ApolloHoax.net
Apollo Discussions => The Reality of Apollo => Topic started by: Peter B on May 11, 2013, 09:40:28 PM
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There's a plot point in a novel (I won't mention it in case you haven't read it) which relies on astronauts hiding from enemies by being buried under the lunar soil.
Would it be possible to do that with an Apollo-style space suit? Would it affect something like the suit's sublimator, or simply cause the astronaut to overheat in a few minutes? Or is it something which would be survivable for several hours with a minimum of improvisation?
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As long as the sublimator wasn't blocked, and the sun wasn't at a high angle, the possibility could exist. But how easy it would be to bury somebody there, and hide the tracks leading there, that's another story.
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A space suit is already excellent thermal insulation so you won't make yourself any hotter with burial. But for the same reason you must have continuous active cooling, so the sublimator would have to be vented to vacuum and kept supplied with feedwater.
If they couldn't find you by your tracks or disturbed soil, they could look for the cloud of water vapor coming from your sublimator. It wouldn't be visible but it would be readily detectable with instruments.
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If the sun was high on the sky, the soil would be heated, and you'll get some conductive heat transfer.
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I have to admit that the first thing that popped into my head, after reading the thread title, was the old misdirection joke "a plane crashes right on an international border, where do they bury the survivors?".
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I have to admit that the first thing that popped into my head, after reading the thread title, was the old misdirection joke "a plane crashes right on an international border, where do they bury the survivors?".
"In Russia, a helicopter crashed in a graveyard. So far, 112 people has been found dead."
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Thanks folks, that was more promising than I expected.
Regarding the telltale tracks, that was unlikely to be an issue. The astronauts were buried at locations marked with coloured rocks. The bad guys saw the coloured rocks but assumed they were something to do with a science experiment (which seemed reasonable to me), so there would be nothing suspicious about seeing tracks. Given the way the Apollo astronauts managed to obscure their tracks, I imagine it would be relatively easy to disguise the number of people walking around to minimise any suspicions of "two went in but only one came out".
Regarding the detectability of water vapour, that might have been a little less reasonable. You'd think the bad guys might at least be expecting an ambush, so might have some sort of IR device which would presumably reveal the water.
Finally, ensuring access to vacuum for the sublimators, might also have been tricky. The story makes it seem like they were simply lying in holes in the ground and got covered up. If it was necessary to insert a pipe into the soil or leave a little hole to ensure part of the sublimator was uncovered, then presumably that would have had the potential for being noticed by the bad guys...
Still, it was a ripper scene in the novel (from a time when the author wrote OTT but still just about plausible thrillers).
Thanks folks.
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How much water vapor was there? How much did they carry for an 8 hour EVA? And did the exhaled water vapor get used?
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5 litres of water
See Wikipedia page for more details :
http://en.wikipedia.org/wiki/Primary_Life_Support_System
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How much water vapor was there? How much did they carry for an 8 hour EVA? And did the exhaled water vapor get used?
Only the feedwater in the tank went through the sublimator into space. Condensation from the suit gas loop was stored and later dumped. I guess it couldn't be used for evaporation (simulating what happens in normal perspiration) because some sweat might have gotten in there and clogged the sublimator pores.
Water is one of the most valuable substances imaginable on the moon, so expending it just to cool a spacesuit does seem rather wasteful. It worked for short Apollo stays, but a permanent base will require something more sustainable.
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How much water vapor was there? How much did they carry for an 8 hour EVA? And did the exhaled water vapor get used?
Only the feedwater in the tank went through the sublimator into space. Condensation from the suit gas loop was stored and later dumped. I guess it couldn't be used for evaporation (simulating what happens in normal perspiration) because some sweat might have gotten in there and clogged the sublimator pores.
Water is one of the most valuable substances imaginable on the moon, so expending it just to cool a spacesuit does seem rather wasteful. It worked for short Apollo stays, but a permanent base will require something more sustainable.
Indeed. I seem to remember a comment somewhere in one of the documents that water (for cooling) was one of the most time-limiting factors on the lunar missions, much more so than oxygen.
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Just curious, is it a Clive Cussler book? That bit sounds familiar.
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Just curious, is it a Clive Cussler book? That bit sounds familiar.
Congratulations. One virtual banana for you. :-)
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Indeed. I seem to remember a comment somewhere in one of the documents that water (for cooling) was one of the most time-limiting factors on the lunar missions, much more so than oxygen.
The consumables (oxygen, water, LiOH, battery) were all sized to run out around the same time (with margin) as there's no point in carrying the extra weight of, say, a lot more oxygen that you can't use anyway.
But if you wanted to extend the endurance of the PLSS by increasing all of the consumables, the one that would probably get you first would be the cooling water, simply because you need so much of it. The tanks took up a good fraction of the PLSS interior volume. After that probably comes lithium hydroxide, as it's also quite bulky.
The OPS carried quite a bit more O2 than the main tank in the PLSS because it had to be sized for a once-through scuba-like mode in case the PLSS died.
Oxygen should be abundant in a future lunar base because it can easily be made from lunar soil; like the earth, it's about half oxygen by weight. So in principle you could do without LiOH and probably cooling water in a future EVA suit by simply squandering oxygen in an OPS-like once-through mode, assuming you can physically carry enough with you. This might actually be practical for surface treks with short EVAs from a pressurized rover. It could carry lots of O2 as LOX.
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Like many of the discussions here, this got me thinking. Could abundant oxygen production on the moon allow the use of simpler but less efficient life support systems?
I see a range of numbers, but a typical human breathing air exhales 4-5% CO2 and 15-16% O2. Since air is about 21% O2, this means that in a once-through scuba-type mode your oxygen efficiency would be about 25%. Assuming these figures would stay the same for pure O2 at reduced total pressure (i.e., same O2 partial pressure), you'd have to carry about 4x as much O2 as in a conventional rebreather-type life support system where the CO2, H2O and trace gases are scrubbed so the unbreathed O2 can be breathed again.
