20

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?

geoffc
  • 79,523
  • 12
  • 227
  • 419
radex
  • 1,642
  • 1
  • 12
  • 23
  • 1
    only peripherally related but still funny as heck http://www.astrodigital.org/space/stshorse.html – Sam Nov 06 '17 at 20:55

3 Answers3

30

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.

geoffc
  • 79,523
  • 12
  • 227
  • 419
  • Not the down voter, but this answer seems to have lots of info without demonstrated relevance. The last sentence needs explanation. IMO the word amusingly would be appropriate in chat, but not in a factual answer. –  May 31 '15 at 11:42
  • 16
    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. – Hobbes May 31 '15 at 13:33
  • 4
    @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). – geoffc May 31 '15 at 22:10
  • The reality is that to take something over 14' tall (total vehicle height, not just your cargo) down the road involves a lot of headaches and sometimes isn't even possible (if you can't avoid going under a bridge somewhere.) Since there is transporter and then ground clearance below the rocket 12' is about the limit. – Loren Pechtel May 31 '15 at 22:13
  • @Hobbes In what way is it relevant that geoffc is amused? –  May 31 '15 at 23:38
  • 11
    @andy256 Come on, it's not 'against the rules' to show emotion here. He's pointing out something ironic, it's perfectly alright. – Vedant Chandra Jun 01 '15 at 03:23
  • 1
    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. – radex Jun 01 '15 at 14:40
  • Railroad gauges are a limiting factor for Soviet/Russian boosters' diameters. The BVGD blocks for R-7 rockets were designed with this constraints in mind - besides other factors, of course. – Junior Miranda Jun 02 '15 at 17:14
  • Interesting to note is that 8K82 Proton is only 7,44m wide in its first stage, wich is ~20 meters long; The rest of the body has a 4,15m diameter, going to 3,6 meters on Block D/DM fourth stage. – Junior Miranda Jun 02 '15 at 17:16
  • @JuniorMiranda Is that 7.44 m width with the side fuel tanks? They look like strapon boosters on Proton, but are really the fuel tanks and engines. Consider the main core instead, I would think. – geoffc Jun 02 '15 at 17:18
  • yes the 7.44m mark refers to the side tanks of 1rst stage - on Proton. The very reason for they use the 6 tanks was the Soviet rail gauge in the 60´s. Proton was conceived as an ICBM for a huge H Bomb. – Junior Miranda Jun 03 '15 at 01:38
  • 5
    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. – pjc50 Nov 06 '17 at 10:36
  • 2
    I was going to ask why SpaceX didn't transport the Falcon 9 by rail to take advantage of the higher clearances... and then I found out that, while the maximum allowed height is indeed greater for rail, the maximum width is limited to only 3.25 meters, which would require making the Falcon 9 even heavier. – Vikki Jun 10 '18 at 15:15
7

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.

Vedant Chandra
  • 6,364
  • 3
  • 34
  • 58
-3

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.

Lachlan
  • 1
  • 1
    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. – Nathan Tuggy May 27 '17 at 03:09