raven
Jupiter
That ain't Earth, kiddies.
Posts: 509
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Post by raven on Feb 2, 2012 13:17:20 GMT -4
The Apollo parking orbits were all quite low to maximize efficiency. It would have been even more efficient to launch directly into a lunar trajectory, but the launch windows would have been extremely limited. So much so that they wouldn't have made it to the moon by the end of the decade.
I know this means you need to get the timing pretty exact and any delays in launch would create further delays, but why would have it meant not getting to the moon by the end of the decade? |
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Post by forthethrillofital on Feb 2, 2012 18:52:41 GMT -4
Just the other day, I was talking to Jarrah White. We all know his stance on Apollo. Now I'm no expert in radiation by any means of the word, and it was just last week that I even got the idea what MeV and REM was. Well Jarrah offered the following theories on why the Apollo shielding should have been about 10cm thick. I'm sure there are plenty of people here who know a lot more about radiation than me, so I'd like to tell you guys what the calculations were and I want you to debunk the holy living hell out of me. Jarrah says the outer belts had an average of 10-100MeV. I told him we could work on the principle that they were 50MeV. Now, here is what he told me. To find the "aerial density?", we must multiply 50 by .545cm^2. We have 27.25cm^2. Now he said we have to divide that by the density of the material itself. Aluminum has a density of 2.7cm^3. We get about 10.09cm^3 worth of shielding. The LM had paper thin shielding. Jarrah says this is also without all the "secondary" radiation you'd get from particles bouncing off each other and fragmenting. According to him, with these numbers, the Apollo astronauts should have absorbed 1200REM, when the official log says they absorbed 2REM. Like I said, I'm no expert, so of course the data means little until I can get a better understanding from the Pro-Apollo side (like me). By no means, I hope everyone realizes, does this mean I have doubts about Apollo. Well anyway, I was hoping some of you radiation guys could come and tell me why you don't need 10cm. Thank you! I have read that a greater concern than VA Belt radiation was the not unlikely occurrance of a high dose of radiation being delivered via a sporadic solar flare. I read that flares releasing lethal radiation doses are not uncommon. It is argued Apollo missions would not be launched into the teeth of such dangerous circumstances. This seems very reasonable to me. |
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Bob B.
Bob the Excel Guru?
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Post by Bob B. on Feb 2, 2012 20:00:48 GMT -4
I have read that a greater concern than VA Belt radiation was the not unlikely occurrance of a high dose of radiation being delivered via a sporadic solar flare. I read that flares releasing lethal radiation doses are not uncommon. I fixed it for you. |
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Post by Data Cable on Feb 2, 2012 20:21:38 GMT -4
I have read that a greater concern than VA Belt radiation was the not unlikely occurrance of a high dose of radiation being delivered via a sporadic solar flare. I read that flares releasing lethal radiation doses are not uncommon. It is argued Apollo missions would not be launched into the teeth of such dangerous circumstances. This seems very reasonable to me. I have read that lightning can be very dangerous, even lethal. Therefore, anyone claiming to have left their house during a thunderstorm must have been lying. This seems very reasonable to me. |
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Bob B.
Bob the Excel Guru?
Posts: 3,072
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Post by Bob B. on Feb 2, 2012 20:56:30 GMT -4
I’d like to understand how to calculate absorbed radiation dose and equivalent dose. I think I’ve got a handle on it but I’d like confirmation. Below is a sample calculation and I request reviews of my method. Please correct me if I’m doing anything wrong. Thanks.
Suppose an 80-kg man is subjected to beta radiation (electrons) for a period of 10 minutes. The energy and flux of the radiation is,
Energy = 1 MeV / electron
Omnidirectional Flux = 106 electrons/cm2-sec
Since the flux is omnidirectional, a body will be exposed to this flux from all directions. I presume, therefore, that a person’s entire skin area is the absorbing surface. An adult male has about 2 square meters of skin, or 20,000 cm2. The total energy absorbed is the particle energy times the flux, times the area, times the duration.
Absorbed energy = 1 MeV/electron × 106 electrons/cm2-s × 20000 cm2 × 600 s = 1.2×1013 MeV
Next I convert MeV to joules,
1.2×1013 MeV × 1.602×10-13 J/MeV = 1.92 J
The absorbed dose is the absorbed energy divided by the mass of the body, where 1 Gray = 1 J/kg.
Absorbed dose = 1.92 J / 80 kg = 0.024 Gy (2.4 rad)
To obtain equivalent dose, the absorbed does is multiplied by the quality factor for the particular type of radiation. For beta radiation, the quality factor is 1, therefore
Equivalent dose = 0.024 × 1 = 0.024 Sv (2.4 rem)
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Post by ka9q on Feb 3, 2012 1:21:45 GMT -4
Bob, I haven't checked your math but overall it looks right assuming the 1 MeV electrons aren't so energetic as to ever pass completely through the body.
