# Two Apollo questions **URL:** \ **Category:** Factual Questions\ **Created:** [July 17, 2009, 2:30pm UTC](https://boards.straightdope.com/t/two-apollo-questions/503339 "2009-07-17T14:30:19Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![MrFloppy](https://avatars.discourse-cdn.com/v4/letter/m/43a26b/32.png) [@MrFloppy](https://boards.straightdope.com/u/MrFloppy)\ **Post date:** [July 17, 2009, 2:30pm UTC](https://boards.straightdope.com/t/two-apollo-questions/503339/1 "2009-07-17T14:30:19Z")
First of all, as a avid moon landing hoax debunker on various sites, I’m quite looking forward to the LRO photos being released at noon today. The will show the landing site of at least one Apollo mission with resolution down to 1 meter. Anyhoo… 1. During the TLI portion of the flight, why, after the burn and separation of the S-IVB did they turn the LM/CM/SM around so that the LM faced away from the moon? 2. Why was a ‘figure eight’ trajectory preferable for the flight. IOW, the flight made a figure eight around the earth and moon. Why not a simple ellipse or oval trajectory with the earth and moon at the focii?? ---
**Author:** ![keeper0](https://avatars.discourse-cdn.com/v4/letter/k/b19c9b/32.png) [@keeper0](https://boards.straightdope.com/u/keeper0)\ **Post date:** [July 17, 2009, 2:47pm UTC](https://boards.straightdope.com/t/two-apollo-questions/503339/2 "2009-07-17T14:47:24Z")
I am posting an answer to #2 based on my basic physics understanding, so I could be wrong. - the figure eight path is notably shorter than an ellipse. The linear speed of the Lunar Orbiter and module relative to earth is fixed by fuel/weight/thrust. Traveling at that speed along an ellipse would have added significant time to the mission. ---
**Author:** ![Earl\_Snake-Hips\_Tucker](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/earl_snake-hips_tucker/32/3269_2.png) [@Earl\_Snake-Hips\_Tucker](https://boards.straightdope.com/u/Earl_Snake-Hips_Tucker)\ **Post date:** [July 17, 2009, 3:15pm UTC](https://boards.straightdope.com/t/two-apollo-questions/503339/3 "2009-07-17T15:15:20Z")
Here’s a WAG on #1): They were going to have to do that anyway, since I think they would have to fire the CSM engine again to slow them down to drop them into a nice lunar orbit. However, there might be a more practical reason as to why they chose that time of the mission, as opposed to nearer the Moon, to execute that maneuver. ---
**Author:** ![MrFloppy](https://avatars.discourse-cdn.com/v4/letter/m/43a26b/32.png) [@MrFloppy](https://boards.straightdope.com/u/MrFloppy)\ **Post date:** [July 17, 2009, 3:22pm UTC](https://boards.straightdope.com/t/two-apollo-questions/503339/4 "2009-07-17T15:22:58Z")
> [@Earl\_Snake-Hips\_Tucker](#): > > Here’s a WAG on #1): They were going to have to do that anyway, since I think they would have to fire the CSM engine again to slow them down to drop them into a nice lunar orbit. However, there might be a more practical reason as to why they chose that time of the mission, as opposed to nearer the Moon, to execute that maneuver. Granted the CSM engine will have to pointing that way to slow them down but the astronauts always complained that they never saw the moon until they got there, I was wondering why they couldn’t fly towards it and turn around at the ‘last minute’. ---
