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The big news in this article is the extremely large carbon-fiber fuel tank. However, the drawing in the article does not show it well. Instead what stands out like a sore thumb in the drawing is the smaller sphere, in a location which suggests it would be suspended inside the "fuel tank".

It reminds me of a similarly shaped (i.e. spherical) object in science fiction, but I don't think it is related. (See below).

Is the sphere a helium tank, coated in carbon and suspended in oxygen? Or have I got it all wrong?

enter image description here

above: SpaceX large rocket conceptual design, found here.

enter image description here

above: Similarly shaped sphere, conceptual, from here

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Firstly, the image you provided is not of the BFR, but the BFS (Big F***ing Spaceship) or ITS (Interplanetary Transport System) spaceship (Musk is purportedly not happy with either name).

Elon Musk answered a question in his Reddit AMA on r/SpaceX recently that describes the exact purpose of the spheres. Long story short, yes, they are propellant tanks. They are not so much suspended but held in place inside the tanks. Enough of me talking though.

Question

ITS Spaceship design question II.: The ITS Spaceship has two mystical spherical tanks, marked green in this slightly edited image. The whole tank design looks very exciting, and there's rampant speculation on this sub about the purpose of those spherical tanks:

are they for landing fuel?
... or are they storing 'hot' gaseous propellants as part of the autogenous propellant pressurization system?
... or are they used for on-orbit propellant densification to store vapor before it's liquefied again?

All of the above perhaps?

Answer by Elon Musk

Those are the header tanks that contain the landing propellant. They are separate in order to have greater insulation and minimize boil-off, avoid sloshing on entry and not have to press up the whole main tank.

As an addendum, SpaceX's Interplanetary Transport System does not use Helium, instead opting for autogenous pressurization whereby you use the propellant to pressurize itself (gaseous Methane and gaseous Oxygen, respectively).

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  • $\begingroup$ OK I can adjust the title. Is the outer surface of the sphere carbon? Is the material immediately outside of the sphere oxygen? Let's converge on an actual answer to the question so I can accept it - thanks! $\endgroup$
    – uhoh
    Oct 25, 2016 at 0:35
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    $\begingroup$ @uhoh: material is probably carbon, but has not been published yet. Outside of the sphere is oxygen (LOX at first, GOX later). $\endgroup$
    – Hobbes
    Oct 25, 2016 at 6:58
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    $\begingroup$ The quote provided by @EchoLogic is all we have at the moment. Reading it again, there might be a layer of insulation on the outside of the sphere. Methane is in the bottom tanks, IDK if the bulkhead is shared. $\endgroup$
    – Hobbes
    Oct 25, 2016 at 8:03
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    $\begingroup$ @Hobbes Elon mentioned that they may have to coat the interior of the main BFR/BFS tanks with a layer of invar to prevent deleterious effects from LOX/LCH4. If this turns out to be the case, I would not be surprised to see the same coating used on the exterior of these internal spherical tanks also. $\endgroup$
    – marked-down
    Oct 25, 2016 at 8:11
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    $\begingroup$ @uhoh I can't include material because I don't have a source for it that the spherical tanks are definitively carbon fiber. Only SpaceX can answer that. $\endgroup$
    – marked-down
    Oct 25, 2016 at 18:36

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