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Post by ka9q on Feb 3, 2012 1:31:32 GMT -4
I know this means you need to get the timing pretty exact and any delays in launch would create further delays, but why would have it meant not getting to the moon by the end of the decade? It has to do with the relative positioning of the launch site and the moon. To do a direct injection from KSC into a lunar trajectory, the injection point (where the upper stage shuts down) has to be at the antipode of the moon's position at arrival. That's a latitude equal to the negative of the moon's declination. I really should check the references first, but here's my understanding from memory. KSC is at about 28.5 deg north. You have some control over launch azimuth, but physical and range safety limits still mean that the moon's declination would definitely have to be negative (in the earth's southern hemisphere) at arrival. Add to that the constraint that you want a landing site on the near side in early solar morning, and the relative orientation of the moon's orbital plane to the earth's rotational axis could mean no launch windows for months or maybe years at a time. But with a parking orbit, relatively small changes in the exact time of TLI could put the latitude of the injection point anywhere within the extremes of the parking orbit, thus accomodating a wide range of lunar declinations and allowing launch windows every day for several days each month to any given lunar destination. Again, that's from memory and I may have gotten it wrong. BTW, some form of parking orbit (or coast phase) is practically standard practice in any launch beyond LEO (including earth escape) that you'll see on NASA TV. This explains the popularity of pressure fed hypergolic upper stages because they are very easily restarted; just close the propellant valves and open them again at the right time. Sometimes they'll do a third burn at apogee after spacecraft separation to cause the spent stage to re-enter on the next perigee instead of becoming another piece of space junk. But restarts are much trickier with a big cryogenic stage like the S-IVB. Several things have to happen in sequence and at the right times, such as repressurizing the propellant tanks, restarting the turbopumps (how?) and re-igniting the propellants quickly enough to avoid a "hard start" that could damage the combustion chamber or nozzle. Many large rocket engines use hypergolic cartridges or small solid rocket engines for these tasks, making it difficult to do them more than once. Another way is to just separate the burns into separate stages. I have wondered if the designers of the Saturn V considered making the S-II larger so that it could achieve parking orbit, allowing the first and only firing of the S-IVB to occur at TLI. |
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raven
Jupiter
That ain't Earth, kiddies.
Posts: 509
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Post by raven on Feb 3, 2012 2:00:14 GMT -4
With a lighter payload, I could see it being possible, Skylab was lifted into orbit after all with only the first two stages of the Saturn V. Perhaps if NASA had gone for the three module GE Apollo D-2, it might have been workable. Maybe, I'm not an engineer. And thank you for taking the time to answer the question in such detail, I really appreciate the effort.  |
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Post by nomuse on Feb 3, 2012 2:47:50 GMT -4
Just the other day, I was talking to Jarrah White. We all know his stance on Apollo. Now I'm no expert in radiation by any means of the word, and it was just last week that I even got the idea what MeV and REM was. Well Jarrah offered the following theories on why the Apollo shielding should have been about 10cm thick. I'm sure there are plenty of people here who know a lot more about radiation than me, so I'd like to tell you guys what the calculations were and I want you to debunk the holy living hell out of me. Jarrah says the outer belts had an average of 10-100MeV. I told him we could work on the principle that they were 50MeV. Now, here is what he told me. To find the "aerial density?", we must multiply 50 by .545cm^2. We have 27.25cm^2. Now he said we have to divide that by the density of the material itself. Aluminum has a density of 2.7cm^3. We get about 10.09cm^3 worth of shielding. The LM had paper thin shielding. Jarrah says this is also without all the "secondary" radiation you'd get from particles bouncing off each other and fragmenting. According to him, with these numbers, the Apollo astronauts should have absorbed 1200REM, when the official log says they absorbed 2REM. Like I said, I'm no expert, so of course the data means little until I can get a better understanding from the Pro-Apollo side (like me). By no means, I hope everyone realizes, does this mean I have doubts about Apollo. Well anyway, I was hoping some of you radiation guys could come and tell me why you don't need 10cm. Thank you! I have read that a greater concern than VA Belt radiation was the not unlikely occurrance of a high dose of radiation being delivered via a sporadic solar flare. I read that flares releasing lethal radiation doses are not uncommon. It is argued Apollo missions would not be launched into the teeth of such dangerous circumstances. This seems very reasonable to me. Notice flares can disrupt communications and even cause outages down here on Earth. The Earth's magnetic field does not sufficiently ameliorate them. And yet, every space-faring nation has seen fit to send astronauts up to the ISS to hang around for months on end hoping a flare won't hit. |
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Post by ubique on Feb 3, 2012 7:38:12 GMT -4
