CSM/LM separation details

[12oh2alarm]

Here's a picture taken shortly after Apollo 11's CSM/LM separation, AS11-37-5443,

The CSM is below the LM. To the uninitiated this looks counter-intuitive. After
all, the LM is supposed to go down to the surface. Why was it done that way?
I know there was a visual inspection of the LM by Collins ("The Eagle has wings.").
Maybe inspection is easier with a black steady background instead of a gray moving one.
Or was it due to orbital mechanics? Can anyone shed some light on this?

[Peter B]

Part of the answer I suspect is that the undocking and inspection process took time. And the two spacecraft appear to have been orbiting the Moon aligned with the distant stars rather than the Moon (Stellar Inertial rather than Orb Rate), I assume to do with reasons of navigation. Thus, while the two spacecraft might have been parallel to the surface of the Moon at the time of undocking, by the time they've reached the time the photograph was taken they've travelled some way around the Moon.

If you go to the Apollo 10 page of the Apollo Flight Journal, there's a chart at the top of the "Snoopy Goes Solo" page which seems to illustrate this.

[Kiwi]

I recall learning about the "why", but cannot find where. Meantime until the experts turn up, here are some excerpts from typescripts I have, which give some explanations.

Apollo By the Numbers – Apollo 11 Missionary Summary

Lunar Orbit/Lunar Surface Phase
During the second lunar orbit, at 078:20, a scheduled live color television transmission was accomplished, providing spectacular views of the lunar surface and the approach path to landing site 2.

After two revolutions and a navigation update, a second service propulsion retrograde burn was made. The 16.88-second maneuver occurred at 080:11:36.75 and circularized the orbit at 66.1 by 54.5 n mi. The commander and lunar module pilot then transferred to the LM and, for about two hours, performed various housekeeping functions, a voice and telemetry test, and an oxygen purge system check. LM functions and consumables checked out well. Additionally, both cameras were checked and verified operational. The pair then returned to the CSM. At 095:20, they returned to the LM to perform a thorough check of all LM systems in preparation for descent.

Undocking occurred at 100:12:00 at an altitude of 62.9 n mi. This was followed by a CSM reaction control system 9.0-second separation maneuver at 100:39:52.9 directed radially downward toward the center of the Moon as planned. The LM descent orbit insertion maneuver was performed with a 30.0-second firing of the descent propulsion system at 101:36:14.0, which put the LM into an orbit of 58.5 by 7.8 n mi.

The 756.39-second powered descent engine burn was initiated at 102:33:05.01. The time was as planned, but the position at which powered descent initiation occurred was about 4 n mi farther downrange than expected. This resulted in the landing point being shifted downrange about 4 n mi.

Apollo 11 Press Kit, page 33

Lunar Orbit Insertion (LOI)

   The first of two lunar orbit insertion burns will be made at 75:54:28 GET at an altitude of about 80 nm above the Moon.  LOI-1 will have a nominal retrograde velocity change of 2,924 fps and will insert Apollo 11 into a 60 x 170 nm elliptical lunar orbit.  LOI-2 two orbits later at 80:09:30 GET will adjust the orbit to a 54 x 65 nm orbit, which because of perturbations of the lunar gravitational potential, will become circular at 60 nm at the time of rendezvous with the LM.  The burn will be 157.8 fps retrograde.  Both LOI maneuvers will be with the SPS engine near pericynthion when the spacecraft is behind the Moon and out of contact with MSFN stations.  After LOI-2 (circularization), the lunar module pilot will enter the lunar module for a brief checkout and return to the command module.

Lunar Module Descent, Lunar Landing

   The lunar module will be manned and checked out for undocking and subsequent landing on the lunar surface at Apollo site 2.  Undocking will take place at 100:09:50 GET prior to the MSFN acquisition of signal.  A radially downward service module RCS burn of 2.5 fps will place the CSM on an equiperiod orbit with a maximum separation of 2.2 nm one half revolution after the separation maneuver.  At this point, on lunar farside, the descent orbit insertion burn (DOI) will be made with the lunar module descent engine firing retrograde 74.2 fps at 101:38:48 GET.  The burn will start at 10 per cent throttle for 15 seconds and the remainder at 40 per cent throttle.

