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One factor when designing a launcher is the total $\Delta V$ required. For a given $\Delta V$, engineers can define the propellant quantity. From this quantity they can define the volume of the launcher. That's the way I imagine firsts steps when designing a brand new rocket. I may be wrong, don't hesitate to correct me.

A launcher is basically a cylinder. For a given volume it can be either thinner and longer or fatter and shorter. I decided to name "aspect ratio" the ratio between height and diameter for the rest of the question. I don't know if there is a better name. I use this number to define if a rocket is thin or fat.

I collected the following data (ordered from the fatter to the thinner) for some launchers without strap-on boosters:

  • sea dragon
    • diameter: 23m
    • height: 150m
    • aspect ratio: 6.5
  • vega
    • diameter: 3m
    • height: 30m
    • aspect ratio: 10
  • saturn V
    • diamater: 10.1m
    • height: 110.6m
    • aspect ratio: 10.6
  • long march 2C
    • diameter: 3.35m
    • height: 42m
    • aspect ratio: 12.5
  • delta IV
    • diameter: 5m
    • height: 63-73m
    • aspect ratio: 12.6-14.4
  • diamant
    • diameter: 1.34m
    • height: 18.95-21.6m
    • aspect ratio: 13.9-15.9
  • new glenn
    • diameter: 7m
    • height: 98m
    • aspect ratio: 14
  • electron
    • diameter: 1.2m
    • height: 17m
    • aspect ratio: 14.2
  • atlas V
    • diameter: 3.81m
    • height: 58.3m
    • aspect ratio: 15.3
  • falcon 9 block 5
    • diameter: 3.66m
    • height: 70m
    • aspect ratio: 19.1

Aspect ratio (height/diameter) varies from less than 7 (short and fat) to almost 20 (long and thin). I fail to see any logic in this choice (no obvious correlation with payload capacity, designed date or origin country, and there is no size constraints as they are all launched outside contrary to ICBM that must be short enough to fit in there silo/submarine). My question arises given this variety in aspect ratio.

How do launcher designers decide the aspect ratio of their launchers? What criteria do they take into account to decide if their launcher should be thinner or fatter?

Related questions:

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  • $\begingroup$ It's not that simple. You could, for example, use a cannon shot as Jules Verne calculated once. Or you could lift off and maintain 1 m/s speed until you go exoatmospheric, and only then generate the desired delta-V to go into orbit. So you need basically calculate the total work necessary by integrating speed vs.position along the planned trajectory $\endgroup$
    – Carl Witthoft
    Commented May 13, 2020 at 12:24
  • $\begingroup$ Related: space.stackexchange.com/questions/40546/… $\endgroup$
    – Organic Marble
    Commented May 13, 2020 at 12:26
  • $\begingroup$ Related: space.stackexchange.com/questions/21669/… $\endgroup$
    – Organic Marble
    Commented May 13, 2020 at 12:27
  • 4
    $\begingroup$ The question isn't overly broad, I don't know why people are VTC. $\endgroup$
    – Russell Borogove
    Commented May 13, 2020 at 16:29
  • 2
    $\begingroup$ I agree, it seems clearly on topic and not a duplicate. Also well researched. $\endgroup$
    – Organic Marble
    Commented May 13, 2020 at 17:11

1 Answer 1

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There are several conflicting factors influencing aspect ratio, so it’s no surprise that it’s hard to find obvious correlations.

A longer, skinnier rocket has less frontal area thus experiences less air resistance.

A shorter, fatter rocket has more volume (potential fuel tankage) per surface area (dry structural mass).

A larger rocket suffers less from air resistance than a smaller one at the same aspect ratio, which helps explain Sea Dragon and Saturn V -- their air resistance penalty is lower, so they are optimized more for volume/surface.

In general your list shows a trend for newer launchers to be longer and skinnier; materials science and computer modeling of mechanical stress has made it possible to substantially lighten rocket tankage, reducing the weight-per-volume penalty for long skinny rockets. Falcon 9 has an exceptionally good dry mass ratio despite its length, for example, even compared with the Zenit, which has a generally similar type of construction.

I assume that Vega and Sea Dragon are outliers at the stubby end because their skins are much thicker than the others — solid rocket casing for Vega, cheap steel for Sea Dragon — thus their designs are balanced more toward reducing surface area.

It’s not correct that an orbital launcher has no dimensional constraints just because it isn’t silo-launched. The rocket body must be built, transported, and assembled. The ceiling height of NASA's Michoud assembly facility may have contributed to the selection of the lunar orbit rendezvous mode for the Apollo moon landings; it wouldn’t have been possible to build the larger rocket needed for direct ascent there.

Once an agency has built a successful rocket with a particular aspect ratio, it will often make a longer version of the same rocket at the same diameter, so it can use the same factory tooling. Falcon 9 is one of the more extreme cases, having grown from 48 m to 70 m long in the Block 5 version. If the original version of a given design happened to hit the optimal aspect ratio for aerodynamic and structural considerations1, the stretched versions will depart from that optimum in order to keep development and production costs down.

1: Ron Howard narrator voice: "It did not hit the optimal aspect ratio."

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  • $\begingroup$ OK, so in short: frontal area, internal volume to skin area ratio, assembly building dimension, and factory tooling reuse. $\endgroup$
    – Manu H
    Commented May 15, 2020 at 19:59
  • $\begingroup$ There are probably other factors as well. Shorter cable runs in squat stages, better aerodynamic stability & control authority for longer ones, etc. $\endgroup$
    – Russell Borogove
    Commented May 15, 2020 at 20:19

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