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  Mercury - Gemini - Apollo
  Apollo 11 lunar module off-course landing

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Author Topic:   Apollo 11 lunar module off-course landing
Jim_Voce
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Posts: 103
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Registered: Jul 2016

posted 07-26-2016 11:55 PM     Click Here to See the Profile for Jim_Voce   Click Here to Email Jim_Voce     Edit/Delete Message   Reply w/Quote
Everyone here knows about the dicey last minute landing of Neil Armstrong in the Sea of Tranquility. When the Eagle was coming down for its landing, it was coming down into a field of boulders. So Armstrong hovered for a clearing.

The field of boulders was unexpected and came about because the Eagle's descent from lunar orbit was off-course. Does anyone recall what caused the Eagle to be off course after it separated from the command module?

Jim Behling
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From: Cape Canaveral, FL
Registered: Mar 2010

posted 07-27-2016 11:53 AM     Click Here to See the Profile for Jim Behling   Click Here to Email Jim Behling     Edit/Delete Message   Reply w/Quote
The LM fly around inspection.

Headshot
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From: Streamwood, IL USA
Registered: Feb 2012

posted 07-27-2016 12:24 PM     Click Here to See the Profile for Headshot   Click Here to Email Headshot     Edit/Delete Message   Reply w/Quote
Didn't sloshing in Eagle's propellant tanks slightly shift its center of gravity and contribute to the trajectory error? I believe baffles were added to Intrepid's tanks by the time Apollo 12 flew.

cycleroadie
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From: Apalachin, NY USA
Registered: May 2011

posted 07-27-2016 12:39 PM     Click Here to See the Profile for cycleroadie   Click Here to Email cycleroadie     Edit/Delete Message   Reply w/Quote
An article in Popular Mechanics where they talked about the mission with many of those involved included this quote from Steve Bales, the Guidance Officer for the mission:
The lunar module could do a lot of things independently, but it had to rely on people on the ground to tell the computer where it was starting on its landing position. On the backside of the moon there had been some venting — some perturbations unknown to us. The time we gave them to light the engine was about 4 or 5 seconds late.

nasamad
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Posts: 2054
From: Essex, UK
Registered: Jul 2001

posted 07-27-2016 12:58 PM     Click Here to See the Profile for nasamad   Click Here to Email nasamad     Edit/Delete Message   Reply w/Quote
I thought there was still some pressure in the tunnel when Eagle and Columbia separated and this caused Eagle to separate a couple of mph faster than planned.

May be I'm thinking of a different mission, I'm going to look it up when I get the chance (unless some nice person gets and shares the answer here first).

Jeff
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From: Fayetteville, NC, USA
Registered: May 2009

posted 07-27-2016 02:50 PM     Click Here to See the Profile for Jeff   Click Here to Email Jeff     Edit/Delete Message   Reply w/Quote
You're thinking of the correct mission. From the pages of the Apollo Lunar Surface Journal Gene Kranz states:
The principal error induced by maneuvering of the spacecraft was, however, the incomplete vent of the tunnel propagated over one orbit after separation. We made a change in all future missions to get a MCC go-nogo on tunnel delta P before giving the crew a Go to undock.

Jeff
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From: Fayetteville, NC, USA
Registered: May 2009

posted 07-27-2016 02:54 PM     Click Here to See the Profile for Jeff   Click Here to Email Jeff     Edit/Delete Message   Reply w/Quote
Here's the complete statement from Gene again taken from the Apollo Lunar Surface Journal.
In a 2002 e-mail, Kranz elaborates, "Floyd's note was correct on the velocity-induced position error at the start of the descent. There were several interrelated navigation problems, i.e. known deficiencies in the R2 lunar (gravitational) potential model, down-track (along the flight path toward the landing site) and cross-track (left or right, perpendicular to the flight path) propagation errors (errors that start out small but become larger as the flight proceeds), and errors induced by maneuvering of the spacecraft. The principal error induced by maneuvering of the spacecraft was, however, the incomplete vent of the tunnel propagated over one orbit after separation. We made a change in all future missions to get a MCC go-nogo on tunnel delta P before giving the crew a Go to undock. Page 82 of the Apollo 11 post mission report says ...'because of uncoupled attitude maneuvers such as hot fire tests, undocking impulse, station keeping, sublimator operation and possible tunnel and cabin venting. The net effect of these perturbations was a sizeable down-range miss.' To my recollection, the trajectory reconstruction determined that with the exception of the tunnel venting, most of the other perturbations were essentially self canceling. Further the post mission review indicated that the delta P gauge was too gross, the markings misleading and the tunnel had to be vented earlier in the timeline and the valve left in the tunnel vent position rather than returned to off.

nasamad
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Posts: 2054
From: Essex, UK
Registered: Jul 2001

posted 07-27-2016 03:38 PM     Click Here to See the Profile for nasamad   Click Here to Email nasamad     Edit/Delete Message   Reply w/Quote
Thanks Jeff, I was trying to look on the ALSJ myself as its my first place to look for this kind of info but the formatting of the site doesn't agree with my iPad for some reason.

Nice to know my mind is working better than my iPad though.

oly
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Posts: 218
From: Perth, Western Australia
Registered: Apr 2015

posted 07-28-2016 01:26 AM     Click Here to See the Profile for oly   Click Here to Email oly     Edit/Delete Message   Reply w/Quote
This article also gives reference to the higher than expected velocity and other causes:
As soon as Eagle had disconnected from Columbia, it was propelled faster than planned because of the residual oxygen expelled from the airlock. As a result, the LM overflew the planned landing site.

Cozmosis22
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Posts: 767
From: Texas * Earth
Registered: Apr 2011

posted 10-07-2017 06:09 PM     Click Here to See the Profile for Cozmosis22     Edit/Delete Message   Reply w/Quote
According to this UPI actual teletype printout during the Apollo 12 mission they had an interesting explanation of the Eagle landing site overshoot. Can't recall this being discussed elsewhere. Any thoughts?
Because of Apollo Eleven's four-mile miss, the Apollo 12 flight planners have made some changes.

Apollo Eleven's computer information was out of date before its final descent. Armstrong and Aldrin were too busy preparing for the historic touchdown to work up on the computer the information obtained in 16 minutes of radio contact before the landing. Apollo 12 is to have more than 30 minutes of radio contact with the Earth before landing. This, it is said, is plenty of time to feed the latest tracking data into the lunar module's computer.

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