Why DID Eagle Land Long?

[kiwigavin]

OK, I know Neil took over manually to flypast the boulder field but why were they on target for that field instead of the planned landing site?

A lot of the players eps. Gene Kranz say that there was residual atmosphere in the docking module which meant that when Eagle separated from Columbia it picked up a small bit of extra push. But Eagle then stopped relative to Columbia and performed the inspection roll. So it would have then started PDI etc from the same velocity start point.

Or am I missing something?

Thanks,

Gavin

PS Pete, Dick and Al get my vote, everytime.

 

[CalliArcale]

I could well be wrong, but my understanding was that they were on target for a landing well within the target landing ellipse when they encountered the boulder field. One of the major (and largely unavoidable) hazards of landing spacecraft on other worlds is that you have to pick your landing site without knowing exactly what the site looks like. Sure, they try to find as smooth and flat a spot as possible, but a boulder much smaller than the smallest boulder resolvable from space could scotch the whole mission. That's probably what happened to Beagle 2 on Mars, though obviously we have no way to know. The second MER, Opportunity, had a landing that is often described as a "hole in one" because it landed right in a tiny crater that nobody could see from orbit at the time. It was hugely fortuitous, but it could easily have been disastrous had the rover been unable to climb out of the crater, or had the crater punctured an airbag or made it impossible for the lander to open.

Another possible explanation for Apollo 11's boulder field is that their navigation may have been off due to lunar mascons (mass concentrations). The Moon is pretty lumpy, which means that its gravitational field is also lumpy. This makes landings a bit more challenging, because your trajectory may be altered in ways you don't expect. I don't think that's what happened to Apollo 11; I think Apollo 11 just encountered a boulder field that nobody on the ground knew about. But it's a possibility to consider.

 

[SkylarkDuQuesne]

Calli, your insight into so many various topics never ceases to amaze me!! Heck, your *parents* were kids when this happened!! :shock: How do you know the moon is lumpy?? Lunar mascons, indeed. Your brain must just absorb knowledge; you can't possibly have the time to read up on such things, chasing those kids around. :mrgreen:

 

[CalliArcale]

I just read too much stuff.

Actually, I never thought of the lunar mascons being a problem until a couple of days ago, when I was reading about Luna 15. Luna 15 was launched a few days before Apollo 11 in a last-ditch effort to steal thunder from the America by returning lunar samples first. Unfortunately, the ambitious probe crash-landed on July 20. The actual reason for the crash is unknown, but the two leading theories include faulty altitude data programmed into the system and the presence of lunar mascons which the system may not have been able to adequately detect and compensate for. So that's what got me thinking about it with Apollo 11. I have no idea if any of my theories are accurate, but I always thought that they were actually on-target but saw a boulder field that flight controllers hadn't known about. That happens all too often with planetary landers, after all.

 

[MeteorWayne]

That is the explanation that I undertand, Calli. I haven't read what Gene Krantz has written about the subject yet, though.

 

[CalliArcale]

Heck, your *parents* were kids when this happened!! :shock:

Actually, as I think about it, they were adults by then -- barely. ;-) Dad was 19, and Mom was 18. I came along exactly six years after Neil Armstrong took his one small step for man.

 

[drwayne]

I became aware of lunar mascons much later, when I found out about, and began to wonder why
lunar orbits are unstable and begin to decay, unless they are at certain inclinations.

Wayne

p.s. I am quite a bit older than Calli by quite a bit, I did see it on TV.

p.p.s Make that Call is much younger than me, that sounds so much better.

 

[kiwigavin]

Sorry to zero the residuals on this thread but...

If it was mascons or an unspotted boulder field why does this item about the escaping atmosphere on separation altering the orbit crop up?

 

[MeteorWayne]

That's my question as well. To me it seems unlikely it would have a residual effect, but I need to examine the descent timing more closely to be sure.

 

[silylene]

You can see why a manual landing was needed:

from this great new press release: "New Photos Reveal Apollo 11 at First Moon Landing Site " http://www.space.com/news/090717-lro-apollo11-images.html

 

[StrandedonEarthsince1970]

Well, I did find one reference to the the separation puff changing the landing target. I suppose it could be that by the time the LEM stopped for inspection, the modules had orbited that much farther along.

#4 in the article "Ten Things You Didn't Know About the Apollo 11 Moon Landing": http://www.popsci.com/military-avia...s-you-didnt-know-about-apollo-ii-moon-landing

 

[jcf2001]

Early in the descent Neil mentioned that they were going to be a little long. You can hear this if you listen to the
descent audio.

So, it wasn't flying over the bolder field that caused them to land long. It was something else that occurred early during the descent phase.

Also I think they still landed in the landing ellipse but they targeted the center of the ellipse. They landed long relative to this target point at the center of the ellipse.

I never heard of a reason why they landed long.

The extra atmosphere explanation could be the answer. It all depends when it leaked out to give the extra push.