**Author:** ![Cliffy](https://avatars.discourse-cdn.com/v4/letter/c/59ef9b/32.png) [@Cliffy](https://boards.straightdope.com/u/Cliffy)\ **Post date:** [July 17, 2009, 3:35pm UTC](https://boards.straightdope.com/t/two-apollo-questions/503339/5 "2009-07-17T15:35:59Z")
They’re doing (or at least planning for) mid-course correction burns throughout that part of the flight. For some reason I think those would tend to be deceleration burns, so the “rear” facing attitude would facilitate that. Plus, as **Earl** says, they’re going to have to face that way to enter lunar orbit. No reason not to do it early, instead of when they have a million other items to prepare for. I also think it might have had something to do with better communications with Earth. I’ve read some of the comm logs from Apollo 15, and they’re constantly switching antennas. Communicaton with Houston required a non-trivial amount of attention and wasn’t as reliable as is shown in the movies (even the good ones like Apollo 13 and From the Earth to the Moon). BTW, the Shuttle also orbits in a rear-facing attitude, IIRC. As for the figure 8, I think you get to use some of your orbital momentum to propel you along, instead of having to waste fuel to zero it out. This is why Apollo 13 went all the way around the moon instead of just hitting the brakes and burning back home when the tanks blew. –Cliffy ---
**Author:** ![Terminus\_Est](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/terminus_est/32/3087_2.png) [@Terminus\_Est](https://boards.straightdope.com/u/Terminus_Est)\ **Post date:** [July 17, 2009, 3:52pm UTC](https://boards.straightdope.com/t/two-apollo-questions/503339/6 "2009-07-17T15:52:38Z")
The CSM has to turn around to dock with and extract the LM. It’s unnecessary to turn the whole thing around, since they’re already pointed in the right direction for midcourse corrections and especially to burn for insertion into lunar orbit. As for the flight profile, remember that the moon is in orbit around the earth, so the path the spacecraft takes doesn’t really look like a figure-8: [http://science.ksc.nasa.gov/mirrors/images/images/pao/AS10/10075090.jpg](http://science.ksc.nasa.gov/mirrors/images/images/pao/AS10/10075090.jpg) ---
**Author:** ![MonkeyMensch](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@MonkeyMensch](https://boards.straightdope.com/u/MonkeyMensch)\ **Post date:** [July 17, 2009, 4:17pm UTC](https://boards.straightdope.com/t/two-apollo-questions/503339/7 "2009-07-17T16:17:54Z")
It **really** doesn’t look like a figure-8. A real scale drawing of the vehicles path looks like a fairly straight line to and back from the moon. Consider the Earth as the size of a golf ball: The moon is dinky ball about the diameter of your pinky finger’s nail and four feet away. The obital path can be approximated by lines tangent to the two balls. In reality it would be slightly curved in the near vicinity of the balls, but not much. ---
**Author:** ![Colophon](https://avatars.discourse-cdn.com/v4/letter/c/f05b48/32.png) [@Colophon](https://boards.straightdope.com/u/Colophon)\ **Post date:** [July 17, 2009, 4:35pm UTC](https://boards.straightdope.com/t/two-apollo-questions/503339/8 "2009-07-17T16:35:32Z")
> [@MrFloppy](#): > > First of all, as a avid moon landing hoax debunker on various sites, I’m quite looking forward to the LRO photos being released at noon today. Psst: they’re [here](http://www.nasa.gov/mission_pages/LRO/multimedia/lroimages/apollosites.html)! Apollo 14 is superb. ---
**Author:** ![Terminus\_Est](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/terminus_est/32/3087_2.png) [@Terminus\_Est](https://boards.straightdope.com/u/Terminus_Est)\ **Post date:** [July 17, 2009, 4:53pm UTC](https://boards.straightdope.com/t/two-apollo-questions/503339/9 "2009-07-17T16:53:03Z")
> [@Colophon](#): > > Psst: they’re [here](http://www.nasa.gov/mission_pages/LRO/multimedia/lroimages/apollosites.html)! Apollo 14 is superb. Obviously photoshopped. ---