That's actually not bad considering how much you can get rid of: no CO2 absorption system, no water condenser/separator, no charcoal filter. Depending on the workload you might also be able to get rid of the cooling system too by relying on gas cooling supplemented by the heat of vaporization of LOX. (Gas cooling was the original plan for Apollo, but water cooling was added when studies showed that at very high workloads it would take less power to drive a water pump than a suit blower at the necessary speed.)
But it would require you to use a scuba-type mouthpiece or a small face mask to limit mixing between exhaled and inhaled air. If instead you simply purged the cabin continuously through a vent, assuming good ventilation keeping exhaled air thoroughly mixed in the cabin, your vent rate would depend on the maximum tolerable CO2 concentration. That appears to be about 1%, so to maintain that level you'd have to continuously vent CO2 as fast as its produced, and you'd have to vent roughly 99 times as much O2 along with it. (The actual figure is a bit better because CO2 has a higher molecular weight than O2.)
So if we take the oxygen efficiency of the conventional rebreather as 100%, the efficiency of a scuba system would be about 25% and that of a purged and vented cabin with ventilation would be about 1%. Still, if oxygen is truly plentiful -- and there's no reason to think that it wouldn't be on the moon -- this might be a worthwhile tradeoff to simplify the life support system, especially if it got rid of the need for other consumables that might not be as available. I know the ISS is now using a renewable CO2 scrubber, and I know that compact cooling systems that don't expend water are under development, but it would be even better if you could simply do without them.
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Followup -- I don't think it's quite as bad as 1%. It's the partial pressures that matter, and in a single-gas system without a diluent you could withstand a greater percentage of CO2 in the total.
1% CO2 in sea level air is about 1 kPa of partial pressure. If the O2 is at 21 kPa, this is more like 5% of the total, by volume, so the vents would purge about 20x as much O2 as CO2, not 99x.
So the efficiency figures would be:
Rebreather: 100%
Scuba: 25%
Cabin purging and venting: 5%
A typical adult crewmember consumes 840g/day, so this would increase it to 16.8 kg/day per crewman. Stored as LOX (density 1141 kg/m3, that would occupy 14.7 L. This might be entirely tolerable in a PLSS or pressurized rover if it eliminated the need for cooling water and LiOH cartridges.
On the other hand it would eliminate the option of recovering the exhaled CO2 and especially the H2O for possible recycling. Given the scarcity of water on the moon, this might be a problem.
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How about once-through scuba-style for an EVA suit, where simplicity and endurance matter most, and CO2 scrubbing for a rover and any fixed facilities?
I don't think use of a mouthpiece would be too objectionable in a suit, though you'd have to be sure you could recover it if it fell out of your mouth.
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CO2 scrubbing at a base certainly makes sense. The Shuttle and ISS already use regenerative alternatives to non-renewable LiOH for CO2 scrubbing, but the hardware is more complex than LiOH and probably too much to put into a PLSS.
The Apollo astronauts suited up several hours before launch and breathed pure O2 from that point to begin flushing the N2 from their systems to avoid possible decompression sickness during launch as the cabin pressure rapidly dropped. You may remember them carrying portable O2 supplies on their way to the pad. They contained a supply of LOX, and maybe that's how they also cooled themselves. So there's precedent for a simple once-through LOX-based system.
Because oxygen will be abundant in any lunar base, scrubbing of exhaled air will probably be motivated not by conserving O2 but by the need to conserve C and especially H2, two elements that are rare on the moon.
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I have to admit that the first thing that popped into my head, after reading the thread title, was the old misdirection joke "a plane crashes right on an international border, where do they bury the survivors?".
Oh I dunno, wherever they happen to be when they eventually die?
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Like many of the discussions here, this got me thinking. Could abundant oxygen production on the moon allow the use of simpler but less efficient life support systems?
I see a range of numbers, but a typical human breathing air exhales 4-5% CO2 and 15-16% O2. Since air is about 21% O2, this means that in a once-through scuba-type mode your oxygen efficiency would be about 25%. Assuming these figures would stay the same for pure O2 at reduced total pressure (i.e., same O2 partial pressure), you'd have to carry about 4x as much O2 as in a conventional rebreather-type life support system where the CO2, H2O and trace gases are scrubbed so the unbreathed O2 can be breathed again.
That's actually not bad considering how much you can get rid of: no CO2 absorption system, no water condenser/separator, no charcoal filter. Depending on the workload you might also be able to get rid of the cooling system too by relying on gas cooling supplemented by the heat of vaporization of LOX. (Gas cooling was the original plan for Apollo, but water cooling was added when studies showed that at very high workloads it would take less power to drive a water pump than a suit blower at the necessary speed.)
But it would require you to use a scuba-type mouthpiece or a small face mask to limit mixing between exhaled and inhaled air. If instead you simply purged the cabin continuously through a vent, assuming good ventilation keeping exhaled air thoroughly mixed in the cabin, your vent rate would depend on the maximum tolerable CO2 concentration. That appears to be about 1%, so to maintain that level you'd have to continuously vent CO2 as fast as its produced, and you'd have to vent roughly 99 times as much O2 along with it. (The actual figure is a bit better because CO2 has a higher molecular weight than O2.)
It's probably more a matter of partial pressure than percentage. The partial pressure of CO2 (pCO2) in the venous blood returning to the lungs is about 6.7 kPa; if the pCO2 of inhaled air approaches that level, the CO2 in the blood will not diffuse out and be exhaled. CO2 builds up in the blood and causes all kinds of nasty effects (even biology is physics, huh?). It's just an educated guess, but I would think that a pCO2 around 5.0 kPa would be a safe margin.
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5 kPa corresponds to 5% at normal pressure. That's definitely too much CO2. The sources I see say you can breathe up to 1% indefinitely with no noticeable symptoms; that's 1 kPa at normal pressure. This is still quite a bit higher than the standards for indoor air quality, which limit CO2 to about 0.25%. See the Wikipedia article on carbon dioxide.
I realized after posting my article that it is indeed the partial pressure that matters, so you're right about that. In a single gas system with lower total pressure, the O2 efficiency would be better than 1%; it would be more like 5%. The same would be true with air except that in addition to venting 95% of your oxygen, you'd also vent quite a bit of diluent gas. I'm assuming nitrogen is extremely scarce on the moon so you wouldn't want to do this.
