Why does the cargo fairing have a much larger diameter than the lower part of the rocket?

Of course a larger diameter lets thicker payloads fit. But then the first, inter, and second stage could have been designed lower in order to decrease overall surface area (and thus aerodynamic drag) and also to ease the landing (lower center of gravity -> easier balancing).

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    Note that at landing time, the first stage already has a remarkably low center of gravity despite its skinny-ness; the tanks are practically empty and the engines are heavy. – Russell Borogove Nov 6 '17 at 16:30
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    And the payload has been removed, and any fuel is at the bottom. – Tim Nov 6 '17 at 18:32
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    Low center of gravity does not make for easier balancing for rockets. - This is because rocket stability is fundamentally different from a pendulum. The force is not based on a static direction (as with gravity), but rather based also on the direction of the rocket. web.archive.org/web/20091018232320/http://geocities.com/… – paul23 Nov 6 '17 at 23:17
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    @paul23 Yes, but that's not relevant here. We're talking about the stage landing on the barge, not a rocket flying up. Low center of gravity certainly helps there - as does having the center of mass close to the control surfaces. – Luaan Nov 7 '17 at 10:15
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    @Luaan Well it's relevant, due to the fact it first has to go up before going down. (And you wish to have the center of gravity in front of the center of lift, potentially as far as possible). -- I do not know enough about the details of the design to judge whether these are driving parameters, but "cog before col" always needs to be full filled. – paul23 Nov 7 '17 at 12:58
up vote 53 down vote accepted

The diameter of the stages is the largest size that can be transported by road without extensive "outsize load" issues (permits, having to move traffic lights and signs out of the way etc.). This makes the rocket much cheaper to transport.

The fairing size (5.4 m) is dictated by the standard satellite diameter set by the Shuttle and Ariane 5.

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    fairing diameter has to be 5.4m to accomodate standard ~4.6m payloads. – Hobbes Nov 6 '17 at 17:49
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    As the linked answer says, the reason is not width - it's height. Bridges are mostly limited to a low bed truck + 12ft payload. – Tim Nov 6 '17 at 18:36
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    Two thousand years from now, the influence of these devices are going to still be showing in the new designs and someone's going to trace it all back to Roman carts in Britain. – corsiKa Nov 6 '17 at 22:31
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    @corsiKa. Road width is the first thing that's mentioned, so we can cut to the chase and just say that it is four Roman horses wide I guess? – Mad Physicist Nov 7 '17 at 15:03
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    @LorenPechtel. I bet 14' is what you get when you stack four Roman horses on top of each other :) – Mad Physicist Nov 8 '17 at 18:53

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