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On this satellite image (credit: Google Maps) it is clear that only 3 out of four bays were ever used, since the 4th is not even connected to the crawlerway: VAB on Google Maps

(Yes, I made the screenshot deliberately a bit too wide to have the crawler transporter in the image.)

Only 3 mobile launch platforms were made, and with only two launch pads, more than 3 don't seem necessary. So why the effort of building 4 bays - maybe next to each other to avoid the loop-around? Was the program budget already cut after the commissioning of the VAB? Was there an optimistic future use envisioned where they'd be launching Saturn-V rockets every other week?

One VAB and three MLPs

(source: ArsTechica/NASA)

Question: what was the design decision driver to make the VAB with 4 bays?

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    $\begingroup$ The bottom photograph is awesome. $\endgroup$
    – Organic Marble
    Commented Aug 2, 2019 at 20:37
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    $\begingroup$ The fourth bay can be built at a slight discount. Assume that the bay floorplans are basically square, one unit on a side. If you arrange three bays in a line, you're going to be building an 8 unit perimeter's worth of exterior walls. If you arrange three bays in an L-shape, it's also 8 units of perimeter. If you arrange four bays in a square, it's still 8 units! (Four in a line would be 10 units.) You have to build more floor and ceiling and interior stuff, of course, all roughly proportional to the number of bays, but you get the fourth bay for approximately zero extra wall construction. $\endgroup$
    – Russell Borogove
    Commented Aug 2, 2019 at 20:57
  • $\begingroup$ @RussellBorogove Good observation. I wonder if that holds up to the extra costs of the crawler way that has to loop around the building though - that was not a simple pavement. $\endgroup$
    – Ludo
    Commented Aug 2, 2019 at 21:05
  • $\begingroup$ I don't know much about construction, but I'd think the crawlerway had to be pretty cheap by comparison to everything else. $\endgroup$
    – Russell Borogove
    Commented Aug 2, 2019 at 21:07
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    $\begingroup$ In this image, it looks like the road was built (or being built) to both west doors. It is being used as a parking lot in the image. Image: assets.atlasobscura.com/media/…; Web page: atlasobscura.com/places/vehicle-assembly-building. I'm still looking for an image that shows the west road completed. $\endgroup$
    – prl
    Commented Aug 4, 2019 at 2:31

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There's a lot of background on VAB design in Moonport.

Your "optimistic future" is pretty much spot on - the original projection was for 36 flights a year! An early VAB design to support this had six high bays.

enter image description here

(six-bay design with barge instead of crawler)

After the annual flight rate prediction was reduced to 24, the design was changed to only have four high bays - but to not preclude the future expansion to six.

See the chapters "LC-39 Plans Take Shape", especially the subchapter Plans for a VAB, and "Funding the Project", subchapter Updating LC-39 Requirements.

Likewise, the number of planned launch pads went from three to four to even five, but in the end only two were built. Here is a picture with four bays in the VAB and three launch pads, with a fourth one tentatively drawn in with dotted lines:

Drawing of VAB with three launch pads and fourth planned

(Drawing of VAB with three launch pads and fourth planned; Mike Jetzer/heroicrelics.org)

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    $\begingroup$ I'm just getting into this historic spaceflight research and have much to learn/find/be distracted by. Bookmarked for future reading! $\endgroup$
    – Ludo
    Commented Aug 2, 2019 at 20:58
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    $\begingroup$ The reason the road circles so far north of the building is to allow space for more bays to be added. $\endgroup$
    – prl
    Commented Aug 4, 2019 at 2:34
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    $\begingroup$ There was provision for a third pad, too. The crawlerway turn-off for it still exists. (See the satellite view here: google.com/maps/place/…) You can see plans and other photos by searching for lc-39c. $\endgroup$
    – prl
    Commented Aug 4, 2019 at 2:47
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    $\begingroup$ @prl I've seen photos of traffic signs from the 60s referring to pad C. $\endgroup$
    – Organic Marble
    Commented Aug 4, 2019 at 4:47
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    $\begingroup$ @prl: Makes you wonder what they did to the ground that this exit is still visible after 50 years. $\endgroup$
    – Ludo
    Commented Aug 4, 2019 at 10:02

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