**Author:** ![MikeS](https://avatars.discourse-cdn.com/v4/letter/m/919ad9/32.png) [@MikeS](https://boards.straightdope.com/u/MikeS)\ **Post date:** [July 17, 2009, 4:58pm UTC](https://boards.straightdope.com/t/two-apollo-questions/503339/10 "2009-07-17T16:58:31Z")
For the “figure-eight” question, I would guess that it has something to do with the lunar rotation. You generally want to launch and land your rockets on a planet or moon in the same direction as it’s rotating, since that way you don’t have to change your velocity as much when you go from orbiting to standing on the surface. Since the Moon is rotating in the same direction as the Earth, you need to “cross over” in a figure-eight to get yourself going around the Moon the right direction; an “elliptical” orbit like you describe would cause you to be going around the Moon opposite its rotation. Granted, the Moon’s rotational period is pretty slow since it’s tidally locked to the Earth (only about 4.6 m/s at the lunar equator, compared to the escape velocity of 2.4 km/s), but why not give yourself that extra bit of delta-vee? ---
**Author:** ![mlees](https://avatars.discourse-cdn.com/v4/letter/m/ba8739/32.png) [@mlees](https://boards.straightdope.com/u/mlees)\ **Post date:** [July 17, 2009, 5:01pm UTC](https://boards.straightdope.com/t/two-apollo-questions/503339/11 "2009-07-17T17:01:23Z")
Heh. I caught myself doing the same thing you folks are doing: talking about the Apollo 11 mission in the present tense. 🙂 (Some of you anyway. It seems that way to me: post #5, for instance.) ---
**Author:** ![UltraVires](https://avatars.discourse-cdn.com/v4/letter/u/ecccb3/32.png) [@UltraVires](https://boards.straightdope.com/u/UltraVires)\ **Post date:** [July 17, 2009, 5:07pm UTC](https://boards.straightdope.com/t/two-apollo-questions/503339/12 "2009-07-17T17:07:14Z")
> [@Colophon](#): > > Psst: they’re [here](http://www.nasa.gov/mission_pages/LRO/multimedia/lroimages/apollosites.html)! Apollo 14 is superb. Those shadows don’t look right and there is no air on the moon. ---
**Author:** ![Dreamy\_B](https://avatars.discourse-cdn.com/v4/letter/d/f07891/32.png) [@Dreamy\_B](https://boards.straightdope.com/u/Dreamy_B)\ **Post date:** [July 17, 2009, 6:21pm UTC](https://boards.straightdope.com/t/two-apollo-questions/503339/13 "2009-07-17T18:21:52Z")
> [@Earl\_Snake-Hips\_Tucker](#): > > Here’s a WAG on #1): They were going to have to do that anyway, since I think they would have to fire the CSM engine again to slow them down to drop them into a nice lunar orbit. However, there might be a more practical reason as to why they chose that time of the mission, as opposed to nearer the Moon, to execute that maneuver. I was considering this very question yesterday while looking at [wechoosethemoon.org](http://wechoosethemoon.org). To me it would make sense to leave the LM stored behind the safety of the shields rather then subject it to the atmosphere of space for the duration of the TL flight. The only advantage that I can think of is by docking the LM, this gives the astronauts a little more room to move about the cabin for the day or two it takes to reach the moon. I suppose this allows them to perform various house keeping chores on their way. ---
**Author:** ![carnivorousplant](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/carnivorousplant/32/3563_2.png) [@carnivorousplant](https://boards.straightdope.com/u/carnivorousplant)\ **Post date:** [July 17, 2009, 6:33pm UTC](https://boards.straightdope.com/t/two-apollo-questions/503339/14 "2009-07-17T18:33:44Z")
> [@Dreamy\_B](#): > > The only advantage that I can think of is by docking the LM, this gives the astronauts a little more room to move about the cabin for the day or two it takes to reach the moon. You know the damn thing is going to work, so you don’t have to metaphorically turn around and come back if it doesn’t. 🙂 And of course, you have a working lifeboat if need it. ---
