I am wondering about the basic design considerations when building a rocket body.

  1. is it made as a separate hollow cylinder with standalone tanks inserted inside?
  2. is it made so that the tank walls are the fuselage walls?

It seems to me that some rockets have it like option 1 (e.g. Falcon 9).

The SpaceX's Starship seems to have only single common wall between the fuselage and the tank, i.e. the tank is the fuselage and vice versa.

Is the second option common or it it new to Starship? Or is it the other way around? Or is it somehow different altogether?

Schematic depiction of the two approaches to rocket fuselage

  • $\begingroup$ I added some information about the common bulkhead part of the question. It's been used a few times. If the fluid temps are a lot different it can be tricky, requiring insulation. $\endgroup$ Feb 19, 2020 at 23:28
  • $\begingroup$ Some rockets (eg. Atlas) don't only have the tanks become the wall structure but are actually built like giant metal balloons using the pressure of the tank as part of the structure. Google "rocket implosion" $\endgroup$
    – slebetman
    Feb 20, 2020 at 7:22

1 Answer 1


Normally the tank is the wall is the structure. This minimizes weight.

There are usually cylindrical regions connecting the different propellant tanks - sometimes called "intertanks". This portion of the structure does not contain propellant as the rest does.

The shuttle external tank and the Saturn V are examples of that style; I do not know about Falcon but I would be surprised if it is not. There is a blueprint here showing Falcon 1 had integral tankage but I am unsure of the provenance. The Falcon 9 user guide Table 2-1 seems to indicate integral tanks, but the document does not explicitly say this.

enter image description here

I do know of some counterexamples from the past.

One is the old Soviet N-1 rocket, which had spherical propellant tanks inside a fuselage / fairing.

enter image description here

Another design not seen much recently is "cluster" tankage as was used in the Saturn-1B.

enter image description here

Image sources 1, 2, and 3

Edit to address question edit:

Common bulkheads have been used instead of interstages. Two examples are the second stage of the Saturn V and the Centaur-G developed for the shuttle. The Falcon 9 user's guide linked above states that its first stage uses an "insulated common dome".

  • $\begingroup$ I am wondering if the added weight of separate fuselage would add a lot more strength and therefore reusability potential, if at the expense of smaller payload - which in fully reusable rocket does not matter as much. Just welding two tanks together seems awfully fragile. $\endgroup$ Feb 19, 2020 at 22:57
  • 2
    $\begingroup$ @PunchyRascal The problem is twofold: It is harder to make the load be properly shared, and even a miniscule increase in tankage weight will destroy the viability of the vehicle. In general, this type of monocoque-ish design is something that lets you make very strong, light structures. $\endgroup$
    – ikrase
    Feb 20, 2020 at 7:46
  • $\begingroup$ @PunchyRascal The Falcon 9 is single-walled, extremely lightweight, and appears to be quite reusable, so the approach of separate tanks inside a fuselage seems unnecessary. Many rocket families are built the same way and reliably withstand 4g to 6g accelerations. $\endgroup$ Feb 22, 2020 at 15:51
  • 1
    $\begingroup$ I guess I am just amazed how they manage to connect those tanks in such a way that they create a single body, resisting all kinds of forces. I am wondering how Starship, supposed to be even more rugged, will fare seeing as it uses the common wall, too. $\endgroup$ Feb 22, 2020 at 20:36
  • $\begingroup$ @PunchyRascal The shuttle external tank was the backbone for the whole ascent vehicle, connecting the orbiter, solid boosters and tanks all together (it did have a big structural beam in the intertank). $\endgroup$ Feb 22, 2020 at 21:37

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