 

[Shaky]

Hello All,

I have read so many Apollo books I can't remember which mentioned this particular idea, but I believe a few did. Here is a paraphrasing of what I remember reading. Certainly I am no authority on this and I can't quite figure out out it would make sense.

They hadn't fully de-pressurized the docking tunnel between the LEM and CM and when they separated the extra pressure/air gave the LEM a little push. This was enough to change the LEM's velocity to have them long. Armstrong based his reading on a crater sighting. Of course 11 lacked the doppler based targeting that made 12 and the rest.

 

[Chryseplanatia]

Y'know, I just spent way to many hours editing down the video of the Ap11 landing, and sure enough, right around PDI, Armstrong announces that they are going to land long. Not sure what he was looking at- the LM window etchings aren't of any use at that altitude as I recall? But after following this discussion, I guess it's pretty clear it was not merely the appearance of the boulder field. And that was after sailing over that big crater...

Maybe he did it so that Pete Conrad could look good on Ap 12.

 

[Shaky]

Just checked Chaikin - he mentions that Neil made a number of observations during the early part of descent and all the position checks/landmarks were coming up 2 seconds or so early - about a mile a second was their speed. Now I am going nuts trying to find some reference to that extra air in the tunnel idea. Kranz, Kraft, Chariots of Apollo... and more? I have 30 books here I could look through! Oh well, never tire of reading them.

 

[CalliArcale]

This is a fascinating thread! Thank you all so much! Obviously I was wrong about the boulder field; there is more to this story, and it's fun learning bits and pieces like this!

 

[davf]

I was always under the impression it was due to mascons. Mascons are regions of high density material inside the moon. At the time, their perturbations to the orbit was not fully appreciated. Again, I'm going by memory here, but I also thought that analysis of the Apollo 11 LM and particually CSM orbits gave the data they needed to refine their orbital models resulting in the ability to hit their target more accurately as proven in Apollo 12. I'll have to look through some stuff to find where I'm remembering that from....

 

[davf]

Aha! First book I opened had it: Exploring the Moon: The Apollo Expeditions by David M Harland.. AWESOME book , btw and highly recommended and readable.

"The perturbation of the orbits of the Lunar Orbiters had shown that the distribution of mass within the Moon was surprisingly lumpy. ALthough the flight dynamics team believed they had understood this effect, Apollo 8 had been perturbed, and if a landing had been attempted Frank Borman and Bill Anders would have found themselves descending on unfamiliar terrain. The Apollo 10 LM made the insertion into the descent orbit, and followed through to Power Descent Initiation (PDI), at which point it shed the descent stage and rehearsed an abort. At this lower altitude, the magnitude of the perturbation was more pronounced, and if Tom Stafford and Gene Cernan had tried to land they too would have come down off target.

Despite the pre-PDI navigational fix, when the Eagle pitched upright Neil Armstrong was astonished to see that he was coming in long. "

The crater they were coming into, btw, was called 'West Crater' because it was at the western end of the elliptical landing site.


Following up with Apollo 12 later in the book:
"Apollo 11 had proved the LM's ability to land on the Moon, but the fact that it had strayed so far off target was embarassing, and the flight dynamics team were eager to demonstrate that they could hit a 'pin-point' site. The ability to land within a kilometre of a given location was a prerequisite to being able to follow a specific geological traverse. The flight dynamics team had devised a simple method for the LM to correct for the perturbations of the mascons. The PGNS would be told to head for a 'different' llanding site, but in reality this redirection would take it to the intended target. The engineers were so confident that this would work that they reduced the size of the ellipse. "

 

[kiwigavin]

Aha - starting to make more sense. Thanks Dave. It seems my initial post about the puff of air might have been the red herring. But I still think someone mentions it in a book. I'll keep hunting.

 

[davf]

I don't doubt that it has been repeated and possibly grown into a good hanger story!

 

[CalliArcale]

So it *was* mascons! Wow.

 

[kiwigavin]

... possibly grown into a good hanger story!

Shurely "..VAB story"?

Anyway thanks for the mascon steer guys. I'm still rummaging around the bookcase trying to find where I read about the puff of air. Sadly 'Er Indoors wants we outdoors trying to garden our swamp. Will keep searching.

Cheers chaps.

 

[Pink21]

Great..Mascons.. I think it has been in a good hanger...


Good Day


Regards

Albert

_________

Pret immobilier

 

[BrianBoru]

Hello All,

I have read so many Apollo books I can't remember which mentioned this particular idea, but I believe a few did. Here is a paraphrasing of what I remember reading. Certainly I am no authority on this and I can't quite figure out out it would make sense.

They hadn't fully de-pressurized the docking tunnel between the LEM and CM and when they separated the extra pressure/air gave the LEM a little push. This was enough to change the LEM's velocity to have them long. Armstrong based his reading on a crater sighting. Of course 11 lacked the doppler based targeting that made 12 and the rest.

That is the official NASA explanation. I also think it's plausible.