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  Mercury - Gemini - Apollo
  Apollo transposition and docking maneuver

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Author Topic:   Apollo transposition and docking maneuver
David C
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Posts: 433
From: Pasadena
Registered: Apr 2012

posted 03-24-2016 02:43 PM     Click Here to See the Profile for David C     Edit/Delete Message   Reply w/Quote
On Apollo 10, John Young performed the "classic" transposition and docking maneuver after separating the CSM from the S-IVB. This consisted of a +X (CSM "forward") translation, then coasting to gain safe separation until arrested by a -X RCS input.

The CSM then pitched through 180 degrees to face the S-IVB. On completion of the rotation another +X translation brought the CSM towards the S-IVB and simultaneously a 60 degree left roll to the docking attitude was performed — and the entire maneuver culminated in docking. The roll is clearly seen on the Data Acquisition Camera footage taken through window 4.

On all subsequent missions it appears that after the pitch back is initiated, the CMP engaged the DAP and executed a Verb 49 to maneuver automatically to the docking attitude. DAC footage does not always make this very clear since the DAC was usually only turned on relatively late in the sequence, however it is particularly clearly seen on the Apollo 11 window 4 and Apollo 17 window 2 footage.

Does anyone know the reason for this procedure change? I imagine it saved time and possibly fuel, but seems difficult for the CMP to cross check until after completion.

indy91
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From: Germany
Registered: Feb 2016

posted 03-24-2016 03:29 PM     Click Here to See the Profile for indy91     Edit/Delete Message   Reply w/Quote
Saving fuel is probably the main reason. From the Apollo 11 Technical Debriefing, section 5.17. Transposition:
Collins: This is something that I'm sure Apollo 12 and other flights will want to massage. I'm firmly convinced that the way to save gas on that maneuver is to let the DAP do it. Make it a totally automatic DAP maneuver. The price you pay for that is that you never know whether it's going to pitch up or down. This is not important. In an effort to save gas and to assure that we always pitched up, I ended up wasting some gas.

David C
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Posts: 433
From: Pasadena
Registered: Apr 2012

posted 03-25-2016 09:30 AM     Click Here to See the Profile for David C     Edit/Delete Message   Reply w/Quote
Thanks Indy, I've read all the technical debriefings and they raise more questions than they answer. When trying to clarify the maneuver I've reviewed DAC footage, flight plans, voice transcripts and technichal debriefings to try and get a complete picture. It seems that all the X translations were manually flown, and all the rolls were flown by the DAP, but executed at different stages. Some pitch backs were manually initiated, others are not completely clear, but they were probably manual. Certainly, Collins recommendation was not universally followed by subsequent missions - if at all.

As an aside, Collins only refers directly to pitch, both in the debriefing and in his book. However DAC footage makes it clear that the Verb 49 was performed during the pitch back.

So I suppose that to clarify my question - what was the reason for the differences in when the Verb 49 was performed? Was it just CMP personal choice?

indy91
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From: Germany
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posted 03-25-2016 05:21 PM     Click Here to See the Profile for indy91     Edit/Delete Message   Reply w/Quote
I can't really provide a definitive answer about the difference in techniques either.

About the Verb 49 specifically, the computer always chooses the shortest way to the desired attitude. And when you initiate the +/- 180° pitch maneuver, the DAP could either choose pitch up or pitch down, whatever is slightly shorter. Collins doesn't seem to mind either way, the DAP knows it's way. To ensure the direction of the pitch maneuver, the Apollo 15 Launch Checklist suggests starting the pitch up manually and let the computer do it as soon as it is clear that it will continue to pitch up. If you are with me on the checklist for CSM Transposition, "MAN ATT (PITCH) - ACCEL CMD" actually inhibits automatic commands from the AGC to the RCS. So the pitching is initiated manually, while the switch settings are already done for automatic control, and then rotational control is given completely to the DAP all the way until docking. All the manual maneuvering around is done with translations.

I fail to see any clear evidence that a manual or automatic procedure is much superior. This Apollo 15 procedure already seems quite refined. It all comes down to saving time, saving propellant, situational awareness for the CMP, best practices gained from experience of past missions. And then whatever the guy who writes the checklists thinks.

David C
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Posts: 433
From: Pasadena
Registered: Apr 2012

posted 03-25-2016 06:09 PM     Click Here to See the Profile for David C     Edit/Delete Message   Reply w/Quote
It's a shame that the DAC footage is not more comprehensive so that a clear comparison between what was written and what was actually performed could be made for every mission. The flight plans and checklists seem to leave it up to the CMP as to when exactly to execute the Verb 49.

It's interesting that on 14 Roosa specifically mentions in the debriefing that he initiated the pitch and then Shepard proceeded with the Verb 49. I wonder if this task sharing was common?

indy91
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From: Germany
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posted 03-25-2016 06:34 PM     Click Here to See the Profile for indy91     Edit/Delete Message   Reply w/Quote
Here a snippet I found on the Apollo 15 Flight Journal:
Scott, from 1998 correspondence - "The TD&E is pretty much a manual out-the-window operation, and the actual thrusting time, maneuvers, etc. are left to the pilot [Worden]. The checklist here is more of a guideline rather than a rigid set of procedures as in other situations. Much like the lunar landing, the pilot in the left seat flies mostly visually using the outside scene for reference."
I'd say this shows the way they looked at the TD&E procedure. The task sharing in this case might have been the CMP looking out of the window almost all the time and using the THC to control the spacecraft, and letting the CDR handle the computer and maybe switches. This stuff certainly was crew preference. If you practice such a procedure as many times as they did, you will find some techniques that work best for your crew. Armstrong e.g. gave Collins the task to watch the clock during TLI and yell cutoff at him at the manual shutdown time (nominal burn time + 6 seconds). It would be rather challenging for Armstrong to monitor all the G&C instruments and also the clock for this backup procedure. It's just something you notice during training I guess.

David C
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Posts: 433
From: Pasadena
Registered: Apr 2012

posted 03-25-2016 06:50 PM     Click Here to See the Profile for David C     Edit/Delete Message   Reply w/Quote
Just like airplane flying, there's more than one way to skin a cat.

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