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The interesting question What are the most difficult challenges SpaceX will face getting astronauts to Mars by about 2025? links to Elon Musk's position paper Making Humans a Multi-Planetary Species which can be found (in both HTML and PDF formats) at:

http://online.liebertpub.com/doi/full/10.1089/space.2017.29009.emu

Figure 9 gives you a more direct comparison. The thrust level is enormous. We are talking about a lift-off thrust of 13,000 tons, so it will be quite tectonic when it takes off. However, it does fit on Pad 39A, which NASA has been kind enough to allow us to use because they oversized the pad in doing Saturn V. As a result, we can use a much larger vehicle on that same launchpad. In the future, we expect to add additional launch locations, probably adding one on the south coast of Texas, but this gives you a sense of the relative capability. However, these vehicles have very different purposes. This is really intended to carry huge numbers of people, ultimately millions of tons of cargo to Mars. Therefore, you really need something quite large in order to do that. (emphasis added)

It's an interesting read, and a good way to start to understand the issues involved and how SpaceX amy approach them. However one item caught my eye. Stating that the proposed ITS rocket design (now downsized to the Mars vehicle version of the BFR (Big Falcon Rocket)) would fit on — and presumably be launch-able from — NASA's Launchpad 39A, it mentions in passing that NASA "oversized the pad in doing Saturn V", and that as a result SpaceX will be able to "use a much larger vehicle on that same launch pad."

Question: I am wondering if there is any information, or even educated speculation about the thinking at the time. Was 39A simply over-designed for Saturn V's weight and thrust out of an abundance of engineering caution, or was it in fact built to be able to accommodate a rocket substantially larger than Saturn V?

According to the linked article and Figure 9 (shown below), SpaceX believes, and presumably NASA agrees that 39A (of course with suitable modifications to some systems) should be able to handle a rocket with about 3.5 times the weight and 3.6 times the thrust of the Saturn V.

enter image description here

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    $\begingroup$ NASA did think about building larger rockets, so they may have been planning ahead: see Saturn C-8 and Nova. Also, variants of Saturn MLV with strap-on boosters could have had a larger diameter (diagram on this page is interesting, IMHO). $\endgroup$ – DarkDust Nov 9 '17 at 14:03
  • $\begingroup$ @DarkDust: the Saturn C-8 and the Nova would have been a little larger than a Saturn V but much smaller than the proposed Mars Vehicle when comparing gross mass. $\endgroup$ – Uwe Nov 9 '17 at 14:32
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    $\begingroup$ Just a note: I think the info above is for the ITS, which is no longer being developed. The ITS was 12M diameter. The BFR is a much smaller rocket, at 9M. $\endgroup$ – jgalak Nov 12 '17 at 2:40
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    $\begingroup$ @jgalak thank you for pointing that out! I saw "2017" and didn't realize how quickly the idea was "evolving". Do you think it reads accurately now? $\endgroup$ – uhoh Nov 12 '17 at 2:55
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    $\begingroup$ @uhoh - yes, that seems to be good. $\endgroup$ – jgalak Nov 12 '17 at 14:27
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Launch Pads 39A and 39B were initially built for either the Saturn C-8 or the Nova rockets, each of which has a lift off thrust of about 7000 tons, about twice that of the Saturn V. These were alternative designs to allow for a return directly from the Moon to Earth, but ultimately scrapped in favor of the Saturn V Lunar Rendezvous design.

Note also that the Space Shuttle actually has a higher thrust on lift off, namely about 6000 tons, then the Saturn V.

I suspect that when the pad was designed, it was designed to support 7000 tons of thrust at liftoff, with a fair bit of margin. I'm not sure that the design can fully support the BFR, but it should be close.

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    $\begingroup$ 7000 tons, surely? $\endgroup$ – Hobbes Jan 10 '18 at 13:37
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    $\begingroup$ Woops... Fixed! $\endgroup$ – PearsonArtPhoto Jan 10 '18 at 13:44
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    $\begingroup$ The space shuttle had a higher thrust on liftoff, unless they've pulled them out of their museums and started launching them again since I last checked. $\endgroup$ – Sean Jun 12 '18 at 1:50
  • $\begingroup$ The space shuttle actually didn’t produce more thrust than the Saturn V. When released, the Shuttle’s two SRBs generate around 2.8 million pounds thrust each – while the three SSMEs generate 393,000 pounds thrust each, for a total close to 6.8 million pounds total (3075 tonnes). Problem is, many sites now quote the Shuttle SRBs as producing 3.3 million pounds of sea level thrust, which they could not do. Even the resulting 7.8 million pound figure would still fall short of the later Saturn V flights – last of which generated 7.89mlbf at T+0.00. This is 35.1MN or 3,579 tonnes equivalent. $\endgroup$ – Alastair Haslam Dec 25 '18 at 10:40

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