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Were the Saturn V(s) assembled in the Vehicle Assembly Building (VAB), on the Crawler transporter or were they assembled on the ground and then kept over the transporter to be taken to the launch pad? If they was assembled on the transporter, was the transporter in a "trench" - meaning the surface on which Saturn V would sit, was at ground level?

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    $\begingroup$ I'm going to answer as a comment because the information derives from growing up near the Cape and visiting the VAB and being on site for a launch. The stack was assembled on the launch platform, which was sitting on pylons inside the VAB. The transporter pulled under the platform and the platform was lowered onto the transporter. The stack, platform, and transporter proceeded to the launch pad where the assembly had to go up a ramp to the pad, while keeping the stack and platform level. Once on the pad, hydraulics lifted the platform, allowing the crawler to move to a safe distance. $\endgroup$
    – BobT
    Mar 31 at 3:45
  • $\begingroup$ @BobT: Thanks. Seeking some additional info. You said that the assembly had to go up a ramp, to the pad. I was wondering about how was the assembly managed to be held vertical, so as to ensure that the CG of the assembly will pass vertically through the base area. It would have had some levelling arrangement - whether manual or automated in closed loop. Can you provide any details about the same? $\endgroup$
    – Niranjan
    Apr 1 at 0:17
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    $\begingroup$ No idea about the leveling control. Probably going pretty slow, so manual control would be simpler and more fool proof. Here's a pic of the assembly going up the ramp. You can see that the upper deck of the crawler is higher at the downhill end, keeping the platform and stack vertical $\endgroup$
    – BobT
    Apr 1 at 1:51
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    $\begingroup$ The Jacking/Elevation/Leveling (JEL) system on the crawler did what the name says - lifted the mobile launcher off its supports and set it down on them; leveled the pad on the ramp, etc. Read about it here ntrs.nasa.gov/api/citations/19760012108/downloads/… $\endgroup$ Apr 1 at 3:21
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    $\begingroup$ The pad surface is 12 meters above ground level on top of an artificial hill. I don't have a reference for this but I always assumed this was done to allow for the construction of the flame trench below the pad (in coastal Florida, if you just dig a hole, it quickly fills with water). $\endgroup$ Apr 2 at 13:26

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This is covered well in the Saturn V News Reference Vehicle Assembly and Launch.

Saturn V stages go into the Vehicle Assembly Building (VAB)...

The (first) stage is positioned above the launch platform of the Mobile Launcher and lowered into place.

The upper stages are prepared for mating...The mating operation consists of stacking the stages.

When the launch vehicle is ready, the Apollo spacecraft is brought to the VAB and mated.

At this point the Mobile Launcher is not yet mounted on the crawler. After the successful completion of testing on the stacked vehicle,

...the transporter is moved into position below the mobile launcher. Hydraulic jacks engage the fittings on the mobile launcher and raise it approximately 3 feet so that it clears its mount mechanisms. Then the transporter moves out of the VAB...

For details, see the link.

This picture shows a first stage being lifted up by the cranes.

enter image description here

(NASA photo via popsci.com)

No trench, the crawler / mobile launcher drove into the VAB on its floor level and the stages were lowered from above with cranes. This is a shuttle picture but you can see the crawler leaving the VAB at floor level.

enter image description here

(NASA photo via wikipedia)

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    $\begingroup$ Great explanation. Thanks also for providing links. $\endgroup$
    – Niranjan
    Apr 1 at 0:22
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    $\begingroup$ Should point out also - you couldn't really dig a trench deep enough for that in coastal Florida - it'd be filled with water. $\endgroup$ Apr 1 at 13:44
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The Saturn V is stacked on the "Mobile Launcher", but it is important to realise that the "Mobile Launcher" is, by itself, not mobile. As you can see in this contemporary photo from when the Saturn mobile launchers were built, the mobile launcher is only a platform with the launch tower:

mobile launchers under construction

(NASA via here)

It is moved by the crawler, which moves under it (note that the platform has "legs" raising it from the ground). The crawler itself has a flat top surface and just slides under it:

crawler

(Apollo Expeditions to the Moon chapter 6)

The stacking process is outlined in this flow chart:

flow chart

(Moonport chapter 19)

The relevant items are:

  1. L/UT [Launch/Umbilical Tower] is moved into high bay

and

  1. Booster [S-IC] transferred to erection bay & erected on L/UT

Steps 18, 19 and 20 stack the S-II and S-IVB stages and the instrument unit.

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    $\begingroup$ Awesome picture of the 3 mobile launchers. $\endgroup$ Mar 31 at 12:20

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