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I've also been thinking about the cooling problem; if a PLSS stored LOX, could you use it for astronaut cooling?
My physiology references say that a typical crew member consumes 840g O2/day. Each liter (presumably at normal temperature and pressure) represents 5 kcal of metabolism assuming a mixed diet (carbohydrates and fats have different requirements). 1 L O2 @ NTP is 1.331 g, so that's 5 kcal/1.331 g or 15.73 MJ/kg. 840 g of daily O2 consumption represents 13.2 MJ or 3156 kcal, which seems about right for an active astronaut.
The heat of vaporization of O2 is 6.82 kJ/mol; the molecular weight is 32, so that's 213 kJ/kg. It then has to be warmed (310-90) = 220 K after boiling, so for a specific heat of about 0.91 kJ/kg-K. that's 213 + 200.2 = 413.2 kJ/kg to convert LOX to breathable O2. Your 840 g/day of O2 consumption would therefore require 347 kJ/day or 4 W to warm, but you put out 13.2 MJ/day or 153 W, so there isn't anywhere near enough heat capacity in the LOX you breathe to absorb your metabolic heat.
But that's in a rebreather with 100% efficiency. Again we're assuming we can squander LOX because it will be plentiful on the moon, so you'd have to boil about 39x as much as you'd actually breathe. But this is not far from the 20x metabolic consumption you'd need in a simple purge system that maintains CO2 at 1%. You could just reduce the CO2 level to about 0.5%
So the bottom line is that if you're prepared to use LOX really inefficiently you could provide metabolic cooling and eliminate the need for CO2 scrubbing.
But wait, there's more. The significant temperature difference between the human body and LOX (220K) means you could (in theory, anyway) run a heat engine off the difference and produce useful work with it, possibly eliminating the battery in your PLSS as well! The Carnot efficiency of a heat engine operating between 310 K (body temperature) and 90 K (LOX boiling point) is 1 - (90/310) = 71%. My figures assume a metabolic rate of 153W, so this could (again at 100% of Carnot efficiency, which is too optimistic) produce 109 W of electricity. But the Apollo PLSS drew a nominal 41W (8.4 W for the water pump, 21.8 W for the fan and 10.9 W for the communications system, which could certainly be improved with newer technology) so you'd need a heat engine with only 38% of Carnot efficiency. So it's entirely plausible that the whole PLSS could be powered off the astronaut's own metabolic heat.
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5 kPa corresponds to 5% at normal pressure. That's definitely too much CO2. The sources I see say you can breathe up to 1% indefinitely with no noticeable symptoms; that's 1 kPa at normal pressure. This is still quite a bit higher than the standards for indoor air quality, which limit CO2 to about 0.25%. See the Wikipedia article on carbon dioxide.
I realized after posting my article that it is indeed the partial pressure that matters, so you're right about that. In a single gas system with lower total pressure, the O2 efficiency would be better than 1%; it would be more like 5%. The same would be true with air except that in addition to venting 95% of your oxygen, you'd also vent quite a bit of diluent gas. I'm assuming nitrogen is extremely scarce on the moon so you wouldn't want to do this.
I guess that educated guess was only half-educated. I neglected to consider that alveolar air has a considerably higher level of CO2 than inhaled air because of anatomic dead space - alveolae don't empty completely at exhalation, so there's always a blending. I was partly right in that the alveolar pCO2 needs to stay below around 5 kPa to assure adequate CO2 exhalation. The empirical evidence seems to be that, as you said, 1 kPa in inhaled air is about the maximum that is tolerable - higher and that and it eventually builds up and becomes toxic.
BTW, I happened on an interesting bit of trivia. According to my A&P text, in microgravity the gas exchange capacity of the alveoli is 28% greater than in 1 gee. (Spacelab)
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How would that energy be extracted? Pleiter-elements?
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How would that energy be extracted? Pleiter-elements?
You mean my heat engine working off body heat vs LOX? It could be anything capable of working between those two temperatures; maybe a Stirling engine. Thermocouples and Peltier devices would probably be too inefficient. NASA Glenn has been working on a Stirling engine for converting Pu-238 RTG heat to electricity that's much (4-5x) more efficient than the thermocouples used on current RTGs. Since it has moving parts and is considerably more complex than the thermocouples, they're only doing this because of the critical shortage of Pu-238.
The Carnot limit is a theoretical limit on conversion efficiency that applies to any heat engine, set only by the ratio of the hot side and cold side temperatures. As always it's difficult to approach the theoretical limits in practice.
It's easy to see intuitively how the Carnot limit works. Kelvin is an absolute temperature scale so there's no heat energy at all in something at 0K. So if you start with heat at, say 310K and dump waste heat at 90K, you are not extracting 90/310 = 29% of the heat input, leaving 71% that you can extract.
An interesting point about my heat engine idea is that if you really could produce a heat engine with 71% efficiency, only 29% of the astronaut's body heat would reach the LOX, the rest having been converted to useful work. That would reduce the LOX boil rate by over a factor of 3. So you could reduce your LOX consumption rate back down to that required to maintain a 1% CO2 concentration or even higher.
Edited to add: Actually it seems more complicated than that. Any electricity generated from a heat engine working between body temperature and LOX temperature would be used to power systems within the PLSS like pumps, fans and radios. They would in turn generate additional waste heat that would have to be removed, e.g., by boiling LOX, but some of that waste heat could again be used to generate more elecricity! The only energy that wouldn't have to be removed is that radiated by radio transmitters, which would be a small fraction of the total.
But you'd still eliminate the need for a battery and all the extra waste heat the PLSS systems would generate from that battery power.
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I neglected to consider that alveolar air has a considerably higher level of CO2 than inhaled air because of anatomic dead space - alveolae don't empty completely at exhalation, so there's always a blending. I was partly right in that the alveolar pCO2 needs to stay below around 5 kPa to assure adequate CO2 exhalation.
