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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, 2017 at 14:03
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    $\begingroup$ @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, 2017 at 14:32
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    $\begingroup$ If the Mars Vehicle would use engines of the same thrust as the F-1 of Saturn V, more than 18 are needed instead of 5. But only about 5 would fit in the diameter of the first stage. Is there information about the number of first stage engines of the Mars Vehicle? If Mars Vehicle should be ready at 2025, its first stage engines should be ready for the first tests soon. $\endgroup$
    – Uwe
    Nov 9, 2017 at 14:50
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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, 2017 at 2:40
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    $\begingroup$ @uhoh - yes, that seems to be good. $\endgroup$
    – jgalak
    Nov 12, 2017 at 14:27

2 Answers 2

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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, 2018 at 13:37
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    $\begingroup$ Woops... Fixed! $\endgroup$
    – PearsonArtPhoto
    Jan 10, 2018 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. $\endgroup$
    – Vikki
    Jun 12, 2018 at 1:50
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    $\begingroup$ 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. $\endgroup$ Dec 25, 2018 at 10:40
  • $\begingroup$ @AlastairHaslam: I was actually referring to the use of the present tense for a retired vehicle, but thanx for the info! :-) $\endgroup$
    – Vikki
    Jul 15, 2021 at 0:55
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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?

The former. NASA confirmed the choice for Lunar Orbital Rendevouz on 11 July 1962. With this announcement NASA also confirmed the C-1B (later Saturn-1B) for Earth-orbital tests and the C-5 (later Saturn V) as launch vehicle for the lunar missions. This removed Nova as option:

Nova, like Dyna-Soar, seemed to evaporate as other issues were settled that placed a premium on the development of its nearest competitor, the C-5. On 11 July 1962, NASA officially endorsed the C-1B as a two-stage Saturn for Earth-orbital tests of Apollo hardware. At the same time, NASA confirmed the choice of the LOR mode for the lunar mission, thereby focusing development on the C-5. Early in 1963, NASA Headquarters announced a new nomenclature for its large launch vehicles. The C-1 became Saturn I, C-1B became Saturn IB, and C-5 became Saturn V. Nova was not even mentioned.

(source: Stages to Saturn, chapter 3)

In fact, Nova was already cancelled effectively when Michoud was chosen as the assembly factory for the launch vehicle's lower stages:

So Michoud, with a mammoth building that contained 0.17 square kilometers under one roof as part of a 3.5-square-kilometer complex along the water's edge, was selected on 7 September 1961.* Designed as a shipyard, it had become a cargo aircraft factory in 1943 and a tank engine plant during the Korean conflict. Here the Chrysler Corporation and The Boeing Company would construct the first stages of the Saturn C-1 and, later, of the C-3, C-4, or C-5 (or whatever model was chosen).

With footnote:

*) Although the Saturn versus Nova debates continued, the selection of Michoud ended all chances of clustering eight F-1 engines in the first stage - unless the plant roof were raised. The fact that only four or five barrels could be put together did not worry Marshall, as this number would be more than enough to support assembly in earth orbit, that center's favored mode. Proponents of direct flight had essentially lost their vehicle; but they continued to argue for another year, anyway.

(source: Chariots for Apollo, chapter 2)

So, while Nova wasn't officially cancelled until 1964, LC-39 was specifically designed for the Saturn V, its dimensions in particular driving the requirements of the VAB. The VAB design contract was awarded in late August 1962, after the choice for LOR/Saturn V was made. The design of the pads themselves only came later, as shown by this contemporary milestone chart from December 1964:

LC 39 milestone chart

(source: NASA, from: Moonport, chapter 11)

The chart shows that pad requirements and design were not completed until late 1963. By this time, Nova was already effectively cancelled.

In the same chart, you can see that the mobile launcher design was finished finished just before the pad requirements were finalized and the crawler design wasn't ready until early 1964. Hence, it seems most plausible that they played it safe and over-engineered the pad to accommodate changes in the total weight of crawler + mobile launcher + Saturn/Apollo.

Many systems at the time were over-engineered, as computations were done by hand and computer simulations were not yet prevalent. There's an anecdote about the Saturn's instrument unit that illustrates this: each subsystem design team was asked for required cooling capacity. Each team over-estimated to be on the safe side and then the cooling system design team put a safe margin on top of that. As a result, the coolant froze on the first flight of the IU, and heating capacity was added to avoid that on future flights (from David Woods, "NASA Saturn V Owners' Workshop Manual", 2016, p122-123).

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