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Comment by nknight | Hacker News Reader

I have a sophisticated layman's understanding of stuff like orbital mechanics, launch energies, etc., but one thing I've never been entirely sure of: Why are rockets always round?

I'm guessing it's the least-questions-asked shape as far as aerodynamics goes, and gives good weight efficiency assuming you can build what you need into a round shape. Is there anything more fundamental than that?

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A rocket is a series of pressure vessels, pressurized fuel tanks, pressurized crew compartment, etc.. Round shapes spread the pressure around the entire structure. The ideal shape for pressurized vessel is spherical, however for aerodynamic reasons cylinders with rounded ends are used.

I would guess that it's a whole lot of reasons including the reasons you stated. I would also add a couple (and there's probably more):

Round is structurally strong (which, when it comes to aerospace engineering, is really just the opposite side of the "light weight" coin).

Symmetry of shape and mass makes the stability and control equations a hell of a lot simpler to deal with, and a round shape makes it easier to be (nearly) symmetrical in more ways than with other shapes..

I suspect weight and aerodynamics play the most important part in determining shape. What I've always wondered is why we've never effectively used wings on our spacecraft to get part of the way there. Spaceship one used something like this with its "stage 1" vehicle.

I haven't done the math, but I always assumed it would be less expensive (as in fuel and weight) to start the orbital boosters from 50,000ft after climbing via traditional wings, as opposed to the brute force option of simply producing as much thrust as possible and climbing straight up.

Granted for extremely large payloads the structural requirements for building wings that can support the weight might be impractical, but wouldn't it have been easier for the space shuttle to "launch" from 50,000ft?

My educated guess: it's because a cylinder is the easiest shape to form sheet metal into. It also avoids multiple seams which are failure points. As a side effect, the stress concentrations at sharp corners are avoided.

Because it makes sense for any pressure vessel to be round.

The roundness is mostly due to the fact that propellant almost always requires being stored in a pressure vessel, more so for cryogenic propellants. It's far, far easier to make a pressure vessel that's at least axisymmetric. Thus, the fuel tanks tend to be spheres or cylinders. And since the fuel tanks are by far the largest components of launch vehicles the rocket itself tends to be cylindrical.

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