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Post by echnaton on Dec 7, 2009 22:26:11 GMT -4
Nice work. This thread has resulted in some very clear explanations.
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Sabine
Mercury
A closed mouth gathers no foot
Posts: 12
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Post by Sabine on Dec 11, 2009 13:35:09 GMT -4
That is absolutely brilliant! I like how you can see the "window" start to close as the thicker part of the belt rotates into the plane of the trajectory. That, sir, is definitive. These videos and visuals have really clarified this whole issue for me, but this is something I'm still not clear on: why does that window start to close? I guess I was under the impression that these belts don't move.
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Post by randombloke on Dec 11, 2009 13:56:30 GMT -4
The Earth does, and so do capsules, therefore, relative* to the Earth and/or the capsule, the belts do move. there is only a limited period in each Earth orbit where firing the boosters now instead of +/-5 minutes from now will put you both on the proper trajectory to intercept the moon and also avoid the thickest part of the belts.
That point moves relative to the Earth, the belts and the capsule because everything is in motion up there.
*i.e. taking either as your static reference frame.
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Post by Jason Thompson on Dec 11, 2009 17:36:12 GMT -4
I guess I was under the impression that these belts don't move. The belts don't move relative to the surface of the Earth, but once you're in orbit your motion is independent of the surface. Because the Earth rotates under you the belts rotate with it, and since the axis on which the belts rest is offset compared to the Earth's rotational axis that introduces some extra complexity to the system. In addition to that, the fact that the belts are formed by interaction with the solar wind means that as they rotate they change shape, being compressed on the sunward side and stretching out on the opposite side (so, to use fm's absurd oversimplification, the altitude to which they extend over any given point on the surface varies throughout the day as the Earth rotates). It's a very complex system, which is why fm's insitence on simplifying it has resulted in so many 'but you can't do that' responses.
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Sabine
Mercury
A closed mouth gathers no foot
Posts: 12
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Post by Sabine on Dec 12, 2009 7:08:00 GMT -4
Thanks Jason and randombloke. I see what you mean about the earth's rotation and the different planes. I thought it must be something like that, but with so many components moving in relation to each other, it's a bit confusing to the lay person. Actually, quite confusing, but it is after all "rocket science".
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Bob B.
Bob the Excel Guru?
Posts: 3,072
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Post by Bob B. on Dec 12, 2009 15:24:40 GMT -4
Nice work, drewid. I'd like your permission to add this to my web page. If permission is given, I may need you to write an introduction to your animations.
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Post by drewid on Dec 12, 2009 16:46:58 GMT -4
Sure, no worries.
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Post by cos on Dec 14, 2009 10:49:16 GMT -4
FM,
Do you now understand how Apollo safely navigated the Van Allen Belts? I've certainly learnt something from this thread (thanks to Bob B, drewid et al.) and it would nice to know that you had too.
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Post by LunarOrbit on Dec 14, 2009 20:49:51 GMT -4
FM's silence speaks volumes.
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Post by Obviousman on Dec 15, 2009 2:24:48 GMT -4
As people have said, I hope FM has learnt something and acknowledges it. I know I have - it's made things a lot clearer and dispelled some misconceptions I had.
FM: acknowledging you were wrong and have learnt something from others is not the mark of a stupid person; it's the mark of a wise person.
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Post by fm on Dec 23, 2009 17:04:06 GMT -4
FM's silence speaks volumes. ;D No, no it only says that I have end of the year deadlines, the holidays and other important issues that need to be addressed first. Yes I did learn something, at the same time I still have questions and comments I will post in the near future. Remember, its the Belts, Outer Space and the Moon that I wanted to get addressed regarding space being or not being "awash" with radiation. But before I go... is there any NASA literature that states why NASA choose that particular inclination of around 30 degrees for the TLI? Was it because they wanted to minimize their stay in the belts, or was it for another reason? Happy Holidays !
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Bob B.
Bob the Excel Guru?
Posts: 3,072
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Post by Bob B. on Dec 23, 2009 21:48:30 GMT -4
But before I go... is there any NASA literature that states why NASA choose that particular inclination of around 30 degrees for the TLI? Was it because they wanted to minimize their stay in the belts, or was it for another reason? You can't have an inclination less than the latitude of the launch site; therefore, launching from Kennedy Space Center is going to put the spacecraft in an orbit with a inclination of not less than about 28.5 degrees no matter what you do. So Apollo was already well positioned to avoid the VARB as a consequence of the launch site latitude. Launching into Earth orbit with an inclination of about 32.5 degrees was common NASA practice going all the way back to the first orbital Mercury flight. Off the top of my head, I don't recall the exact reason for this. The early Apollo missions followed this same practice, though some of the later missions began to deviate from this a bit as necessary to reach their landing sites (for example, Apollo 17 had a parking orbit inclination of 28.526 o and a translunar inclination of 28.466 o, the least of all the missions). Since they were already in an advantageous inclination, the engineers were ingenious enough to take full advantage of it to avoid the worst parts of the VARB. The better question might be, if the launch site was nearer the equator, would they have used the extra propellant needed to get into the higher inclination or would they have chosen a different method to mitigate the radiation of the VARB, such as more shielding? Since this wasn't an issue, I guess we'll never know the answer.
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Post by trebor on Dec 24, 2009 7:12:18 GMT -4
Remember, its the Belts, Outer Space and the Moon that I wanted to get addressed regarding space being or not being "awash" with radiation. "awash" is a meaningless term. Please be more specific. As for the Moon, the Russian lander Luna 9 measured a radiation dose on the surface of 30 millirads per day. 5 months worth of exposure to that level of radiation 'may' give you an increased chance of developing cancer.
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Post by drewid on Dec 24, 2009 8:23:16 GMT -4
Quite The earth's surface is "awash with radiation", It would be hard to see stuff / listen to the radio / watch TV if it wasn't.
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Post by tedward on Dec 24, 2009 8:54:25 GMT -4
I hate it when you see comments like "awash". Well, not hate it, just that it appear to be used as the old 50's radiation killer mutant mentality to get across to the uninformed that space is deadly so we could not have gone there. Am I wrong in this case?
Satellite comms are up there in abundance beaming their rays at us. Various frequencies are beamed at us 24/7, sat nav, TV signals, earth monitoring stuff, TV and Radio terrestrial transmitters and cell phone sites saturate us from an earthly perspective as does other transmission services like pagers and taxi radios and this is just picking a few from the transmitted spectrum.
Am I dead yet?
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