I found similar information in the physiology literature. The partial pressures of CO2 and O2 remain remarkably constant in the alveolae as you breathe, and as you say you only exchange a fraction of that air on every breath assuming you're breathing normally.
There always have to be partial pressure gradients between the blood and the alveolae, and between the alveolae and the ambient air so that the gases continue to flow in the direction you want. It's like needing a voltage drop, however small, across a conductor to get it to carry the current you want.
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I neglected to consider that alveolar air has a considerably higher level of CO2 than inhaled air because of anatomic dead space - alveolae don't empty completely at exhalation, so there's always a blending. I was partly right in that the alveolar pCO2 needs to stay below around 5 kPa to assure adequate CO2 exhalation.
I found similar information in the physiology literature. The partial pressures of CO2 and O2 remain remarkably constant in the alveolae as you breathe, and as you say you only exchange a fraction of that air on every breath assuming you're breathing normally.
There always have to be partial pressure gradients between the blood and the alveolae, and between the alveolae and the ambient air so that the gases continue to flow in the direction you want. It's like needing a voltage drop, however small, across a conductor to get it to carry the current you want.
More so with the alveolar-capillary interface, since there's an actual physical barrier there and the osmotic pressure is required to move the molecules across it. The ambient-alveolar differential is just a result of the fact that the alveola are anatomical dead ends, so there's always a little "stale air" in them. The body does have mechanisms to cope with that, though - if CO2 levels begin to rise, you automatically breathe deeper, which ventilates the alveola more completely. According to the literature I looked at, the body can evidently compensate for quite a long time for slightly elevated ambient CO2 levels.
The body also tries to maintain a certain range of pCO2 in the blood - it's an important component of pH balance, which is supremely important. I need to pull up some of the NASA Biomedical research - it would be interesting to see what data they were able to gather on physiological changes while living for 10 - 12 days in microgravity and a low pressure O2 atmosphere.
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Speaking of blood pH and overventilation, just the other day I was wondering how dogs can pant without going into hypocapnia. They pant to lose heat since they don't sweat through their fur, and that wouldn't always be directly associated with metabolic rate, e.g., on a hot day. My wife, a nurse, didn't seem to know the answer offhand.
Perhaps they've evolved some different mechanisms for controlling blood pH. Maybe we humans lost those mechanisms since we don't need to pant to lose heat.
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need to pull up some of the NASA Biomedical research - it would be interesting to see what data they were able to gather on physiological changes while living for 10 - 12 days in microgravity and a low pressure O2 atmosphere.
Check out "Biomedical Results of the Apollo Program" or something like that. That's your bible on the topic. If you can't get it because of the NTRS shutdown let me know and I'll dig up my copy.
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Speaking of blood pH and overventilation, just the other day I was wondering how dogs can pant without going into hypocapnia. They pant to lose heat since they don't sweat through their fur, and that wouldn't always be directly associated with metabolic rate, e.g., on a hot day. My wife, a nurse, didn't seem to know the answer offhand.
Perhaps they've evolved some different mechanisms for controlling blood pH. Maybe we humans lost those mechanisms since we don't need to pant to lose heat.
As it happens, I asked a vet about this once. "Thermic panting" is shallow enough that air movement is confined almost entirely to the oronasopharynx, trachea, and broncheal tree, so it really doesn't increase gas exchange very much. There is also some vascular shunting that goes on, but it's mostly just the shallowness of the breathing.
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That's an excellent answer! Makes perfect sense.
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That's an excellent answer! Makes perfect sense.
I have to come up with one occasionally - biology/medicine doesn't come up that often in this group.
Perhaps you can provide me with an excellent answer. The radar 'ground tracking' that was done during Apollo - I gather it was based on the telemetry signal (well, one of the signals) from the LM and CSM rather than the traditional ping-and-return approach? How accurate was that in terms of guidance, i.e., measurement of their lunar orbit, velocity and altitude during descent, etc? If they were getting accurate data from 240,000 miles away, well, that's pretty freakin' amazing. I've looked at some of the Apollo guidance & navigation documents but they're pretty much gibberish to me. Can you point me toward a primer-level document or source?
I'll say this for hunchbacked - he's better than most as prompting me to go trotting off to do research (which is more than he does, apparently.)
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It wasn't radar - they tracked the actual telemetry transmissions from the vehicles. The big radio telescopes had enough resolution to follow the actual descent and hover by Apollo 11. I'll search for some info.
Edit: http://depletedcranium.com/fascinating-recording-of-apollo-11-at-jodrell-bank-released/
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I think what Noldi400 is asking is how NASA tracked the spacecraft; the Jodrell Bank info, while fascinating (and yet another blow on the smear-on-the-ground corpse that is HB denialism) is not that.
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Yes, Jodrell Bank didn't supply their data to NASA, but did the to satisfy their own curiosity.
Edit: Honeysucklecreek has some information. They apparently had the ability to measure the distance to within 1 meter, but errors from outside the system increased the error margin to 15.2 meters.
http://www.honeysucklecreek.net/station/technical.html
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I think this passage is the key:
Once the station had locked onto the signal, another special processor called a Tracking Data Processor, (TDP) accepted the ranging data, the speed of the spacecraft relative to the station from the doppler, and the antenna angles relative to the station's geographical location, and coded this information for transmission to Goddard. It was coded in both high-speed data at 2,400 bits per second and in teletype code onto paper tape.
So they had the distance pretty accurate, the speed pretty accurate, the direction to the spacecraft too. All they missed was the attitude, and direction of travel. The former was in the datastream, the latter could be calculated.
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Here's how it worked; I know this stuff pretty well.
The spacecraft had an "S-band transponder", basically a special kind of radio transceiver (transmitter and receiver). What made it special was a linkage between the receiver frequency and the transmitter frequency. When the receiver didn't detect a signal, the transmitter used a local oscillator to transmit on its nominal frequency; this could drift due to manufacturing tolerances, temperature, etc.
But when the receiver did receive an uplink signal from earth, it "phase locked" onto it and produced a transmitted frequency that was exactly 240/221 times the received signal frequency. This became the carrier of the downlink signal, which was designed so that the carrier was always present even with modulation (this isn't always true with some modulation methods).
