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Listening to the mission control audio loops on Apollo In Real Time in the immediate aftermath of the Apollo 13 explosion, I hear flight director Kranz asking about "delogs":

056:26:01   FLIGHT  EECOM, from FLIGHT.
056:26:02   EECOM   FLIGHT, EECOM.
056:26:03   FLIGHT  Have you got anybody getting a delog on this thing downstairs?
056:26:07   FLIGHT  NETWORK, from FLIGHT.
056:26:09   NETWORK FLIGHT, NETWORK.
056:26:10   FLIGHT  Bring me up another computer in the RTCC, will you?
056:26:14   NETWORK We got one machine on the RTCC and we got dual CPs downstairs.
056:26:18   FLIGHT  Okay, I want another machine up in the RTCC and I want a bunch of guys capable of running delogs down there.
056:26:23   NETWORK Roger that.

From the context I assume it might be something to do with reviewing telemetry data, but I have never come across the term "delog" in my Apollo studies before.

Kranz keeps returning to the topic:

056:40:59   FLIGHT Okay, now has anybody started the delog of the initial problem? You've got a delog going? Have you got people that are going to be in a position to evaluate it?

...and...

056:44:29   FLIGHT  EECOM, I don't think we're going to come to any solution here until we get back to the initial set of conditions, so I hope you got a set of guys looking at the delog pretty soon.

At the shift change, with Kranz's team handing off to Glynn Lunney's, he mentions the delogs again:

057:06:12   FLIGHT  Okay. All flight controllers, I'm handing over to Glynn. I assume the majority of all the team guys are pretty much briefed and up to speed as best we can. Now what I'd suggest is the white team do two things: they go over the delogs - okay - let me go back over this again. We're handing over to Glynn. I'd suggest the white team goes back and starts going through the delog of the data. In other words, let's see if we can go back to the initial conditions and work on that problem to see if we can find out what happened and we may find some better clues as to what to do and let the fresh guys come on and try to figure out where do we go from here.
057:06:51   FLIGHT  And the delog is in way now. Roger.

What's a delog, and why is Gene Kranz concerned with it half an hour after the explosion?

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    $\begingroup$ I haven't found a definition, but it might help to know that in other sources the same thing is also spelled "DLOG" or "D-log". See for example the handover at 057:06 in the Flight Journal. $\endgroup$ – TooTea Apr 14 at 11:45
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While the Mission Control Center's (MCC) Real Time Computer Complex (RTCC) was designed, as the name says, to receive and process data to the consoles in real time, it also recorded the voice and telemetry data ("logging").

"Delogging" was the the process of going back into the recorded data and extracting desired parameters for a specified time period.

I couldn't find a reference for the Apollo MCC, but here is one describing the logging/delogging functionality for the early Shuttle MCC.

5.3.2.2.10 Logging. The checkout software logging function shall provide the capability for recording data on magnetic tape for historical and/or analytical purposes. This function shall pro­vide a means for selectively logging data at each of the data mon­itoring points within the checkout system. Logging shall be se­lective in nature; such that hardware interface, data flow (input and/or output), and data type parameters may be specified. Appli­cation tasks shall initiate data logging requests to the logging task to perform the required logging functions. All-tape control and data blocking functions shall be performed by the logging task, while all logging control and data building functions shall be performed by the applications tasks. The checkout system shall use the ALT NIP TPC logging software.

....

5.3.2.2.15 Delogging. The checkout software delogging function shall provide the capability to selectively delog the contents of checkout system log tapes that have been created as described in paragraph 5.3.2.2.10. This function shall be performed as a back­ground delogging task and requires no application software inter­action. Delogging shall be selective in nature, such that data type, data format, and start/end time parameters may be specified to control the format and content of the delog line printer out­put. The TCOS shall use the NIP operational delog software.

MCC System Specification for the Shuttle OFT Timeframe

Kranz is enquiring about going back into the recorded data from the time of the explosion and pulling data from it. From this Apollo Experience Report, it sounds like back then the delog consisted of printed replicas of the controller's screen formats at 1 Hz intervals.

For special events and particular problem times, printouts (DELOG) can be made of the display television formats by personnel in building 30.These will cover the complete format page once each second.

(Building 30 is the MCC).

The Apollo 13 Final Report Panel 3 Flight Operations and Network Addendum 1 mentions such a delog (this is likely the exact one Kranz was talking about, since it appears to be the time of the explosion):

A study of a RTCC DELOG during this time frame revealed the following:

  1. Battery 2 current was off-scale high for two seconds.

  2. Battery 1, 3, and 4 currents reached a maximum of 'between 0 and 37 amperes during the same period.

  3. CDR bus volts dropped to a minimum of 28.9 volts during one sample.

  4. LMP bus volts dropped to a minimum of 27.7 volts during one sample.

  5. There were no Battery MAL or MASTER ALARM indications.

enter image description here

This MCC Progress Report from 1969 mentions that the function has been added, but gives no details.

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    $\begingroup$ Disk storage was small and expensive in the Apollo era. A state of the art hard drive back then cost over a quarter of a million dollars (1969 dollars) for less than a quarter of a million bytes. Compare that to now, when I can go to Amazon and buy a four TB drive for less than a hundred dollars (2020 dollars). That's over a million dollars per gigabyte (1969 dollars) versus less than 2.5 cents per gigabyte (2020 cents). The data archive in question was almost certainly boatloads of 9 track tapes. Data retrieval would have been a complex and certainly not instantaneous operation. $\endgroup$ – David Hammen Apr 14 at 12:57
  • $\begingroup$ @DavidHammen agreed, even the early Shuttle MCC document refers to using tapes. $\endgroup$ – Organic Marble Apr 14 at 13:01
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    $\begingroup$ Awesome answer! $\endgroup$ – Russell Borogove Apr 14 at 14:09
  • $\begingroup$ BTW, inflation 1969–2020 is roughly 7.2, so that would be over 7 million dollars (2020) per GiByte in 1969. $\endgroup$ – Jörg W Mittag Apr 14 at 16:00
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    $\begingroup$ Tape concepts even "persisted" up into the mid 80s and beyond. We used a CDC Cyber-750 and 760 mainframe computer with a "tape operating system", even though it actually used disk drives. You had to "rewind" your executable and input files after a run, so that the file pointer would be back at the front of the file before running it again. I can still type "rewind,*" really really fast! :) $\endgroup$ – Matt Jessick Apr 15 at 18:27

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