why are we still designing rockets with the same shape (cylinder/nose cone)? Do we still not have the technology to send up boxier/flatter objects with thrusters on like the 4 corners, making for more stable landing and re take off?



why are we still designing rockets with the same shape (cylinder/nose cone)? Do we still not have the technology to send up boxier/flatter objects with thrusters on like the 4 corners, making for more stable landing and re take off?

In: Physics

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.

Corners make for turbulent flow. They are by no means more efficient. No matter where you put the thrusters.

Boxier things have more wind resistance. More wind resistance means that 1) it takes a lot more fuel to take off, and 2) a lot more heat is generated during re-entry. Both of those things are bad enough that it’s not worth it.

We always could send up boxier rockets. A boxier rocket is less efficient for a bunch of reasons and has no benefits. Why would we do such a thing? How would thrusters on corners be better than thrusters on the cylindrical body of the rocket?

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.

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.

I guess the aerodynamics of a rocket matters a lot so having nose coner rather than a box is easier for it to fight air resistance and the density isn’t as much as a square or ‘boxier object’ considering it was made from the same materials.

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.

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.

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.

necessity is the mother of all invention. When there is enough need to go into space, i’m sure human innovation will make more sophisticated designs to endure the demands and strains of space-travel.

Nose cone: Aerodynamics, as many others have touched on. You’ll see that the actual satellites are almost never cone-shaped (unless they’re designed to come back down), and also the cone is usually ditched as soon as we’re out of the atmosphere.

Cylinder: Mostly manufacturing. Rolling metal into a circle is cheap and easy, only needs a single weld (=weak point), and a circle shape can also hold the most pressure for the least material. OFC a sphere would even be better, but you can’t mount something on top of a sphere that easily.

“more stable”: Another shape would give you no real advantage, if not even a disadvantage. On the launch pad, the rocket is held in place by clamps, so you don’t really care about how intrinsically stable it is. Unless there’s a hurricane it won’t tip over. In flight, aerodynamics is everything, and here again, corners = bad. And on landing, you have legs, which again make the shape of the rocket irrelevant.

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