The ground used a highly stable oscillator (an "atomic clock") to generate its uplink, and by producing a local reference signal 240/221 times its own transmit frequency, could compare that with the incoming downlink signal from the spacecraft. If the spacecraft were at constant distance from the ground, the two signals would be exactly the same frequency and phase. But if the spacecraft moves toward or away, the phase will rotate at some rate. Phase rate is the same as frequency, so there's a Doppler frequency shift.
The phase will rotate 360 degrees for every change in round trip distance of one wavelength, which at S-band is about 13 cm. The round trip distance changed one wavelength for every half wavelength of actual ground-spacecraft distance. And phase changes considerably less than 360 degrees could easily be detected.
So you can see how incredibly precise this method was; it could easily detect the changes in velocity caused by a simple urine dump (which led to an unfortunate misunderstanding during Apollo 13).
Doppler tracking was continuous, and if you knew at time t how far it was, by counting RF cycles you could keep track of its current distance. But how did you find the actual distance to start? With a separate mechanism called PN ranging. The ground could optionally transmit a fast (a little less than 1 Mb/s) pseudorandom data sequence to the spacecraft, which (if enabled by the crew) would repeat that signal on the downlink. The sequence was long enough to not repeat during the several seconds it could take the signal to reach Apollo and return, so the ground could compare it with its transmitted sequence and see to within 300 m (actually much less) what the round trip distance was and remember this figure for updating with Doppler. Then it could turn off the PN ranging signal.
PN ranging is widely used today; it's the basis of GPS, for example, and the Qualcomm CDMA digital cellular system that I worked on. GPS and CDMA all use different PN sequences from Apollo, but the "chip" (random bit) rates are remarkably close: 996 kHz (I think) for Apollo, 1.023 MHz for GPS, and 1.2288 MHz for CDMA. It made E911 position determination relatively straightforward, and if a CDMA phone was involved that's how the police found the Tsarnaev brothers in Watertown last month...
That you can continuously update a one-shot PN ranging measurement with Doppler is one of my disagreements with Hunchbacked. He insists the PN signal has to be on all the time, and the fact that it can be turned off is one of his many discovered "incoherences". I've tried to explain how the Doppler tracking is coherent and continuous so the PN ranging only need be done at first acquisition and after any loss of signal, but to no avail...
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So they had the distance pretty accurate, the speed pretty accurate, the direction to the spacecraft too. All they missed was the attitude, and direction of travel. The former was in the datastream, the latter could be calculated.
Right. The accuracy of the direction measurements depended on the size of the ground station antenna and thus its beamwidth. Apollo used either 85' or 210' (25.9 or 64m) antennas. At S-band the smaller dish had a beamwidth of +/- 0.15 deg at the 3dB points and the larger had a beamwidth of about +/- 0.06 deg at the 3dB points.
I've written a program to determine a satellite's orbit from measurements like these; you design it to accept whatever information you have, with a weighting on each measurement to indicate how confident you are in that number. Then you turn the crank on a big least-squares fit to produce a state vector (or orbital element set) that most closely matches all those measurements.
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So you can see how incredibly precise this method was; it could easily detect the changes in velocity caused by a simple urine dump (which led to an unfortunate misunderstanding during Apollo 13).
Poor Fred....
Fascinating. I bet this info perplexes many people. I understand the random-sequence-bit - they frame-matched to get a range-estimate.
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That's right. And the Doppler measurement was not much different from how the microwave motion sensors on automatic supermarket doors work. They beam a continuous carrier at 10 GHz toward you and detect your reflection. They can even tell which way you're going by which way the phase difference rotates; this is done with a pair of detectors in quadrature to pick up both the sine and cosine components. Police radars work much the same way except I don't think they bother with the quadrature detectors.
The only real difference between the supermarket door detector and the Apollo transponder was that the former relied on passive reflection while the latter required the spacecraft to actively amplify and repeat the signal. It necessarily had to do this on a different frequency to avoid interference, but even this difference was effectively eliminated by having the ground compare the received signal to that of a local reference transponder that doesn't move.
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As I understand it, the deliberate phase-locked frequency shift eliminates the doppler shift, so the return carrier signals's frequency is independent of the spacecraft's speed and direction. Is that correct?
Edit: No, that didn't make so much sense when I read it again.
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So they had the range to within one wavelength, and the up/Down/left/right position to an area approx. 200 x 200 km (edit: at the far end of the journey)? That explains the need for the rendezvous radar and transponder system on the LM/CSM.
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I'd have to look up the actual numbers, but from the radio link design I'd estimate they had the range to probably several meters (a fraction of a PN chip time) and the range-rate (velocity projected along the line of sight) to something like a millimeter per second (a few degrees per second of carrier phase rotation). These are the accuracies you'd likely get in real time; you could do better if you averaged out the noise over a longer tracking run.
There were several reasons for the rendezvous radar and transponder. One, neither spacecraft could be tracked on the lunar far side, and much of the rendezvous took place there. Two, while the ground can measure the range and range-rate along its line of sight very accurately, it can't measure position and velocity at right angles to that line of sight. It has to rely on antenna angles, and while the beamwidths are small they're still quite large compared to the half degree angular size of the moon at the earth. Three, while you can solve for orbital elements by doing a Kalman filter or a big least-squares fit with whatever measurements you have, you also need a model of all the forces affecting the spacecraft which, at the moon, is largely lunar gravity -- and we didn't have a very good model of lunar gravity in those days. (We're just getting a really good one now from the GRAIL mission.) Four, good orbital estimates require a series of measurements over the longest possible period of time when the spacecraft is not being maneuvered, and neither assumption was a good one during the relatively quick rendezvous sequence.
I'm not sure which was the dominant factor, but #1 and #3 were probably the most important limitations of earth-based tracking during rendezvous.
This isn't to say that earth-based tracking wasn't also done and used as a cross-check; after the DOI burn on at least Apollo 11, the ground tracked Eagle as soon as it came around the edge of the moon as a double-check to ensure that they didn't overburn and put themselves on a lunar impact trajectory. Supposedly there would have been enough time to perform an emergency bailout burn to avoid that.
