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  Correcting for Apollo 17's launch delay

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Author Topic:   Correcting for Apollo 17's launch delay
FordPrefect
Member

Posts: 26
From: Karlsruhe, Germany
Registered: Apr 2005

posted 03-18-2008 07:13 AM     Click Here to See the Profile for FordPrefect     Edit/Delete Message   Reply w/Quote
While doing some research on the Apollo 17 mission, I have detected different versions of how they managed that Apollo 17 would arrive at the moon at the originally planned GET after the launch had been delayed by 2 hours and 40 minutes.

On the website "Apollo by the Numbers," it is stated that the Midcourse Correction burn performed at GET 035:29:59.91 which resulted in a velocity change of 10.5 ft/s was the maneuver required to have 17 arrive at the pre-planned time in lunar orbit.

In the Apollo 17 mission (PAO) transcript, the PAO states that (already) the TLI burn has been modified (prolonged) to compensate the 2 hours 40 minutes behind the flight plan.

The MCC at 35 hours GET increased the velocity in the posigrade direction and thus increasing the perilune altitude to 53 miles for LOI, and to my understanding, accelerating in the direction of travel (posigrade burn) should decrease the time for translunar coast. My question is have both the TLI and the MCC been altered to get back on the scheduled times, or only one of them, or has the MCC been required only due to the launch delay?

I am getting a bit confused trying to understand how they managed to catch up from the delayed launch.

ehartwell
New Member

Posts: 8
From: Waterloo, ON, Canada
Registered: Feb 2007

posted 03-18-2008 12:41 PM     Click Here to See the Profile for ehartwell     Edit/Delete Message   Reply w/Quote
The TLI burn was extended to make up the time. Only a single, tiny midcourse correction was required to trim the outbound trajectory. From PAO at 05:30:
...launch occurred 2 hours 40 minutes late at a ground elapsed time of 11, of a Central Standard Time rather, 11:33 PM. As a result of the late launch-time the translunar injection, that's up through translunar injection, also slipped 2 hours, 40 minutes. We would expect that the translunar injection which is targeted to make up the difference will get us back on the nominal flight plan time by the time the spacecraft arrives at the Moon. In other words, arrival time at the Moon would be at the same Central Standard Time as called for in the Flight Plan at about 1:49 PM Central Standard Time, December 10.

But, the Ground Elapsed Time would be about 2 hours, 40 minutes earlier than that provided for in the Flight Plan, the arrival being at about 86 hours, 14 minutes ground elapsed time. The 2 hour, 40 minute difference being accounted for in a speedier arrival time at the Moon, a translunar injection burn being targeted just slightly longer than would have been the case in a normal launch. The spacecraft getting to the Moon in a total elapsed time 2 hours 40 minutes less, in effect, making up for lost time from the late launch.

In order to get the flight plan back in agreement with the ground elapsed time the GET, or ground elapsed clock, will simply be moved ahead 2 hours, 40 minutes between now and the time spacecraft arrives at the Moon. So, that by the time Apollo 17 is inserted into Lunar Orbit, the GET will once again agree with the flight plan GET and of course, the Central Standard Time of arrival will be the same as was originally planned by virtue of a speedier trip time.

FordPrefect
Member

Posts: 26
From: Karlsruhe, Germany
Registered: Apr 2005

posted 03-29-2008 03:59 AM     Click Here to See the Profile for FordPrefect     Edit/Delete Message   Reply w/Quote
Hello Eric, well, if your assumption is correct, then the information given on "Apollo by the Numbers" is not accurate, as it states:
The 2-hour 40-minute launch delay caused ground controllers to modify Apollo 17's trajectory so that it would arrive at the Moon at the originally scheduled time. They shortened the translunar coast time by having the crew make a 1.73-second 10.5 ft/sec midcourse correction at 035:29:59.91.

TLIGuy
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Posts: 207
From: Virginia
Registered: Jul 2013

posted 12-14-2016 03:31 AM     Click Here to See the Profile for TLIGuy   Click Here to Email TLIGuy     Edit/Delete Message   Reply w/Quote
Reviving an old thread, I would like to know if anyone could offer some additional insight into the +2:40 mission time adjustment caused by the launch delay and how it is identified or noted in the ASJ or at Apollo17.org.

I have always been somewhat confused about the Apollo 17 time adjustment and I'm a bit more confused this morning after watching the Apollo17.org website as Cernan says his last words upon leaving the Moon.

Here is a screen shot (Apollo17.org) at the moment Cernan begins making his final speech before leaving the surface of the Moon. (Note this was taken at 06:34:17 AM here in Italy.)

The speech occurs 7 days 1 minute and 17 seconds after launch on Dec 7 at 12:33am EST making it 12:34:17am EST or 05:34:17 GMT Dec 14.

The MET time under it at the exact moment is 170:41:17. This time appears to reflect correction of the +2:40 and not the actual MET of 168:01:17.

