vq
Earth
What time is it again?
Posts: 129
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Post by vq on Aug 19, 2008 21:45:11 GMT -4
This is an amazing forum; greetings. I have a few questions about Apollo that five minutes on Google and Wikipedia couldn't answer, and I was wondering if someone here could help me out. For the record, I have absolutely no ulterior motives about claiming any phase of the Apollo missions were technologically impossible or something like that. I'm a mechanical engineering intern who spends much of his life writing contingency procedures, so what the Apollo guys had planned in case something critical went wrong fascinates me. How long was the launch window for the LM ascent stage to successfully rendezvous with the CSM? If the ascent engine failed to ignite, were there any procedures or plans to repair it and make another burn attempt? Was there enough spare air for a contingency EVA or any reasonable probability that one would be helpful in repairing the ascent stage engine? Did the exploding bolts connecting the descent stage to the ascent stage fire on the burn command, or was there a firing sequence to allow the ascent stage engine to reach full thrust prior to release similar to rocket liftoffs on earth? If so, where was the ascent stage exhaust routed when in close proximity to the descent stage? Were there contingency plans in place for the CSM to change its orbit and dock with the LM if the orbiting LM was for some reason unable to reach the CSM? How long could the ascent stage support the astronauts in lunar orbit? To sum up, I guess I'm a little ignorant about this particular stage of the mission. Guidance to a concise but detailed resource about the lunar rendezvous would be welcomed as well (I don't mind reading, really...) These are a lot of questions, too; answers to any one of them would still be greatly appreciated. Thanks!
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Post by laurel on Aug 19, 2008 22:05:29 GMT -4
Were there contingency plans in place for the CSM to change its orbit and dock with the LM if the orbiting LM was for some reason unable to reach the CSM? From A Man On The Moon by Andrew Chaikin, page 223, referring to Apollo 11: "The [LM ascent] engine must work, and it must work long enough for Eagle to reach some kind of orbit. Collins was prepared to rescue them if they couldn't make it all the way up to 69 miles. He could drop down to 50 000 feet, but not too much lower than that; some of the lunar mountains were 20 000 or 30 000 feet high." I suspect someone else will be along soon to give you a much more detailed answer.
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Post by PhantomWolf on Aug 20, 2008 2:33:19 GMT -4
The LM engine was pretty simple in design, so it was unlikely not to work. Pressurised Helium would be allowed to flood into the fuel and oxidiser tanks, forcing them out into the combustion chamber. Since the two would spontaneously combust when mixed, there was no need for any ignition system, just mix the two and away you'd go.
Essentially the only reasons for the engine not to work would be a loss of pressure in the helium system or a broke fuel/oxidiser line. In the event of either of those, the crew was most likely not coming home.
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vq
Earth
What time is it again?
Posts: 129
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Post by vq on Aug 20, 2008 2:54:32 GMT -4
The LM engine was pretty simple in design, so it was unlikely not to work. Pressurised Helium would be allowed to flood into the fuel and oxidiser tanks, forcing them out into the combustion chamber. Since the two would spontaneously combust when mixed, there was no need for any ignition system, just mix the two and away you'd go. Essentially the only reasons for the engine not to work would be a loss of pressure in the helium system or a broke fuel/oxidiser line. In the event of either of those, the crew was most likely not coming home. As I recall in Apollo 13, the helium pressurization system on the LM descent stage cracked its relief valve late in the mission. Was this a risk with the ascent stage as well or were they different designs (ie liquid helium vs pressurized gas)?
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Post by Kiwi on Aug 20, 2008 10:31:36 GMT -4
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Post by JayUtah on Aug 20, 2008 12:25:24 GMT -4
So many great answers already. The ignition transient for the APS was only about a third of a second, so there's no problem with the lengthy startup of pump-fed engines. The separate and ignite commands come very close after each other. As a writer of contingency procedures, you would naturally appreciate that this would be needed in an aborted landing.
The overall view of the LM ascent was that the LM pilots were to get it to orbit -- any orbit -- and that it would be the CM pilot's job to do any major maneuvers to rescue the LM from a botched orbit. Michael Collins says there were 18 rendevzous abort scenarios, but he doesn't describe any of them in any of his books.
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Post by Count Zero on Aug 21, 2008 12:32:14 GMT -4
It was considered a possibility (though a remote one) that the LM might have to leave the surface on short notice at any time. Such emergencies included a leak in the Ascent Stage helium pressurization system (which meant they had to use it immediately or not be able to take-off at all) and solar flares. The astronauts practiced emergency takeoffs. Fron there, Jay already said it:
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Post by scubadude402 on Aug 21, 2008 21:17:15 GMT -4
I do remember Edgar Mitchell , on his website, mention that there was a sort of last ditch procedure involving " jumper cables" that could be used, theroreticaly, to connect to the ascent engine if it didnt ignite but he stated that is was very far fetched and they all hoped it would not come to that. joe B
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Post by PhantomWolf on Aug 22, 2008 5:07:48 GMT -4
It was considered a possibility (though a remote one) that the LM might have to leave the surface on short notice at any time. Such emergencies included a leak in the Ascent Stage helium pressurization system (which meant they had to use it immediately or not be able to take-off at all) and solar flares. The astronauts practiced emergency takeoffs. Fron there, Jay already said it: In fact if you read through the landing procedures in the ALSJ you'll see that the first thing they did after landing was to prepare the craft to take off immediately. This ment that if for any reason they had to return and lift off, the craft was ready to go at a moment's notice.