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Back in the late 1970s I did an informal experiment with a TV broadcasting satellite that worked much like the Apollo tracking system. In my college years I worked as a summer intern at the Baltimore PBS station. This was just before domestic communication satellites, and after PBS installed ground stations at their affiliates I paid the engineering team a visit to take a look. It was Friday evening and they were uplinking Wall Street Week to the network. I punched up the downlink of their own signal and put it on a vectorscope using house subcarrier as the reference.
A vectorscope was a special-purpose oscilloscope used in analog TV to display the amplitude and phase of the NTSC color subcarrier in a polar plot. White, grey and black all showed up as a point in the center with saturated colors around the outer perimeter. One point was always supposed to say fixed: the color burst between each horizontal scan line that was a reference sample of the 3.579545... MHz NTSC color subcarrier. Since my scope was synchronized to house subcarrier (coming directly from the station's rubidium frequency standard, which was also used to synchronize the uplink to the satellite) any change in color subcarrier phase in the downlink would show up as a rotation of the color burst (and all the colors, actually) on the scope face. I wanted to see if the satellite was moving -- and it was. I saw a slow steady rotation of the entire chroma signal, indicating the satellite was moving along our line of sight.
I was looking at the color subcarrier, not the C-band (6/4 GHz) RF carrier, so each rotation represented one cycle of the color subcarrier frequency rather than the RF. The wavelength of the subcarrier is about 80 meters, so one full rotation would have been a change of about 40 m in station-to-satellite distance. I don't remember the exact numbers (it's been 35 years!) but it was maybe one rotation every few minutes. That would have been a range-rate of a fraction of a meter per second, and it was easily visible.
Because the Apollo system did the same thing directly on the S-band carrier, each rotation of the carrier phase represented a much smaller change in distance (by a factor of over 600!) so as you can see the system was very sensitive.
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Here's how it worked; I know this stuff pretty well.
The spacecraft had an "S-band transponder", basically a special kind of radio transceiver (transmitter and receiver). What made it special was a linkage between the receiver frequency and the transmitter frequency. When the receiver didn't detect a signal, the transmitter used a local oscillator to transmit on its nominal frequency; this could drift due to manufacturing tolerances, temperature, etc.
But when the receiver did receive an uplink signal from earth, it "phase locked" onto it and produced a transmitted frequency that was exactly 240/221 times the received signal frequency. This became the carrier of the downlink signal, which was designed so that the carrier was always present even with modulation (this isn't always true with some modulation methods).
The ground used a highly stable oscillator (an "atomic clock") to generate its uplink, and by producing a local reference signal 240/221 times its own transmit frequency, could compare that with the incoming downlink signal from the spacecraft. If the spacecraft were at constant distance from the ground, the two signals would be exactly the same frequency and phase. But if the spacecraft moves toward or away, the phase will rotate at some rate. Phase rate is the same as frequency, so there's a Doppler frequency shift.
The phase will rotate 360 degrees for every change in round trip distance of one wavelength, which at S-band is about 13 cm. The round trip distance changed one wavelength for every half wavelength of actual ground-spacecraft distance. And phase changes considerably less than 360 degrees could easily be detected.
So you can see how incredibly precise this method was; it could easily detect the changes in velocity caused by a simple urine dump (which led to an unfortunate misunderstanding during Apollo 13).
Doppler tracking was continuous, and if you knew at time t how far it was, by counting RF cycles you could keep track of its current distance. But how did you find the actual distance to start? With a separate mechanism called PN ranging. The ground could optionally transmit a fast (a little less than 1 Mb/s) pseudorandom data sequence to the spacecraft, which (if enabled by the crew) would repeat that signal on the downlink. The sequence was long enough to not repeat during the several seconds it could take the signal to reach Apollo and return, so the ground could compare it with its transmitted sequence and see to within 300 m (actually much less) what the round trip distance was and remember this figure for updating with Doppler. Then it could turn off the PN ranging signal.
PN ranging is widely used today; it's the basis of GPS, for example, and the Qualcomm CDMA digital cellular system that I worked on. GPS and CDMA all use different PN sequences from Apollo, but the "chip" (random bit) rates are remarkably close: 996 kHz (I think) for Apollo, 1.023 MHz for GPS, and 1.2288 MHz for CDMA. It made E911 position determination relatively straightforward, and if a CDMA phone was involved that's how the police found the Tsarnaev brothers in Watertown last month...
That you can continuously update a one-shot PN ranging measurement with Doppler is one of my disagreements with Hunchbacked. He insists the PN signal has to be on all the time, and the fact that it can be turned off is one of his many discovered "incoherences". I've tried to explain how the Doppler tracking is coherent and continuous so the PN ranging only need be done at first acquisition and after any loss of signal, but to no avail...
PROGRAM ALARM: 1201 . . . . . . . EXECUTIVE OVERFLOW
OK, wow, nicely answered, even if most of it is over my head.
It does match up nicely with the tech reports I've found since asking the question. I got my answer, at least enough of a broad overview that I understand what they were doing, even if I quickly get lost in the technical details.
Sometimes I get the feeling that most HBs think that NASA claims to have basically slapped something together, fired it off in the direction of the moon, and made the rest up as they went along.
Then too, that may not be confined to HBs; I remember someone who accepted the reality of Apollo asking if anyone knew how the crew on Apollo 11 decided who was going to land and who stayed in the CM. Did they draw straws or something? A lot of people, I believe, really don't understand the sheer volume of the planning, designing, testing, training, etc. that went into Apollo, or the extent to which every scrap of data was analyzed and incorporated into subsequent missions.
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And what about the discussion of who went out first? A discussion which was eventually resolved by pointing out, that the hatch in the LM was hinged on the LMP's side, and therefore it would only be possible for the Commander to get out first, since there wasn't room inside for them to swap places, once the spacesuit were on.
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A lot of people, I believe, really don't understand the sheer volume of the planning, designing, testing, training, etc. that went into Apollo, or the extent to which every scrap of data was analyzed and incorporated into subsequent missions.
That is very true. The more you read about Apollo, the more impressive it becomes for just this reason.
I even catch myself sometimes underestimating just how difficult it was to do something because of how easy it is to do now. Even though I was around back then, I have to remember that much of the technology I now take for granted came after Apollo. In fact, Apollo helped spur much of it into existence.
Everybody knows that when engineers do something for the first time they have to work out a lot of unexpected problems when it's not clear what the best solution may be, or even if there is a solution. Much less well appreciated is the amount of effort that goes into solving what turn out to be non-problems before you know that they are, in fact, non-problems.
Edited to add: I'm sure someone will ask me for examples, so here's one. A major design driver in Apollo communications and navigations design was the threat that the Russians might deliberately jam or spoof communications. Some of the procedures (such as the uplink "block" switch the astronauts regularly threw to enable ground computer updates) were based on this assumption.
A great deal of effort went into making it possible for the astronauts to return home on their own in the event of sustained Russian jamming. While it's true that this also protected against a non-malicious but serious communications failure, that never happened either. I think simply adding hardware redundancy (which they also did) was the better way to address that particular risk.
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A lot of people, I believe, really don't understand the sheer volume of the planning, designing, testing, training, etc. that went into Apollo, or the extent to which every scrap of data was analyzed and incorporated into subsequent missions.
That is very true. The more you read about Apollo, the more impressive it becomes for just this reason.
I even catch myself sometimes underestimating just how difficult it was to do something because of how easy it is to do now. Even though I was around back then, I have to remember that much of the technology I now take for granted came after Apollo. In fact, Apollo helped spur much of it into existence.
Everybody knows that when engineers do something for the first time they have to work out a lot of unexpected problems when it's not clear what the best solution may be, or even if there is a solution. Much less well appreciated is the amount of effort that goes into solving what turn out to be non-problems before you know that they are, in fact, non-problems.
Edited to add: I'm sure someone will ask me for examples, so here's one. A major design driver in Apollo communications and navigations design was the threat that the Russians might deliberately jam or spoof communications. Some of the procedures (such as the uplink "block" switch the astronauts regularly threw to enable ground computer updates) were based on this assumption.
A great deal of effort went into making it possible for the astronauts to return home on their own in the event of sustained Russian jamming. While it's true that this also protected against a non-malicious but serious communications failure, that never happened either. I think simply adding hardware redundancy (which they also did) was the better way to address that particular risk.
We ought to start a new thread: "Underappreciated Things About The Apollo Program"
i.e., almost all of the Apollo astronauts were engineers in a relevant field, many of them with advanced degrees. Each of them, beginning with Group 3 at least, was assigned a specialty area that they were heavily involved in throughout the program. IOW, they weren't just pilots waiting around for their flight; they were either heavily involved in (or at least closely monitoring) everything from instrument panel layout to spacesuit design.
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It would be interesting to see the astronauts' academic degrees in a simple table. I think you're right that the later astronauts generally had engineering degrees, but I don't know if that was true for all of the earlier ones when test pilot experience was the main criterion. An engineering background was emphasized later as the spacecraft and missions got increasingly complex.
I really like that introduction Neil Armstrong gave in 2000 when he presented the list of the top 20 engineering achievements of the 20th century. He identified himself as a fellow engineer ahead of everything else he was:
“I am, and ever will be, a white-socks, pocket-protector, nerdy engineer, born under the second law of thermodynamics, steeped in steam tables, in love with free-body diagrams, transformed by Laplace and propelled by compressible flow.”
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It would be interesting to see the astronauts' academic degrees in a simple table. I think you're right that the later astronauts generally had engineering degrees, but I don't know if that was true for all of the earlier ones when test pilot experience was the main criterion. An engineering background was emphasized later as the spacecraft and missions got increasingly complex.
The official NASA bio for the original seven lists six with engineering degrees of some sort, including three in aeronautics. Shepard is listed as graduating from the Naval Academy with a Bachelor of Science but does not state the subject. It appears that a strong technical background was a requirement for attending test pilot school.
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It would be interesting to see the astronauts' academic degrees in a simple table. I think you're right that the later astronauts generally had engineering degrees, but I don't know if that was true for all of the earlier ones when test pilot experience was the main criterion. An engineering background was emphasized later as the spacecraft and missions got increasingly complex.
Good idea - I'll see if I can knock something together. Also, glad to see you guys chewing on HB over his orbital calculations - I was getting a headache.
I'm pretty sure that an undergrad degree was a requirement for Mercury right from the start. Mike Collins writes that for the second group of astronauts the requirements (in part) were certified test pilots with a degree in engineering or one of the biological sciences.
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First Installment.
The Mercury Seven
1 Shepard, Alan BS, Engineering
1 Grissom, Gus BS, Mechanical Eng
BS, Aero Mechanics
1 Glenn, John BS, Engineering
1 Carpenter, Scott BS, Aeronautical Eng
1 Schirra, Walter BS, Aeronautical Eng
1 Cooper, Gordon BS, Aerospace Eng
1 Slayton, Deke BS, Aeronautical Eng
The Next Nine
2 Armstrong, Neil MS, Aeronautical Eng
2 Borman, Frank MS, Aeronautical Eng
2 Conrad, Pete BS, Aeronautical Eng
2 Lovell, James BS, Aeronautical Eng
2 McDivitt, James BS, Aeronautical Eng
2 See, Elliot MS, Engineering
2 Stafford, Thomas BS, (?) USNA
2 White, Ed MS, Aeronautical Eng
2 Young, John BS, Aeronautical Eng
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I'd say there's a pretty strong trend here. Of course, being military officers (or ex-military officers) they would all have at least a BS degree, and if they were always interested in aviation it's quite likely they would have aeronautical engineering degrees. Only a few seem to have MS degrees. The first PhD was almost certainly Buzz Aldrin, but there were also some medical doctors in there somewhere like Joe Kerwin.