I have the following questions:

  • Is there a notation in the Apollo 17 Lunar Surface Journal voice transcripts that note the MET times that correspond to the ASJ voice transcripts reflect the +2:40 time adjustment and do not reflect the actual elapsed MET time from launch?

  • Is the MET time above correct as adjusted to fit the original flight plan and not correct regarding the actual MET from launch?
Thanks for any additional insight that can be provided.

EDIT: I do see at the top of the transcript page "Corrected Transcript and Commentary." Is this referring to the time adjustment?

indy91
Member

Posts: 15
From: Germany
Registered: Feb 2016

posted 12-14-2016 05:57 AM     Click Here to See the Profile for indy91     Edit/Delete Message   Reply w/Quote
The clocks were updated with the +2:40h time adjustment before Apollo 17 reached the moon, at 65h MET. The TLI burn already sent the mission on a about 2:40h faster trajectory to the moon. Because of this they arrived at the moon at the same "actual" time as planned with a launch on time. The clock update then synchronized the mission time with the flight plan, so that it could be used in lunar orbit and later without having to change every planned time in it.

The voice transcript has a note at 65h MET that the MET times are not adjusted with the clock update.

The ALSJ uses the adjusted times, I think, because PDI happens almost exactly at the planned MET from the flight plan.

TLIGuy
Member

Posts: 207
From: Virginia
Registered: Jul 2013

posted 12-14-2016 06:33 AM     Click Here to See the Profile for TLIGuy   Click Here to Email TLIGuy     Edit/Delete Message   Reply w/Quote
Thank you. I understand your explanation. I guess one would only know that that the times are not adjusted only by reading the voice transcript at the 65h.

Here is see the event transcript noting the +2:40 at the 65h. Not being familiar with this I assumed the times from that point forward included the adjustment but they do not because the 168h at the bottom is in line with the 05:34 GMT ending of the 3rd EVA.

Here is the ALSJ transcript I used of the speech Cernan made at 170:41 which appears to be with the corrected mission plan time but not the actual elapsed MET. I did not see anything on this transcript noting that the MET was not the actual elapsed time but time adjusted to the mission plan.

In hindsight using the 170:41 time for my purposes was incorrect because I assumed it was the actual MET. I'm trying to understand why the other event transcripts have consistent times but the one I used (bottom) had the adjustment.

Why is this relevant today you ask? It's relevant because I now realize I provided Captain Cernan with the incorrect time he stepped off the moon when he signed the case back for my collection.

When I began prepping for the Cernan piece there was nothing I could find that stated the exact time he made his final speech on the moon. In order to arrive at the time the math was simple. Take the MET (170:41) and add it to the launch time. Thinking that the 170:41 was the actual MET I made the calculation. Launch time 05:33 GMT + 7 days 2h 41m = 08:14 GMT which is incorrect.

Backing out the +2:40 I get the correct 05:34 GMT time which matches the Apollo17.org time. If I had used the uncorrected time of 168h 1m would have arrived at the correct GMT time.

So in the end the piece is incorrect but the story is great and telling a great story is what my collection is meant to do.

indy91
Member

Posts: 15
From: Germany
Registered: Feb 2016

posted 12-14-2016 07:06 AM     Click Here to See the Profile for indy91     Edit/Delete Message   Reply w/Quote
The corrected MET is really only relevant for the crew and the flight controllers in Houston. So I can understand why the transcript has the actual elapsed time, not even using the hhh:mm:ss format. For a reader of the AFJ and ALSJ it would probably be useful to use the times that the astronauts and CAPCOM are talking about. But the AFJ, in contrast to the ALSJ, does not use the corrected times.

Apollo 16 also had a time correction, because it returned to Earth 24h earlier than previously planned. So on the way home they did a (not exactly) +24h clock update, again mostly for being able to use the flight plan. From that point on the AFJ often mentions both the actual elapsed time and the corrected time.

Blackarrow
Member

Posts: 3160
From: Belfast, United Kingdom
Registered: Feb 2002

posted 12-14-2016 06:11 PM     Click Here to See the Profile for Blackarrow     Edit/Delete Message   Reply w/Quote
This takes me back! The delayed launch of Apollo 17 had many repercussions, some less significant than others. Here is what I wrote in my diary for Thursday 7th December, 1972:
With my heart in my mouth I watched as the countdown reached the final minute. Then, with complete disbelief, I saw the count being held at T-30 seconds by an over-zealous computer. Still awake till 5.00am, then fell asleep but somehow (sixth sense?) woke up at the new T-2 minutes! The 5.30am launch was really spectacular when I saw a recording later...
I had to follow the launch on the radio as the BBC's live launch coverage had ended when it became clear the delay would be hours (at best) not minutes. The launch of Apollo 17 was therefore the only launch to the Moon I didn't see live on TV.

All times are CT (US)

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