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Post by gillianren on May 22, 2008 12:46:05 GMT -4
Four--the four driving factors . . . .
I'm not impressed. The most basic questions remain.
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Post by sts60 on May 22, 2008 14:01:56 GMT -4
Yup. If you actually calculate the angular diameters of Earth and the window you find that at about 130,000 miles distance from Earth they'd have to get over 12 feet away before the angular diameter of a 9" diameter window got smaller than the angular diameter of the Earth. They don't have 12 feet to move away from the window inside the command module, so they could film the entire Earth through that window at any distance from the window that they could be. Jason, I got the same thing - about 12-1/4 feet, with an Earth angular size of 3.49 degrees.
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Post by JayUtah on May 22, 2008 14:24:45 GMT -4
I concur with the computations presented, however it's a red herring because there is evidence to contradict Bart Sibrel's claim that the astronauts were not close to the window. And the fact that Sibrel leaves that evidence out of his first film is an indication to me he realizes that his hypothesis is problematic on this point. The window size issue has subverted support.
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Post by Count Zero on May 22, 2008 18:39:17 GMT -4
From the Apollo 11 Mission Report: "The digital autopilot was used to initiate the passive thermal control mode at a positive roll rate of 0.3 deg/sec, with the positive longitudinal axis of the spacecraft pointed toward the ecliptic north pole during translunar coast." Thus the CSM/LM stack was broadside to the sun, and parallel to the Earth's terminator. This is consistent with the image from the television broadcast:
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Post by scooter on May 22, 2008 20:45:24 GMT -4
Four--the four driving factors . . . . I'm not impressed. The most basic questions remain. I've got to find that Monty Python skit now... Now that Turbonium now understands the difference between spacecraft orientation vs spacecraft trajectory, and how, in space, the orientation is based on other factors, perhaps he will re-think, and perhaps retract the associated, errant, claim?
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Post by Count Zero on May 22, 2008 21:07:02 GMT -4
"A dream is a wish your heart makes..."
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Post by Ginnie on May 22, 2008 21:27:12 GMT -4
Another hit and run thread?
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Post by Czero 101 on May 22, 2008 21:30:50 GMT -4
Another hit and run thread? You weren't actually expecting intelligent discourse and debate were you? Cz
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Post by scooter on May 22, 2008 23:31:02 GMT -4
Turbonium, you really gotta download Orbiter...
While not "aimed" at a point in space, the "trajectory" of the CSM/LM was at a point way in front of the Moon, when this video was taken. Now, we want to meet the Moon, using a highly eliptical Earth orbit with the high point a bit beyond the Moon's orbit. This was created with the TLI burn by the SIVB. The orbit high point coincides with the position where the Moon WILL be some 2.5-3 days hence, when the CSM/LM gets there. Leading the target, to borrow a skeetshooting term. The burn points them at an empty point in space, ahead of the Moon in it's fixed Earth orbit, where the Moon will be when the CSM/LM stack gets there.
There are other complex factors, 2 (3?) body patched conic computations (lots of varying gravitational influences enroute), and the like, but it was all based on basic Newtonian physics, and gobs of precise math.
BTW, the Shuttle spends most of it's time on orbit "upside down and backwards", with no effect on it's orbit. In a vaccuum, orientation has no significance, except during burns, when force is being applied in precise, desired vectors. The shuttle orientation is for TPS protection, the Apollo orientation was for thermal and communications considerations.
edit for spelling
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Post by JayUtah on May 23, 2008 0:31:53 GMT -4
There are other complex factors, 2 (3?) body...
Restricted three-body. "Restricted" means that one of the bodies is of insignificant mass compared to the other two, and is not considered to affect their orbits.
...patched conic computations [...] and gobs of precise math.
The patched-conic approach is analytic and provides a first order approximation that was refined numerically on the CDC supercomputer at Lawrence Livermore National Lab. This was done for several variations on the orbits, such that intermediate results could be fed into the RTSS mainframes and refined for precise time-dependent mission parameters.
But yes, Newtonian physics sufficed for Apollo flight dynamics.
