Someone else will likely be able to explain this more eloquently but here goes: the weight of the fuel in the rocket is a huge issue because you’re using the thrust of the rocket to fight gravity and air friction. Sure, if you had unlimited fuel the shape wouldn’t matter, but it’s prohibitively expensive from a resource point of view. The current shape of rockets is based on minimizing drag.
Our atmosphere is filled with pressurized gasses. Moving through it causes resistance. Even if you just run forward, you have tons of molecules pushing back against you. This is why vehicles are built with aerodynamics in mind, to prevent too much resistance from the air. More resistance requires more energy to push past.
So, with a rocket, the shape needs to be aerodynamic to “slice” through the atmosphere in order to save fuel. To oversimplify, making a craft less aerodynamic is essentially making it harder to lift. That would mean you would need more fuel. However, more fuel equals more weight, so you can get to a point where the craft won’t even be able to launch properly.
In space, the shape wouldn’t matter, as traveling in a vacuum would allow for movement without resistance from air. If you watch space launches, the rocket comes apart after leaving the atmosphere. That’s all the rocket was needed for, and the smaller, non-rocket-shaped craft moves forward once in space.
The biggest components in rockets is the propellant tanks. These are designed to maximize the volume and minimize the forces so that they can be built as light as possible. The most efficient form is a cylinder. It is possible to stack multiple of these cylinders next to each other, a lot of rockets does this. However it does make the rocket more complicated, larger and have more drag. So this is only done when the advantages are greater then these costs. Typically the advantage is that fuel tanks can be detatched in flight.
Rockets are really mass optimized since it’s so hard to get to orbit. We can only get a fraction of the rocket’s overall mass into orbit as payload.
So you want to maximize the surface area to volume ratio of the propellant tanks. The best shape for that is a sphere. Ideally you’d have a big ole sphere with an internal bulkhead to separate the fuel & oxidizer to minimize the material mass needed for the tanks. Spheres are also strong.
Then atmospheric drag comes into play. A true sphere would have too much drag due to it’s larger frontal area. It would also be unstable during ascent. So we “stretch” the sphere into domed cylinders to get the required volume while reducing the frontal area. (This also helps with transportation logistics but that’s besides the point).
You’ll actually see quite a few upper stages and kick stages that use spherical tanks since they are used in space, where drag isn’t an issue.
Ever heard the phrase “flies like a brick”?
Turns out, having an aerodynamic shape lets you use much less fuel and thrust than trying to fly a literal airbrake at Mach 10. Much less being measured in the “hundred tons of fuel” category.
As for stability: cubes tumble like absolute cunts. A pointed tube mostly stays on course by itself. In vacuum, none of it matters, especially since when the fairing is blown off the payload, what is under it is rarely rocket-shaped anyways.
If we were taking off from the moon, ya, we absolutely could do that. Only thing that matters there is the weight to fuel ratio, pick whatever shape you want.
Except on earth we have a lot of atmosphere to get through, so in the name of practicality and efficiency, it has to be aerodynamic. A rocket, an air plane, a bullet. You basically need a cylindrical objected with a cone tip if you want minimal drag.
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