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Compared to most rockets I know of, Space X's Falcon 9 seems unusually tall and skinny. The v1.1 version is nearly 70m tall, yet only 3.6m thick.

For comparison, Atlas V is 58m tall and 3.8m thick, Proton-M is 58m tall and 7.4m thick, etc…

So, what's the reason for Falcon 9's shape? Without any particular knowledge about rocketry, I can imagine a shorter, thicker rocket might be more stable and durable. What are the advantages (and disadvantages) of this approach?

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SpaceX manufactures their booster in Hawthorne, CA. They then truck it on the highways to McGregor, TX for test firing with all 9 engines.

Then it is back on a truck for the drive to Florida to LC-40 at the Air Force station for launch.

The diameter of the booster at 12 feet/3.6 meters is the largest they could go and still be road transportable.

Delta IV in comparison is built in Decatur, AL and shipped by barge to the Cape or Vandenberg for launch as it is too wide.

The original Falcon 9, the 1.0 version was the appropriate size for its width in order to have enough fuel and oxidizer for a mission to orbit based on how long the Merlin-1C engines needed to fire for that mission.

The 1.1 version changed the engine to the Merlin-1D a higher thrust engine, which means it also consumed more fuel and oxidizer so the tanks had to get bigger to hold the extra propellant. They could not go wider, and stay road transportable so they went longer.

The legs are transported separate, and attached at the launch site as they make it just too tall to transport.

Amusingly it is height, not width that is the limiting factor. But for a round stage that distinction is without a difference.

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    $\begingroup$ It could do with some references, but it's all relevant. The height of a road transport is limited by bridges etc. 3.6 m is about the biggest cylinder you can fit on a low loader and drive around the country in a reasonably routine manner. You could go much wider (taking up two lanes) if you're prepared to close road sections for the passage of the transport, but as Geoff said, if you want the stage to be cylindrical, then the extra width available is of no use and you're limited by height restictions. $\endgroup$ – Hobbes May 31 '15 at 13:33
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    $\begingroup$ @andy256 Its amusing to me, since you think of fat not tall as the problem in normal life. (Tall is usually good, fat is usually bad). $\endgroup$ – geoffc May 31 '15 at 22:10
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    $\begingroup$ @andy256 Come on, it's not 'against the rules' to show emotion here. He's pointing out something ironic, it's perfectly alright. $\endgroup$ – Vedant Chandra Jun 1 '15 at 3:23
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    $\begingroup$ Thanks geoffc & @VedantChandra for your answers! It's quite fascinating to me that a design of a rocket would be primarily predicated upon logistical, not "real" technical reasons. $\endgroup$ – radex Jun 1 '15 at 14:40
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    $\begingroup$ Logistics is a real technical reason - every product engineer needs to take into account assembly and delivery of their product. It would be no use building a great rocket that gets stuck on the first bridge after the factory. There are other options (move the factory, build a giant VAB like the shuttle) but they cost money too, and SpaceX is all about cost optimisation. $\endgroup$ – pjc50 Nov 6 '17 at 10:36
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Your assumption is correct, a shorter and thicker rocket is obviously more stable on the ground. However the purpose of a rocket is not to stand strong on the launchpad forever. ;)

When it comes to actually taking off into the stratosphere and beyond, a rocket needs to be aerodynamic. Drag (air resistance) depends on the cross-sectional area of a body, in this case the thickness of the rocket. Therefore by being a thinner rocket, the Falcon cuts through the atmosphere with less drag, wasting less fuel pushing against the atmosphere.

With reference to the comparisons you made; both Atlas V and Proton M were transported via rail, allowing for a thicker width. SpaceX moves it's booster from California to Texas, and then to Florida by road. The thin design is small enough to be loaded onto a truck and driven around.

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    $\begingroup$ The thing about a rocket is that it leaves the appreciable atmosphere pretty fast so aerodynamics are less of an issue. $\endgroup$ – geoffc May 31 '15 at 22:10
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    $\begingroup$ @geoffc The main point is the transport IMO, which you've detailed better in your question. $\endgroup$ – Vedant Chandra Jun 1 '15 at 3:21
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    $\begingroup$ The argument about aerodynamics is an oversimplification. Not only the cross-sectional area of a body but also its overall surface area are major contributors to drag. The latter is in favor of lower and thicker shapes. On the other hand, drag might also be desired to maximize aero braking. $\endgroup$ – Everyday Astronaut Nov 6 '17 at 16:20
  • $\begingroup$ Note that rail transport wouldn't help here, as U.S. freight loading gauges allow no more than 3.25 meters of width, and sometimes even less. $\endgroup$ – Sean Jun 10 '18 at 15:17
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The thing about a rocket is most of the fuel spent is getting out of the atmosphere and a large portion of the energy spent is wasted on aerodynamic forces rockets are limited to how fast they can go in the atmosphere by this and if they could leave the atmosphere at a faster speed the effects of gravity would be lowered it is all a balancing game and aerodynamics are one of the two major resistances during flight.

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    $\begingroup$ The question starts "Compared to most rockets I know of"; other rockets are generally designed by competent teams fully aware of the costs of extra drag, much like Falcon 9's design team. So there is no obvious reason why F9 shifts the balancing game so much in one direction; it is not as though SpaceX is the first organization to realize how much cheaper it would be if that pesky atmosphere wasn't slowing things down so much. $\endgroup$ – Nathan Tuggy May 27 '17 at 3:09

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