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This excellent answer to the question Why is Falcon 9's shape so tall and skinny? mentions California, Florida, Texas and Alabama, within and between-which there must be at least a few limiting underpasses through which Falcon-9 must go, "ere the other side he see".

If aspect ratio were not constrained by transportation on fascinating public roads and infrastructure (if for example there were just a bunch of very long, rocket-friendly but otherwise boring tunnels instead), would the Falcon 9 have evolved to be substantially shorter and/or substantially larger in diameter than it's svelte 3.66 meter OD? If the plan to go to Falcon Heavy was still in place, what might Falcon 9's dimensions have been?

It seems to me that the extra propellants accommodated by every 10 cm in additional diameter might more than compensate for the additional structural weight and aerodynamic body drag, so I don't see anything that would be preventing it from getting somewhat wider, perhaps a lot wider! But I'm no rocket scientist, so I'm asking here.


Of course, if it weren't so skinny, we might not be able to ask questions like Why does the CRS-8 Falcon 9 rocket appear to be swaying before launch?

The unconfirmed SpaceX adjective "flying noodle" comes from comments below the as yet still unanswered question Just how much can tall skinny rockets bend? (roughly, safely)

below: GIF of Falcon 9 faster than normal speed to make the motion more visible.

enter image description here

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  • $\begingroup$ Wow that wobble really reminds me of the physics easing of spacecraft on the launchpad in Kerbal Space Program. $\endgroup$ – Ingolifs Apr 8 at 1:36
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Without knowing how SpaceX arrived at the 3.66 m diameter, this has to be speculation.

  • Most rockets have a lower fineness ratio than the F9v1.2 (i.e. they're fatter). That makes it likely there's an advantage to designing a fatter rocket. A larger frontal area due to a larger diameter (=more air resistance) is compensated for by less weight (fatter tanks have a better volume/surface area ratio) and/or less form drag.
  • The F9 gained ~14 m in length between v1.0 and v1.2. A reasonable fineness ratio became a marginal one. I've no idea how much of the performance increase of v1.2 was foreseen by the time the body diameter of v1.0 was decided upon. Or how constrained the v1.2 is by its length limit: a longer version might have better performance, but has been decided against because it would be too expensive to transport.

What diameter would have been chosen? There are basically 2 choices:

  • make everything the same diameter as the payload fairing (5.4 m)
  • choose a smaller diameter than the payload fairing. Depends on the aerodynamics of this solution (I suspect there's some extra drag due to the decrease in diameter).
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  • $\begingroup$ I mentioned "body drag" because I don't know the word for the drag that comes from the sides of the rocket (circumference $\times$ length) but I thought that for F9 it's much larger than the "frontal" resistance, which is defined more by the size of the fairing/capsule than the F9's skinny diameter. Anyway, your answer here is that you'd expect it to be somewhat larger in diameter and similar height, or...? $\endgroup$ – uhoh May 27 '17 at 11:04
  • $\begingroup$ Another way to ask; considering your two options at the end, would you say that either one could be described as "larger diameter but similar height"? $\endgroup$ – uhoh May 27 '17 at 15:05
  • $\begingroup$ No. There's little point in going to e.g. 3.8 m diameter because that would only make the rocket a bit shorter (i.e. small change in fineness ratio). If you want to take advantage of not being limited to 3.66 m, I'd expect a move to either 5.4 m or something halfway in between (4.5 m). $\endgroup$ – Hobbes May 27 '17 at 16:07
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    $\begingroup$ Depending on speed and air density, aerodynamically ideal ratio of diameter to length seems to be somewhere in the range of 5:1-10:1. Overly skinny doesn't help, but then, once a stage drops off, it's a different ratio but then the vehicle has progressed into a different speed/altitude regime from that at lift-off. $\endgroup$ – Anthony X May 27 '17 at 23:28
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    $\begingroup$ @uhoh I was just going off some vague memory - probably some article I read a long time ago, or perhaps a science show on TV. I'm not qualified to post an actual answer, but perhaps this will inspire someone who is. About the only tidbits of relevance I could add would be that it would be related to drag co-efficient and Reynolds number $\endgroup$ – Anthony X May 28 '17 at 3:50
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Without the transport constraints, a wider rocket would make sense, but I think SpaceX would still end up with a flying noodle, albeit a much larger one.

Changing the diameter of a rocket or aircraft usually ends up being a completely new design - for example the existing landing legs won't fit because the larger rocket is a different shape. You don't do that unless you really have to.

Instead, you increase the length - just add another identical fuselage section, and as long as it hasn't gotten too noodly, it will fly much the same way and use all the same parts.

The noodle then represents the biggest rocket you can build without either building a new factory or having it fold in half on the launchpad. If the boss has a thing for absurdly large rockets, you will be building a noodle at some point.

In the real world, the diameter was chosen based on road size. Without that limit, the relevant constraint becomes what you want your largest rocket to achieve.

For SpaceX, that means ITS - a vehicle with a volume of approximately 9000 cubic meters, compared to around 500 for F9.

So what diameter would SpaceX choose? Some possibilities:

  • 4m - Essentially the real world design. Easy to transport, but Falcon Heavy has a bunch of engineering challenges with loads in different directions from a single core and an 800m tall ITS simply won't work, so you'll need to start over with that.

  • 12m - ITS diameter. at 2m, F9 would resemble a pancake more than a rocket.

  • 10m - ITS volume is achievable with a 115m vehicle, about the same shape as a real F9. The F9 first stage equivalent is still decidedly stubby at under 10m - not ideal in terms of drag, and possibly impractical to fit standard shaped tanks in, but reusability means that it's ok to use a larger rocket if necessary.

  • 7m - Possibly a reasonable compromise - F9 would still be quite stubby, but Falcon Heavy capacity could be simply a lengthened rocket, and ITS could be 3 noodly cores together rather than a comppletely new design.

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    $\begingroup$ My question asks if it "would have been" different. I haven't asked about a redesign. "If the plan to go to Falcon Heavy was still in place, what might Falcon 9's dimensions have been?" Do you think you can address this directly? Right now your answer is just unsupported generalized speculation about rocket design. You mention numbers, but without explanations where they are coming from. $\endgroup$ – uhoh May 27 '17 at 14:55
  • $\begingroup$ Removing the biggest design constraint will get you a significant redesign, unless you decide not to because you already have a factory that makes thin rockets, in which case he answer would simply be no. $\endgroup$ – Quentin Clarkson May 27 '17 at 21:49
  • $\begingroup$ The numbers are just cylinder volume - what diameter lets you go from roughly 500m^3 for F9 to 9000 for ITS. The 7m design ends up looking a lot like a bigger F9/Heavy. 10m is very different until you get to ITS size, but avoids the redesign needed to stick multple rockets together. $\endgroup$ – Quentin Clarkson May 27 '17 at 22:00

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