The Real Time Computer Complex (RTCC) was the central computing facility for the Gemini and Apollo programs, and contained, during the Apollo program, several IBM System 360 mainframes. These systems were used for simulations, offline data processing, and mission support.

IBM System/360 75 in RTCC Source: Mission Control Center Houston Familiarization Manual

For the mission support role, two identical computers were running simultaneously and fed the same data, yet only one at a time was “primary” and used to feed data displays. The second computer ran as a Dynamic Standby system and could take over the primary role at any point through operator intervention.

Was this “failover” capability ever used during the course of a mission, or in some other time-critical moment?

The only reference I have found so far is near the end of this NASA publication which mentions potentially a failure during Apollo 10, but provides no detail. It also isn’t clear if this capability was used any other times.

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    $\begingroup$ That document states about the Apollo 10 incident "...by 13:01, the standby had been brought up..." however, it apparently was not an 'hot' backup. $\endgroup$ Oct 8, 2020 at 3:59
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    $\begingroup$ Apparently that incident during Apollo 10 was so minimal that it doesn't even merit a mention in the mission report... $\endgroup$
    – kgutwin
    Oct 8, 2020 at 13:27
  • $\begingroup$ I looked through what little I have on the Apollo MCC and didn't find anything detailed enough to help. With luck someone will come along and provide resources. $\endgroup$ Oct 8, 2020 at 13:36

1 Answer 1


I have found two instances of a "selectover" from the Mission Operational Computer (MOC) to the Dynamic Standby Computer (DSC), one on Apollo 13 and one on Apollo 15.

Apollo 13:

The single episode of computer faults during this mission occurred during the period of two computer, i. e., MOC/DSC operation, and there was no interruption to mission support. Machine checks occurred in the MOC which was 75D. A selectover to 75A which was the DSC was performed immediately and was followed by a channel to channel high speed restart from 75A to 75F so that two computer support was restored. This incident occurred at a GMT of 06:37:00 on April 15. Operations on 75A as the MOC and 75F as the DSC were continued through the recovery of Apollo 13 flight crew, and without interruption.

Source: Apollo 13 Mission Operations Report

Apollo 15:

At 16:42:24 on July 30, 1971, the MOC 75D failed due to a machine check. At the time of this occurrence, 75A was the DSC so a selectover was performed and there was no interruption to mission support. Investigation revealed a bad logic card in main memory "A". The card was replaced, and the machine was made available for mission support.

Source: RTCC Coordinator's Report for the Apollo 15 mission

I don't have these kind of documents for every mission, so there might have been some more.

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    $\begingroup$ This is great! Is HOC in the first incident a typo for MOC? $\endgroup$ Oct 9, 2020 at 11:24
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    $\begingroup$ @OrganicMarble Yes, I copy and pasted that from the document and didn't see that it had detected it as an H and not an M. Thanks! $\endgroup$
    – indy91
    Oct 9, 2020 at 13:38

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