEM was requiring when it was leaving to the moon, and also when it was coming back from the moon. But of course, I'm an idiot, a naive chump who knows nothing about nothing, and you are the brilliant minds who know everything!
|
|
|
Post by inquisitivemind on Oct 27, 2010 11:05:43 GMT -4
The Space Shuttles fly around the ISS after undocking to take photographs. I wonder if inquisitivemind thinks that is impossible too? It's not the same thing, the space shuttle is not landing on the moon! Furthermore the space shuttle has a more powerful computer than the LEM had! When the LEM is going to the moon, it has no fuel to waste, because ot might need this fuel if the conditions for landing on the moon are difficult.
|
|
|
Post by scooter on Oct 27, 2010 11:11:11 GMT -4
oooh boy, here we go...
|
|
|
Post by inquisitivemind on Oct 27, 2010 11:12:29 GMT -4
I would also like to ask you a question: If you think that the LEM is easier to maneuver than a plane, then why did Armstrong have to much difficulty to pilot it that he had to eject himself from the LEM prototype before the LEM prototype crashed? The LEM is not easier to pilot than a plane, much to the contrary, it's much more difficult to pilot it than a plane.
|
|
|
Post by JayUtah on Oct 27, 2010 11:12:50 GMT -4
I wouldn't go there, the fact that the Shuttle does a blackflip and orbits upside-down and backwards...There's the post-docking inspection maneuver, but the backflip is part of the approach maneuver. The orbiter has to approach the ISS carefully to avoid hosing down the latter's delicate solar panels with its RCS exhaust. with just enough rotation to do a single revolution every orbit would probably blow his mind as it is.LVLH mode in the orbiter's DAP is pretty darn sophisticated, but so was ORDEAL for Apollo. It had to account for constantly changing orbital elements in order for P63 to maintain the right notion of lunar "up."
|
|
|
Post by inquisitivemind on Oct 27, 2010 11:13:22 GMT -4
Yes, we go for a lot of misconceptions from your friends.
|
|
|
Post by JayUtah on Oct 27, 2010 11:18:46 GMT -4
If you think that the LEM is easier to maneuver than a plane, then why did Armstrong have to much difficulty to pilot it that he had to eject himself from the LEM prototype before the LEM prototype crashed?First because the LLRV broke and was thus rendered uncontrollable. Second because the LLRV is not a "LEM prototype." It's a LM Earth-gravity trainer. The LEM is not easier to pilot than a plane, much to the contrary, it's much more difficult to pilot it than a plane.No. You haven't addressed any of the reasons I've given why your statement isn't true. You've simply restated your belief. "The autopilot in Apollo has a somewhat easier job than that found in aircraft." (Frank O'Brien. The Apollo Guidance Computer: Architecture and Operation. p. 316).
|
|
|
Post by echnaton on Oct 27, 2010 11:21:03 GMT -4
I would also like to ask you a question: If you think that the LEM is easier to maneuver than a plane, then why did Armstrong have to much difficulty to pilot it that he had to eject himself from the LEM prototype before the LEM prototype crashed? The LEM is not easier to pilot than a plane, much to the contrary, it's much more difficult to pilot it than a plane. Armstrong crashed in the Lunar Landing Research Vehicle. It was not a prototype LM, but a purpose built training vehicle. The LM could not fly on earth and the LLRV could not fly in space. It helps to get your fact straight before complaining that others don' t know theirs. An analogy for your misunderstanding is to complain that a 747 can't be flown because a Piper cub will blow over in a strong wind.
|
|