Eli5: why are submarines cigar shaped but jets pointy?


Both water and air are fluids but jets and missiles are pointy whereas submarines and torpedoes are cigar shaped. Why is that?

Edit: Grammar

Edit2: why is pointy better for supersonic? Why is round better for subsonic?

In: 29

Hydrodynamics vs aerodynamics…

I’m sure it has to do with the differences in laminar flow of air vs water…

When moving through a fluid at subsonic speeds, it’s better to be have a rounded shape. When moving through a fluid at supersonic speeds, it’s better to be pointy.

Take a look at a subsonic jet, like an airliner. They’re rounded too just like submarines. Their fuselage isn’t that much different in design. Now look at something like an F-22, a supersonic fighter jet. They’re pointy

Speed. At sub sonic speeds round is the best shape for traveling through fluids. Look at propeller aircraft and commercial jet aircraft. Pointy fronts work better in the trans and supersonic speed area. So most sub sonic cruise missles will be round. Super sonic missles will be pointy. Jet aircraft that can reach super sonic speeds will be pointy to achieve that, even if it means less efficiency in their sub sonic cruising speed.

Air is a compressible gas whereas water is usually a non compressible fluid. The air density vs water density is also very different. With those two variables the dynamics at each medium are widely different, and hence you have the hydrodynamics and the aerodynamics.

Making a submarine very pointy creates a weak structure that will not be able to whitstand the compression deep under the sea, and it will also make the submarine hard to steer because of its relatively slow speed (compared to airplanes).

An airplane with a cigar shape is actually possible (as long as it has wings) and those type of designs are used for slower and bigger cargo planes. High speed jets require a geometry that allows them to break the sound barrier more easliy, and thats the main reason for the pointy geometry. Air is usually considered uncompressible at subsonic speeds, once the speed of sound is reached, the air resistance becomes exponentially more “thick” and the calculations for the aerodynamics start considering air as a compressible fluid. A plane can be made to be good at subsonic speed or good at supersonic speed, but not the two at the same time. Pointy geometry is good at supersonic speed.

Water is a completely different fluid than air:

Water has 1000 times the density, and is incompressible. You don’t really hope to cut through water. Just push it forward with a bulbous shape, and it will get out of the way better. What matters in water is to be pointy in the aft part of the vessel. Because water is heavy, you don’t want a sharp rear part, the sharp rear will pull water behind the ship and the ship will feel like it’s towing water.

Subsonic Airplanes have a similar shape, round front, pointy back. But with some major changes on the optimization of it. Let’s say, same logic but different curves.

Only supersonic flight has an advantage into pointy shape: the air will not get out of the way in time, you can’t push it forward, it is compressible and you are moving faster than the speed air can compress. So basically, your only hope to get through is to “poke a hole” in it.
(Its more complex than that, but whatever shape can cut through dirt would cut through supersonic air)

Edit: don’t try to get a “one-reason answer” to the problem.
Each shape is a trade off between “best at cutting, reducing friction surface, reducing cross section, and doing the job (a ship carries people, the fastest possible shape doesn’t allow for enough cargo capacity, or gives a bad handling. Same for planes). That’s why for each job, desired speed, handling, there are different solutions.