The Invasion of the Moon 1969, Peter Ryan, Penguin Books Ltd, Harmondsworth, UK (1969), pages 100-103

GET 100:14 (6.48 p.m. BST)   

PAO:  This is Apollo control at 100 hours 14 minutes.  We are now less than two minutes from reacquiring the spacecraft on the
<101

102>
Diagram:  Columbia and Eagle undock
1  GET 99:30 LOS at start of orbit
2  GET 100:13  undocking
3  GET 100:16 AOS
4  GET 100:36 separation
<102

103>
thirteenth revolution...  We're presently twenty-five minutes away from the separation burn [using the attitude control thrusters of the service module] that will be performed by Mike Collins in the command module to give the LM and the CSM a separation of about two miles...  We'll stand by now to reacquire the spacecraft...  We have acquisition of signal [telemetry].
   
MCC:  'Hello, Eagle, Houston.  We're standing by, over.'  Eagle:  'Roger, Eagle, stand by.'  MCC:  'Roger, how does it look?'  Armstrong:  'The Eagle has wings.'  Safely undocked, Eagle and Columbia were flying side by side as they came from behind the moon.

GET 100:32 (7.04 p.m. BST)  MCC:  'Columbia, Houston, how do you read?'  Collins:  'I hear you loud and clear, Houston, how me?'  MCC:  'Roger, Mike...  On my mark, seven minutes to ignition.  Mark, seven minutes.'  Collins:  'I got you...  everything's looking real good.'

PAO:  This is Apollo control.  We are six minutes, eight seconds from ignition.

MCC:  'Houston, you are looking good for separation.  You are go for separation, Columbia, over.'  Collins:  'Columbia, understand...  We're really stabilized, Neil, I haven't fired a thruster in five minutes.'  Eagle:  'Mike, what's going to be your pitch angle at SEP [separation]?'  Collins:  'Zero zero seven degrees.'  Eagle:  'OK.'  Collins:  'I think you've got a fine-looking flying machine there, Eagle, despite the fact you're upside down.'  Eagle:  'Somebody's upside down.'  Collins:  'OK, Eagle, one minute until T [separation burn time].  You guys take care.'  Eagle:  'See you later.'

GET 100:40 (7.12 p.m. BST)

PAO:  This is Apollo control.  That separation was performed as scheduled.  In the command module a Delta-V [speed change] of about two point five feet per second [was recorded], which gave a separation to the two vehicles of about eleven hundred feet at the beginning of the descent orbit insertion [DOI] manoeuvre [which was to take place behind the moon].

Eagle:  'Going right down US one, Mike.'  (US highway number one is the nickname the astronauts have given to the approach for the landing site on the Sea of Tranquillity).
<103

[gwiz]

A radially downwards manoeuvre sets up a CSM orbit which initially moves below and ahead of the LM, then moves up to be in front of the LM after half an orbit, continuing to above and ahead and returns to the vicinity of the LM one complete orbit later.  This keeps the CSM away from the path of the LM after the DOI manoeuvre, which causes it to depart to the aft of its original position.

If you visualise the pair travelling to the right, the CSM trajectory relative to the original path of both is an anti-clockwise ellipse starting at the 9 o'clock point, while the LM departure is out to the left, curving gently downwards.

[Obviousman]

The CSM is below the LM. To the uninitiated this looks counter-intuitive. After
all, the LM is supposed to go down to the surface. Why was it done that way?
I know there was a visual inspection of the LM by Collins ("The Eagle has wings.").
Maybe inspection is easier with a black steady background instead of a gray moving one.
Or was it due to orbital mechanics? Can anyone shed some light on this?

You're correct; the visual inspection was done that way so that the LM would be seen against the black of space and not the greys / brightness of the lunar surface. That way the lunar surface also acted as a light reflector, "illuminating" the LM.

[Luke Pemberton]

You're correct; the visual inspection was done that way so that the LM would be seen against the black of space and not the greys / brightness of the lunar surface. That way the lunar surface also acted as a light reflector, "illuminating" the LM.

Was there anything that they did not think of? This place is just a hive of knowledge and learning. Again, it just staggers me that even with all the fine detail that was considered, acted upon and documented, there are those that claim it was hoaxed.