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On the Saturn V there was a line of several small black rectangles running vertically on the upper part of the S-II stage. What were they for? enter image description here

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here is a picture from the McMahan Photo Archive. I added it to verify these markings did exist on the actual SV-II . enter image description here

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    $\begingroup$ I think they're just fiducial marks, but can't prove it. $\endgroup$
    – Russell Borogove
    Commented Feb 16, 2020 at 0:46
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    $\begingroup$ I agree to fiducial marks. $\endgroup$
    – Uwe
    Commented Feb 16, 2020 at 10:55
  • $\begingroup$ I checked my pictures of the JSC Saturn V but I didn't have a good one of that area. I'm pretty sure they are just paint. I might be able to stop by tomorrow and confirm. $\endgroup$
    – Organic Marble
    Commented Feb 16, 2020 at 18:29
  • $\begingroup$ They are a different black than the nearby writing, and the top one doesn’t have a break for the change of surface there. Could these have been added at some point? $\endgroup$
    – Bob Jacobsen
    Commented Feb 16, 2020 at 22:46
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    $\begingroup$ @BobJacobsen the image in the question is not sourced. It appears to be computer imagery to me, the top black mark does not conform to the surface. $\endgroup$
    – Organic Marble
    Commented Feb 17, 2020 at 5:27

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This document is for a Saturn 1B but I think it still applies. See below for the Saturn V info.

The vertical bar of black rectangles are called "vertical motion targets".

enter image description here

Source APOLLO/SATURN CONSOLIDATED INSTRUMENTATION PLAN FOR AS-204/LM-1 pp 3-5 & 3-6

Special cameras were pointed at the targets.

4.4.2 FIRST MOTION AND VERTICAL MOTION OPTICAL COVERAGE. This information is provided by 16mm Milliken and 35mm Mitchell cameras. The two Milliken (DBM-4) cameras are located on holddown arms, 180 degrees apart, looking at first-motion targets near the base of the launch vehicle. They will operate at a frame rate of 400 frames per second (fps). A 35mm Mitchell camera will be sited at the 110-foot level of the umbilical tower, centered on the motion targets (Figure 3-3) located on the Position II (Fin 3) centerline of the S-IB/S-IVB interstage. The cameras will provide pictures showing first motion and the first 3 to 5 meters of the vertical movement of the space vehicle.

(emphasis mine)

Once you know what the document is called...here is the same info for a Saturn V (Apollo 15).

enter image description here

One camera at the 160-foot level of the umbilical tower, positioned to include a fixed reference target in the field of view will provide coverage during the first 5 to 7 meters of vehicle ascent. The frame rate is 96 frames per second.

Page 4-4 of APOLLO/SATURN V CONSOLIDATED INSTRUMENTATION PLAN FOR AS-510 (APOLLO 15)

I managed a quick visit to the JSC Saturn V. Here is a personal photo of the targets on its 2nd stage. Obviously it's been repainted over the years but I think they tried to keep it historically accurate.

enter image description here

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    $\begingroup$ 110 feet is 1320 inches. Station 1206 is 1206 + 116 = 1322 inches above the bottom of the engine nozzle end (see heroicrelics.org/info/saturn-v/saturn-v-general.html). I don't know for sure where the umbilical tower base is with respect to the station number references, but it seems to add up with the camera exactly at the top of the topmost marker. $\endgroup$
    – Ludo
    Commented Feb 17, 2020 at 16:28
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    $\begingroup$ I stopped by JSC but the guard told me you now must have a badge to get into the Saturn V building! I blame the pernicious influence of Space Center Houston. $\endgroup$
    – Organic Marble
    Commented Feb 17, 2020 at 18:24
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    $\begingroup$ @uhoh be careful -- the British Imperial pint is different from the US pint :-) . Luckily (or not), the inch is the same. $\endgroup$
    – Carl Witthoft
    Commented Feb 18, 2020 at 13:11
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    $\begingroup$ "English units" is correct: en.wikipedia.org/wiki/English_Engineering_units But then at the end they throw in "meters of vehicle ascent".... $\endgroup$
    – Organic Marble
    Commented Feb 18, 2020 at 13:33
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    $\begingroup$ @OrganicMarble and yet 1 & 2 $\endgroup$
    – uhoh
    Commented Feb 18, 2020 at 14:05

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