The shuttle orientation is for TPS protection...
And for other modes of protection: the orbiter's now-pointless SSMEs are used as shield mass against collisions with debris and micrometeoroids.
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Post by Count Zero on May 23, 2008 0:45:10 GMT -4
Just be sure to use the new cover-sheet for your TPS reports. You got that memo, didn't you? I'll send it again.
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Post by gillianren on May 23, 2008 1:02:36 GMT -4
Gods, Your Excellency, that's just what I was thinking.
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Post by JayUtah on May 23, 2008 1:45:11 GMT -4
Hm, some additional detail, then.
Orbital mechanics problems fit into categories of complexity depending on how many people are on the dance floor and how big they are.
Two-body problems are the easiest because they have closed-form solutions. Closed-form math just means equations you only have to solve once for each problem. Three-(or more)-body problems are harder because they have only iterative solutions. Those are things like Taylor series methods where you feed the results back into the equation repeatedly and solve it several times until the answers converge to a single value.
Restricted forms of those problems are ones where you assume that some bodies are hugely more massive than others. The shuttle orbiting the Earth is a restricted two-body problem. The math for those is simpler than the unrestricted forms. But the restricted three-body problem is still iterative. For Apollo it means that the path of the CSM/LM is given by its interaction with the combination of Earth and Moon gravity fields, but the motions of the Earth and Moon aren't affected by the gravity field generated by the CSM/LM. You find the positions of the Earth and Moon as if the CSM/LM weren't there.
Patched conics. All orbits are conic sections -- that peculiar class of geometric shapes generated by the intersection of a plane with a cone. Because all orbits share some mathematical fundamentals, orbits can be fit together with some nice continuity properties. Algebraic continuity of a certain degree answers velocity questions, since we're talking about an object moving along that path.
C0 continuity is when two curves meet at a point, but not smoothly. C1 continuity is where two curves meet, and their first calculus derivatives (i.e., rates of change) also meet at a single point. C2 means the second derivatives meet, and so forth. So to create a transfer orbit by the patched-conic method, you find two ellipses (one an orbit around one body, and the other an orbit around the other body) that meet with the desired degree of continuity. The geometry and continuity of your two particular choices vary with mission velocity, etc.
That's your first stab at a transfer orbit. You plug those values into the iterative orbit solver as the initial guess, and it solves the restricted two-body problem down to the actual trajectory. Patching conics doesn't give you the whole solution because your ellipses are each chosen with only one body in mind.
In the late 1960s, Control Data Corporation delivered the first of its new model of supercomputer to Lawrence Livermore National Laboratory. You have to understand internal DOE politics: there's a perpetual "arms race" for computing power. This supercomputer was used to solve several different initial patched-conic solutions down to the point where the actual orbits could be computed from the intermediate results on the slower IBM mainframes.
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Post by ineluki on May 23, 2008 7:53:08 GMT -4
The question was about the craft's orientation, not its flight path. Are you saying that a spacecraft's "nose" must always be pointed in the direction of travel, as an aircraft's is? Of course, I saw it on StarWars and anyway, the CSM doesn't have wings, it could never change course...
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Bob B.
Bob the Excel Guru?
Posts: 3,072
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Post by Bob B. on May 23, 2008 9:16:24 GMT -4
Other than during propulsive maneuvers, the direction of the flight path is of no consequence for spacecraft orientation. I’d like to clarify the above statement. After re-reading I fear my statement might be misinterpreted to mean the spacecraft must be aligned with the flight path during propulsive maneuvers, i.e. engine burns. Although this may be true in some instances, it is certainly not true in all instances. The reason for an engine burn is to alter the spacecraft’s velocity vector. The direction and duration of the burns depends on the velocity change that must be added to the initial velocity vector to result in the desired final velocity vector. Most of you probably already understand this, but let’s consider the following example: Suppose that A is our initial velocity vector and C is our desired velocity vector. Adding vector B to vector A results in vector C. Vector B is therefore the velocity we must add to make our course correction. During the engine burn the spacecraft is aligned along the direction of vector B.
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