**Author:** ![MonkeyMensch](https://avatars.discourse-cdn.com/v4/letter/m/82dd89/32.png) [@MonkeyMensch](https://boards.straightdope.com/u/MonkeyMensch)\ **Post date:** [July 17, 2009, 6:37pm UTC](https://boards.straightdope.com/t/two-apollo-questions/503339/15 "2009-07-17T18:37:35Z")
> [@MikeS](#): > > For the “figure-eight” question, I would guess that it has something to do with the lunar rotation. You generally want to launch and land your rockets on a planet or moon in the same direction as it’s rotating, since that way you don’t have to change your velocity as much when you go from orbiting to standing on the surface. Since the Moon is rotating in the same direction as the Earth, you need to “cross over” in a figure-eight to get yourself going around the Moon the right direction; an “elliptical” orbit like you describe would cause you to be going around the Moon opposite its rotation. Granted, the Moon’s rotational period is pretty slow since it’s tidally locked to the Earth (only about 4.6 m/s at the lunar equator, compared to the escape velocity of 2.4 km/s), but why not give yourself that extra bit of delta-vee? But since the Earth and Moon are rotating in the same direction you’re actually going to end up going the “other way” if you cross-over. That is to say, viewed from the North both Earth and Moon rotate counter-clockwise. The Apollo lunar orbits were clockwise viewed from the North. ---
**Author:** ![ElvisL1ves](https://avatars.discourse-cdn.com/v4/letter/e/5daacb/32.png) [@ElvisL1ves](https://boards.straightdope.com/u/ElvisL1ves)\ **Post date:** [July 17, 2009, 8:28pm UTC](https://boards.straightdope.com/t/two-apollo-questions/503339/16 "2009-07-17T20:28:37Z")
> [@Dreamy\_B](#): > > subject it to the atmosphere of space Um, what? ---
**Author:** ![UltraVires](https://avatars.discourse-cdn.com/v4/letter/u/ecccb3/32.png) [@UltraVires](https://boards.straightdope.com/u/UltraVires)\ **Post date:** [July 17, 2009, 9:24pm UTC](https://boards.straightdope.com/t/two-apollo-questions/503339/17 "2009-07-17T21:24:34Z")
> [@ElvisL1ves](#): > > Um, what? I know what he meant. 🙂 ---
**Author:** ![Dreamy\_B](https://avatars.discourse-cdn.com/v4/letter/d/f07891/32.png) [@Dreamy\_B](https://boards.straightdope.com/u/Dreamy_B)\ **Post date:** [July 17, 2009, 10:23pm UTC](https://boards.straightdope.com/t/two-apollo-questions/503339/18 "2009-07-17T22:23:37Z")
> [@ElvisL1ves](#): > > Um, what? Harsh environment. :rolleyes: ---
**Author:** ![CurtC](https://avatars.discourse-cdn.com/v4/letter/c/ce73a5/32.png) [@CurtC](https://boards.straightdope.com/u/CurtC)\ **Post date:** [July 17, 2009, 10:28pm UTC](https://boards.straightdope.com/t/two-apollo-questions/503339/19 "2009-07-17T22:28:20Z")
> [@Terminus\_Est](#): > > As for the flight profile, remember that the moon is in orbit around the earth, so the path the spacecraft takes doesn’t really look like a figure-8: > [http://science.ksc.nasa.gov/mirrors/images/images/pao/AS10/10075090.jpg](http://science.ksc.nasa.gov/mirrors/images/images/pao/AS10/10075090.jpg) Nice diagram. It looks to me, from looking at that diagram, that going to the left of the Moon and orbiting it clockwise would help slow the spacecraft down. Had they approached from the right, it would have been like a classic gravity-assist which would use the Moon’s tug to speed the orbiter up. ---
**Author:** ![John\_DiFool](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_difool/32/19543_2.png) [@John\_DiFool](https://boards.straightdope.com/u/John_DiFool)\ **Post date:** [July 17, 2009, 11:08pm UTC](https://boards.straightdope.com/t/two-apollo-questions/503339/20 "2009-07-17T23:08:55Z")
> [@Colophon](#): > > Psst: they’re [here](http://www.nasa.gov/mission_pages/LRO/multimedia/lroimages/apollosites.html)! Apollo 14 is superb. I think I can make out Alan Shepherd’s golf ball in that shot. Truth now-I saw the launch of 14 from an airliner. [Next page](https://boards.straightdope.com/t/two-apollo-questions/503339.md?page=2)