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Ian Pearse
Mars
Apollo (and space) enthusiast
Posts: 308
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Post by Ian Pearse on Aug 22, 2008 10:42:36 GMT -4
So many great answers already. The ignition transient for the APS was only about a third of a second, so there's no problem with the lengthy startup of pump-fed engines. The separate and ignite commands come very close after each other. As a writer of contingency procedures, you would naturally appreciate that this would be needed in an aborted landing. The overall view of the LM ascent was that the LM pilots were to get it to orbit -- any orbit -- and that it would be the CM pilot's job to do any major maneuvers to rescue the LM from a botched orbit. Michael Collins says there were 18 rendevzous abort scenarios, but he doesn't describe any of them in any of his books. I remember reading in First Man how Michael Collins had a pad with the 18 different scenarios listed on it right by his hand as Eagle was preparing for the ascent. Thankfully he never had to use any of them!
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Ian Pearse
Mars
Apollo (and space) enthusiast
Posts: 308
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Post by Ian Pearse on Aug 22, 2008 10:43:38 GMT -4
So many great answers already. The ignition transient for the APS was only about a third of a second, so there's no problem with the lengthy startup of pump-fed engines. The separate and ignite commands come very close after each other. As a writer of contingency procedures, you would naturally appreciate that this would be needed in an aborted landing. The overall view of the LM ascent was that the LM pilots were to get it to orbit -- any orbit -- and that it would be the CM pilot's job to do any major maneuvers to rescue the LM from a botched orbit. Michael Collins says there were 18 rendevzous abort scenarios, but he doesn't describe any of them in any of his books. I remember reading in First Man how Michael Collins had a pad with the 18 different scenarios listed on it right by his hand as Eagle was preparing for the ascent. Thankfully he never had to use any of them!
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vq
Earth
What time is it again?
Posts: 129
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Post by vq on Aug 23, 2008 2:01:03 GMT -4
Welcome to ApolloHoax, vq. These might help. Great information, someone should put it in a sticky thread at the top of this forum. So it sounds like the CSM had enough spare deltaV to dock with the LM if it was able to attain any sort of orbit... good plan. Glad it never came to that though. From an engineering perspective, the LM was a heckuva machine. I had always assumed that the ascent was a "direct ascent" sort of situation with precise launch timing involved, but it sounds like the LM could fire its engine multiple times in the process of synchronizing orbits. Makes a lot more sense, actually. What was the endurance capability of the ascent stage? I didn't see that info in the links.
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Post by Kiwi on Aug 23, 2008 5:35:26 GMT -4
I had always assumed that the ascent was a "direct ascent" sort of situation with precise launch timing involved, but it sounds like the LM could fire its engine multiple times in the process of synchronizing orbits. Makes a lot more sense, actually. From Apollo 14 onwards a more direct method of lunar orbit rendezvous was used, and this shortened by over an hour the time between the LM lifting off the lunar surface and docking with the CSM, but a number of burns were still made. For more information see the first file and the timeline for each mission in the Apollo By The Numbers thread. apollohoax.proboards21.com/index.cgi?board=apollo&action=display&thread=1356Duration between LM lunar liftoff and docking with the CSM, with credit to Al Shepard for the shortest time: Apollo 11 -- 3:40:59 Apollo 12 -- 3:32:32 Apollo 14 -- 1:47:11 Apollo 15 -- 1:59:03 Apollo 16 -- 2:09:30 Apollo 17 -- 2:15:38
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Post by Count Zero on Aug 23, 2008 11:25:43 GMT -4
.... it sounds like the LM could fire its engine multiple times in the process of synchronizing orbits. Iirc, the APS engine only fired once, for the ascent to orbit. All orbital adjustments were made using the RCS thrusters.
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Bob B.
Bob the Excel Guru?
Posts: 3,072
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Post by Bob B. on Aug 23, 2008 12:08:39 GMT -4
.... it sounds like the LM could fire its engine multiple times in the process of synchronizing orbits. Iirc, the APS engine only fired once, for the ascent to orbit. All orbital adjustments were made using the RCS thrusters. I don't believe this is true. I think that from Apollo 14 onward, the terminal phase maneuver was performed using the APS. For example, take a look at the following "Apollo 14 Lunar Orbit Phase" data: history.nasa.gov/SP-4029/Apollo_14g_Lunar_Orbit_Phase.htmNotice the burn performed at 142:30:54.7 GET -- this is a 88.5 ft/s burn lasting only 3.6 seconds. Achieving that much acceleration surely means the high-thrust engine of the APS was used. There are a few other smaller burns in which the acceleration is only about 1 ft/s 2 -- these are the ones that used the RCS. The data for Apollo 11 & 12 appears to show all low-thrust RCS burns, but the data for Apollo 14-17 all show one apparently high-thrust APS burn.
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