Bob, I haven't checked your math but overall it looks right assuming the 1 MeV electrons aren't so energetic as to ever pass completely through the body. Looking at the penetration depths for electrons in water [1], it seems rather an opposite situation: Tissue near the skin surface gets most of the dose. That's why one should calculate the effective dose, as for example testicles will get a high dose, and they are very vulnerable to radiation damage. [1] www.photobiology.com/educational/len/part2.htm |
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Post by Jason Thompson on Feb 3, 2012 9:06:55 GMT -4
I read that flares releasing lethal radiation doses are not uncommon. Define 'not uncommon', and tell us where you read that. I have read it too, but never yet in a proper scientific tract regarding the Sun's behaviour. How dangerous are those circumstances? How common are such flares, what level of radiation do they deliver, and what means of protecting the crews were there in case such a flare did occur? Of course it does, if all you're going on is 'solar flares can kill and they happen sometimes'. However, answer those questions I posed above, and consider that the astronauts are former test and/or combat pilots: i.e. men who willingly climbed into aircraft without knowing for certain that their return to the ground would be a slow safe one rather than a rapid descent into a fireball that would turn them into a charred and mangled lump of flesh that was only identifiable because the ground crews knew who went up in that plane in the first place. |
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Post by sts60 on Feb 3, 2012 11:48:16 GMT -4
...I have read that a greater concern than VA Belt radiation was the not unlikely occurrance of a high dose of radiation being delivered via a sporadic solar flare. Leaving aside the handwaving of "not unlikely", do you think that perhaps steps were taken to study, characterize, and mitigate the threat of solar flares? I read that flares releasing lethal radiation doses are not uncommon. More handwaving. What exactly is "not uncommon"? More importantly, what does it mean in the context of the mission planning? It is argued...By ignorant people. Not by people who actually understand the subject. ...Apollo missions would not be launched into the teeth of such dangerous circumstances.Loaded language. Every mission requires risk management of some sort, and a great deal of effort went into managing (identifying, characterizing, and mitigating) the risks of the Apollo missions. And they were risky; this was understood and acknowledged - which is not the same thing as passively accepting the risk. This seems very reasonable to me. It is reasonable that it seems reasonable to you, since you are evidently a layman unfamiliar with the program and spaceflight in general. But reality is more complicated, and more interesting. At what point, if any, do you intend to either offer more substantial arguments than uninformed handwaving and unuspported endorsements of the manifestly incompetent and dishonest Patrick1000/fattydash/DoctorTea/etc.? Or, alternately, possibly concede that people knowledgeable about spaceflight in general and Apollo in particular might be right and you might be wrong? ETA: added emphasis |
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Post by JayUtah on Feb 3, 2012 13:12:30 GMT -4
I read that flares releasing lethal radiation doses are not uncommon. did you read this? Citations and references, please. The problem with this argument is that it's impossible to hide data on solar weather. It is collected worldwide by many national observatories since the 1800s and made freely available to the public. Hence it's useless to try to catastrophize this risk; you can simply look up the data for the relevant period. In fact there were only two solar events during the entire Apollo operational period 1969-1972 that had biological significance, and neither of them occurred while an Apollo mission was in progress. This is not an abstract risk; it is quantifiable. And the quantities of record do not establish that a two-week mission every 3-6 months poses a significant risk. "Not uncommon" are weasel words. We have actual data. Weasel words. The only people I have seen argue this are conspiracy theorists who have no relevant education or training. They don't get to make up rules for real people to follow. Who exactly do you say is arguing this? Citations and references, please. But you don't have the proper education and training to judge reasonableness in this situation. Every single qualified expert on solar weather for the past 40 years agrees that the Apollo missions were real. It doesn't matter what you personally think is reasonable. |
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Post by gillianren on Feb 3, 2012 16:30:25 GMT -4
I know this means you need to get the timing pretty exact and any delays in launch would create further delays, but why would have it meant not getting to the moon by the end of the decade? The fun thing is that I've read that NASA was perfectly willing to quibble about what "the end of this decade" meant if they had to. They were willing to get into the "there's no year zero" argument and go to 1970 if they had to. |
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Post by Obviousman on Feb 4, 2012 17:40:24 GMT -4
Bob,
Have you got the papers done by Francis Cucinotta?
JSC-29295 SPACE RADIATION CANCER RISK PROJECTIONS FOR EXPLORATION MISSIONS: UNCERTAINTY REDUCTION AND MITIGATION
(JSC, Jan 2001)
SPACE RADIATION CANCER RISKS AND UNCERTAINTIES FOR MARS MISSIONS
(pp 682 - 688, Radiation Research #156, 2001)
They go into the methodology. If you'd like copies, just PM me.
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