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I'd say there's a pretty strong trend here. Of course, being military officers (or ex-military officers) they would all have at least a BS degree, and if they were always interested in aviation it's quite likely they would have aeronautical engineering degrees. Only a few seem to have MS degrees. The first PhD was almost certainly Buzz Aldrin, but there were also some medical doctors in there somewhere like Joe Kerwin.
Maybe I should have listed their schools - pretty impressive, for the most part: Purdue, MIT, UCLA, Cal Tech, Ga Tech.
I had to put this on pause because I'm currently a bit under the weather. Group 3, though, looked a lot like the first two, but for Group 4 they were recruiting scientists and applicants were required to have a doctorate.
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Since nearly all of them (except for the scientist-astronauts) were military officiers, didn't they generally get their degrees from the service academies?
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The fact that so many Apollo astronauts were themselves engineers makes me wonder about the often-claimed divide between them and the engineers who designed their spacecraft.
I think The Right Stuff has a scene, obviously done for laughs, showing an argument between a Mercury astronaut and an engineer over whether the capsule should have a window. Maybe that movie was especially hard on the engineers because it was told mainly from Chuck Yeager's viewpoint, and he was famously non-academic. Though I always found it hard to identify with the man's pure seat-of-the-pants philosophy I lost a great deal of respect for him after his non-performance on the Rogers (Challenger) Commission. I think his only contribution was to suggest not flying in cold weather.
Another work to play up the astronaut/engineer divide was the excellent book Digital Apollo; its entire theme was the man-machine (i.e., pilot-spacecraft/aircraft) interface. It featured a famous early-60s cartoon showing two alternative approaches to Apollo design: in one, the astronauts inhabit a totally automated spacecraft in a sterile cabin whose only feature is a big ABORT button. They look totally bored. In the other, they're mad at work over a pile of equipment, doing nearly everything by hand.
I think this divide was overstated as everything I've read by those actually involved is that the astronauts and engineers got along pretty well, largely because the astronauts themselves were engineers and actively took part in the design. Each was assigned some area of specialty (life support, pressure suits, guidance, propulsion, etc) and spent quite a bit of time with the contractors. Some, like Roger Chaffee, reportedly went above and beyond the call of duty often asking to meet every line-level worker during a plant visit.
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Those scenes in The Right Stuff also featured the divide between proud oldworld Germans vs brash young Americans to the same humorous effect. It seems there was a large difference in personal attitude between those designing a ballistic missile to be controlled from the ground and those wanting a space ship to be flown by a pilot. Tom Wolfe never let the truth get in the way of a good story.
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The fact that so many Apollo astronauts were themselves engineers makes me wonder about the often-claimed divide between them and the engineers who designed their spacecraft.
I think The Right Stuff has a scene, obviously done for laughs, showing an argument between a Mercury astronaut and an engineer over whether the capsule should have a window. Maybe that movie was especially hard on the engineers because it was told mainly from Chuck Yeager's viewpoint, and he was famously non-academic. Though I always found it hard to identify with the man's pure seat-of-the-pants philosophy I lost a great deal of respect for him after his non-performance on the Rogers (Challenger) Commission. I think his only contribution was to suggest not flying in cold weather.
Another work to play up the astronaut/engineer divide was the excellent book Digital Apollo; its entire theme was the man-machine (i.e., pilot-spacecraft/aircraft) interface. It featured a famous early-60s cartoon showing two alternative approaches to Apollo design: in one, the astronauts inhabit a totally automated spacecraft in a sterile cabin whose only feature is a big ABORT button. They look totally bored. In the other, they're mad at work over a pile of equipment, doing nearly everything by hand.
I think this divide was overstated as everything I've read by those actually involved is that the astronauts and engineers got along pretty well, largely because the astronauts themselves were engineers and actively took part in the design. Each was assigned some area of specialty (life support, pressure suits, guidance, propulsion, etc) and spent quite a bit of time with the contractors. Some, like Roger Chaffee, reportedly went above and beyond the call of duty often asking to meet every line-level worker during a plant visit.
My impression is that most of the conflict was during the Mercury program, when the admittedly old-world engineers regarded the astronaut as almost an afterthought. You don't ( or I haven't, at least) hear nearly as much about it in Gemini and following. Of course, there were multiple occasions where the machines malfunctioned and the astronaut turned out to be the component that saved the mission - not to mention his own hide.
Service Acadamies - some did, some didn't. Gene Cernan, for example, was a product of the NROTC program at Purdue. As it happens, Neil Armstrong was there at the same time, going through Purdue on the "Holloway Plan". I guess flying jets, especially for the Navy, was a fast-growing field in the 50s and more than one route was open for the right candidates. Gus Grissom enlisted as an aviation cadet right out of high school (WWII was still going on), pulled a hitch, got his Mechanical Engineering degree (Purdue again), then re-enlisted in the new USAF. Deke Slayton did something similar, although he enlisted early enough to actually become a pilot and fly fifty-some missions in WWII, then stayed in and got his degree at U. Minn after the war.
Also, I don't know if you've ever read his autobiography, but Yeager seems to have been sort of an intuitive, self-educated engineer (although Ralph Rene makes me kind of leery of that term). He talks about how he always made it his business to be sure he understood the function of every nut, bolt, washer, transverse thingamabobble, and every part of any aircraft he flew; he credits that knowledge with 'saving his ass' more than once.
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Of course, there were multiple occasions where the machines malfunctioned and the astronaut turned out to be the component that saved the mission - not to mention his own hide.
Yes, though one has to point out that it's not so important to bring a spacecraft back to earth when it's not carrying any of those aforementioned hides.
Astronauts have always been more than a little defensive about crewed vs robotic spacecraft (what used to be called manned vs unmanned). I hope we'll be over that when humans do return to the moon because there won't be much point in doing it simply to show they can fly a spacecraft to the moon. We did that over 40 years ago, and it has become routine to send highly capable robots to the moon and planets.
If we send humans back to the moon, it should be for what they can do once they're there, not to show off their skills as pilots in getting there. I'm sure there's a lot more useful scientific work that could be done by professional geologists, for example, and engineers could gain a lot of useful experience in designing, building and operating habitats in such a hostile environment.