Space Exploration Stack Exchange is a question and answer site for spacecraft operators, scientists, engineers, and enthusiasts. It only takes a minute to sign up.
Sign up to join this community
Russian rockets look like this:
They flare them out at the bottom. With their newest rocket, the Proton, the flared shape is gone but the boosters still have caps that angle in towards the main rocket so the shapes fuse together. American rockets look like this:
The boosters look like mini-rockets, with their own nose cones, stages are straight, not flared out. The Chinese do the same. Europe takes the Russian approach with Ariane.
Why the different approach in booster design?
The Soyuz uses conical boosters because there's an aerodynamic advantage.
According to The Red Rockets' Glare:
Engineers gravitated to a conical shape primarily because of the aerodynamic advantages ...but also for 3 other reasons: the large size of the engines at the tail end, the possibility of imparting additional thrust to the central sustainer by the shape of the booster, and the opportunity of decreasing wall thickness.
Another source claims:
But it turned out that the conical-shaped boosters of the Soyuz are an advantage for the aerodynamics, especially the gap between the boosters has an effect of a “negative” wing.
At first glance a cone wouldn't appear to have an advantage: drag depends mostly on frontal area, and a cone has a larger frontal area than a cylinder of the same volume.
But a cylinder of this diameter would be half as long as the conical booster, giving a short, stubby shape that is less efficient than a long, thin cylinder.
And there's a more subtle effect going on here, one specific to having 4 boosters close to each other. Rockets with 2 boosters wouldn't benefit from this effect.
The other conical rocket is the N-1. In this case, there's no aerodynamic advantage. The rocket's shape is due to limitations of manufacturing technology: a spherical tank was easier to manufacture than a cylindrical one, so they used spheres. But they needed two spheres of different diameters to get the fuel:oxidiser ratio correct, so they ended up with conical stages.
According to Foothold in the Heavens: The Seventies, the Soviets were unable to produce cylindrical tanks that were part of the rocket's load-bearing structure: they couldn't manufacture aluminium plate that was thick enough for the job. So they had to separate the rocket's skin from the tank. They chose to use a spherical tank instead of having two cylinders inside one another, to minimize contact between the tank and the outer skin.
From the website linked to the book N-1 for the Moon and Mars:
