# Was the lunar module computer in the ascent stage or the descent stage?

Two Apollo Guidance Computers accompanied each lunar mission. One CPU was housed in the lower equipment bay of the command module, with one DSKY display-keyboard unit adjacent to the CPU, and a second DSKY at the navigator's station of the main display console.

The other AGC was in the lunar module. Obviously its DSKY was in the ascent stage with the astronauts. But where was its CPU: in the ascent stage, or in the descent stage?

• One CPU was in the Command Module, not in the SM. If the other CPU would have been in the descent stage, it could not be used for ascent and rendezvous. – Uwe Sep 11 '18 at 10:41
• @Uwe: Thanks, I've corrected that. The "computer" in the SM is the RCS computer, which is not an AGC. However, it doesn't change the question. – DrSheldon Sep 11 '18 at 10:49
• Considering that the guidance computer was needed for ascent and rendezvous, it could not be left behind on the lunar surface. – Russell Borogove Sep 11 '18 at 14:52
• @RussellBorogove: Logic would suggest that. However, get a good laugh from en.wikipedia.org/wiki/Lunar_Escape_Systems – DrSheldon Sep 11 '18 at 15:00
• Not a guidance system you'd want to rely on as a first choice! – Russell Borogove Sep 11 '18 at 15:17

There is a figure showing the location of the Lunar Guidance Computer LGC within the LM. See page GN-11 in the PDF. Similar figures here (1) (figure 3.2-3 on page 40 or 3-7), (2) (figure 2.1-1 on first page 2.1-1).

The LGC needs connections to the DSKY, the Coupling Data Unit, the Navigation Base and many more other units. Therefore its best place is within the pressurized cabin of the LM in the ascent stage. It is used for ascent and descent, mounting it to the descent stage is not possible.

Placing the LGC outside the pressurized cabin would require several additional gas tight connectors mounted to the cabin walls. To keep it as simple and reliable as possible, these additional connectors should be avoided.

The Landing Radar Antenna (lower right of image) is used for descent only, its best place is at the descent stage near a landing leg.

From this page: "The LGC (with related equipment) was mounted behind the astronauts at the back of the LM cabin. In front of the astronauts was a rigid structure called the "Nav Base" that held an alignment telescope and the Inertial Measurement Unit (IMU) in a fixed geometrical relationship. The computer's Display and Keyboard Unit (DSKY) was mounted like a desk between the two astronauts."

There is indirect evidence that it is in the ascent stage.

According to https://history.nasa.gov/afj/compessay.html, there are several lunar guidance computer programs related to ascent:

P12  Powered Ascent Guidance
P42  APS Thrusting
P71  APS Abort


These would only make sense if the computer were in the ascent stage, as the descent stage is left behind.

Nonetheless, I find it odd that NASA gives very specific documentation about the location of the AGC in the CM, but does not for the LM.

The CM housed the computer in a lower equipment bay, near the navigator's station. Block II measured 24 by 12.5 by 6 inches, weighed 70.1 pounds, and required 70 watts at 28 volts DC. The machine in the lunar module was identical.

• See this PDF Figure 3-2.3. Guidance, Navigation, and Control Subsystem-Major Equipment Location on page 40 of the PDF, 3-7 of the document "Lunar Module, LM 10 Through LM 14, Vehicle Familiarization Manual (31 Mb). Grumman Document LMA790-2. 1 November 1969." The LGC is shown in the LM cabin, the ascent stage. More documents to be found here – Uwe Sep 11 '18 at 12:39
• @Uwe that sounds like an answer! – Organic Marble Sep 11 '18 at 12:50
• well, crop, then save. In the Answer box on this site, use the Image button, and 'drag and drop or click here', follow the instructions from there. – Hobbes Sep 11 '18 at 14:29
• Well, NASA did give information about the location of the LGC in the LM. You only were not able to find it. Nothing to find it odd. – Uwe Sep 12 '18 at 20:56