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Inspired by answers to Could the lunar module have moved to a different spot? question but not talking about that specific relaunch scenario:

Could the LEM stack (descent configuration) be flown without any guidance program running in the LEM Guidance Computer (LGC)? (I think the actual response to a total LGC failure would have been to abort. I could be wrong about that, but that is not the question).

Definition of "could be flown" is:

  • An attitude / attitude rate reference is available to the crew (other than out-the-window)
  • A three-axis velocity reference is available to the crew (h/t to Uwe)
  • An altitude readout is available to the crew (other than out-the-window) (I think this is a yes)

  • The crew can manually control the attitude of the vehicle

  • The crew can throttle the descent engine
  • The crew can shut off the descent engine (I think this is a yes)

All of the above must be accomplished without using any LGC functions.

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  • $\begingroup$ What about horizontal speed, it should be small at touchdown? The landing gear was specifief for less than 4 foot per second. $\endgroup$ – Uwe Mar 17 at 12:53
  • $\begingroup$ @Uwe great comment, thanks, will add. $\endgroup$ – Organic Marble Mar 17 at 13:01
  • $\begingroup$ To be clear, do they have the AGS but not the primary, or are both computers down? $\endgroup$ – Russell Borogove Mar 17 at 17:01
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    $\begingroup$ @RussellBorogove Since I didn't specify any failure but the LGC and the answer already posted went with that, I'll stick with only an LGC failure. I did read the flight rules you supplied in an earlier answer, and it looks like below a certain point, they could be go for landing with neither computer. But not go for staying. $\endgroup$ – Organic Marble Mar 17 at 17:02
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    $\begingroup$ @RussellBorogove I wrote that sloppily. Looks like below the "low gate" go for landing with a dual computer failure. But liftoff at the next opportunity if the PGNS is failed. Pdf page 260 in this document you found: hq.nasa.gov/alsj/a11/A11MissRules.pdf $\endgroup$ – Organic Marble Mar 17 at 17:08
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"Could be flown" is a clear yes, "could be flown to a landing" is another story. Most of the points you are listing can be accomplished using the backup system in the LM, the Abort Guidance System (AGS). I'll be using the LM Apollo Operations Handbook Volume I as the main source for all your specific points.

An attitude / attitude rate reference is available to the crew (other than out-the-window)

The AGS has a subsystem called the Abort Sensor Assembly (ASA). As the AOH says, "the ASA performs the same function as the IMU" (Page 2.1-8). The ATTITUDE MON switch selects PGNS or AGS as source of vehicle total attitude and attitude errors displayed on the FDAI. The displayed attitude rates displayed are always obtained from the rate gyro assembly, which is part of the AGS. So the LGC is not the source for the rates on the FDAI (Page 3-20).

An altitude readout is available to the crew (other than out-the-window)

The MODE SEL switch switches between three source for the altitude/range indicators (tapemeters). LDG RADAR, PGNS and AGS. In the LDG RADAR position the displays show the raw LR data (slant range and rate), in PGNS and AGS mode the data is coming from the computers. (Page 3-22).

A three-axis velocity reference is available to the crew

In AGS mode only two axis are available (altitude and lateral), but the raw data from the LR is showing altitude and altitude rate on the tapemeters and forward and lateral velocity on the X-Pointers. (Page 3-16, X-Pointer indicator, Remarks).

The crew can manually control the attitude of the vehicle

The PGNS and AGS have automatic attitude control modes, but there always is a capability to control the attitude manually using the Attitude Control Assembly (ACA) in different modes, from computer aided to full manual hardover (Page 2.1-30, all modes except Automatic).

The crew can throttle the descent engine

Manual throttle commands can be generated using the Thrust/Translation Controller Assembly (TTCA) which always are added to the auto throttle commands from the LGC (Page 2.1-20, first paragraph).

The crew can shut off the descent engine

This can be accomplished using the engine stop buttons (Page 3-81, Engine stop pushbutton and Page 2.1-140, figure 2.1-51).

What the AGS does not have is the descent guidance programs of the LGC. So landing without the automatic guidance is probably possible by following a nominal altitude and altitude rate chart or so, but landing with any accuracy at the desired landing site without the LGC would be rather difficult.

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    $\begingroup$ Really excellent, well sourced answer. $\endgroup$ – Organic Marble Mar 17 at 14:07
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    $\begingroup$ "Crew members have mentioned that it is a formidable task to land with manual throttle and manual attitude control [i.e. in program P67]. The throttle in particular appears to be hard to handle manually." ibiblio.org/apollo/Documents/… $\endgroup$ – Russell Borogove Mar 17 at 17:04
  • $\begingroup$ I remember from the Sunburst and Luminary book that the automatic throttle-down was a key event telling them guidance was working right. $\endgroup$ – Organic Marble Mar 17 at 17:12
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    $\begingroup$ @OrganicMarble, that's because the automatic throttle-down was a fairly dramatic event. For complicated reasons, the descent engine could only run at either 100% throttle or less than 60% throttle, so there was a significant drop in acceleration when it shifted from full-throttle mode to variable-throttle mode. $\endgroup$ – Mark Mar 17 at 23:37

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