Why did the Vertical/Vehicle Assembly Building have four doors?

Question

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:

(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?

(source: ArsTechica/NASA)

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

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. — Russell Borogove, Aug 02 '19 at 20:57
@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. — Ludo, Aug 02 '19 at 21:05
I don't know much about construction, but I'd think the crawlerway had to be pretty cheap by comparison to everything else. — Russell Borogove, Aug 02 '19 at 21:07
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: https://assets.atlasobscura.com/media/W1siZiIsInVwbG9hZHMvcGxhY2VfaW1hZ2VzL3Zhc3NlbWJ1bGQuanBnIl0sWyJwIiwidGh1bWIiLCIxMjAweD4iXSxbInAiLCJjb252ZXJ0IiwiLXF1YWxpdHkgODEgLWF1dG8tb3JpZW50Il1d/vassembuld.jpg; Web page: https://www.atlasobscura.com/places/vehicle-assembly-building. I'm still looking for an image that shows the west road completed. — prl, Aug 04 '19 at 02:31
@prl: it may never have been completed. On the photo it seems that the crawler roads were being completed up to where the 3rd MLP is being built, after that it looks like regular gravel, not the light-coloured gravel of the crawler road. — Ludo, Aug 04 '19 at 10:01
@RussellBorogove given the crawlerway had to be solid and stable, despite being build straight through a swamp, I suspect it was significantly more expensive than the same surface area of conventional roadway. Could be a good stand-alone question right there. — Criggie, May 04 '21 at 01:05

score 12 · Accepted Answer · edited Aug 04 '19 at 13:36

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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.

(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; Mike Jetzer/heroicrelics.org)

edited Aug 04 '19 at 13:36

Ludo

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answered Aug 02 '19 at 20:56

Organic Marble

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I'm just getting into this historic spaceflight research and have much to learn/find/be distracted by. Bookmarked for future reading! – Ludo Aug 02 '19 at 20:58
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The reason the road circles so far north of the building is to allow space for more bays to be added. – prl Aug 04 '19 at 02:34
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There was provision for a third pad, too. The crawlerway turn-off for it still exists. (See the satellite view here: https://www.google.com/maps/place/28°36'22.1"N+80°37'15.0"W) You can see plans and other photos by searching for lc-39c. – prl Aug 04 '19 at 02:47
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@prl I've seen photos of traffic signs from the 60s referring to pad C. – Organic Marble Aug 04 '19 at 04:47
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@prl: Makes you wonder what they did to the ground that this exit is still visible after 50 years. – Ludo Aug 04 '19 at 10:02
“The best built road in the world,” I heard it called, “even if it is gravel.” – prl Aug 04 '19 at 10:05
Wikipedia even mentions 5 pads were envisioned: https://en.wikipedia.org/wiki/Kennedy_Space_Center_Launch_Complex_39#Initial_design , although without reference. The cited source only mentions four: https://history.nasa.gov/SP-350/ch-6-1.html – Ludo Aug 04 '19 at 10:11
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@OrganicMarble: I stumbled upon a lot of info with respect to the number of pads etc. on this page: http://heroicrelics.org/info/lc-39/lc-39-abcd.html It also has a nice image of the VAB with four bays being linked to three pads: http://heroicrelics.org/info/lc-39/lc-39-abcd/lc-39-a-b-c-d-dotted-line.jpg Do you mind if I add some to your answer? – Ludo Aug 04 '19 at 10:52
Please, go ahead! – Organic Marble Aug 04 '19 at 11:44

Why did the Vertical/Vehicle Assembly Building have four doors?

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