Why can't spaceships go underwater?
It is a somewhat common limitation of spaceships in some universes, that they cannot go underwater. They can land on the planet, take off into orbit, hyperdrive into the next star system, but they cannot dive in water. A different vehicle is required to go underwater.
But exactly why is that so?
In the story, the characters go into the spaceport with their starship, but they need to acquire a submarine to go to an underwater city. I am looking for a rationale on why they can't just go there with their spaceship.
P.S.: If you are going to frame-challenge and prove that spaceships can go underwater, please explain throughly.
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1 answer
Spaceships are not supposed to withstand a lot of external pressure
Have a look at our sister site Space.SE and the question Do spacecraft have similar structural integrity requirements as submarines? for some information about this problem. Here are a few quotes relevant to this question that come from the answers:
Next, the orbital craft. To get there it must be be light. A mild kick could have pierced Apollo's walls, ISS is more sturdy but still it's to withstand 1 bar pressure difference towards the outside (that's equivalent of mere 10m submersion depth), and again - towards the outside, that means no need for cross-beams to prevent buckling; it has natural tendency to bloat like a balloon.
The basic problem is that a submarine is supposed to go quite deep and withstand the pressure of all the water and atmosphere all around it - while a spaceship is supposed to withstand all the pressure from within from going outside.
Spacecraft are designed to contain internal pressure of not more than one atmosphere; submarines are designed to withstand dozens of atmospheres of external pressure.
The difference in requirements are big.
The structural members of a spacecraft hull are predominantly operating in tension, at a significant fraction of the material's yield limits, and the most likely failure mode would be tensile fracture. The structural members of a submarine hull are predominantly operating in compression, and the most likely failure mode would be buckling.
For some interesting twist on your question you might also want to check out a question on WorldBuilding.SE: Would a submarine make a great spaceship?. The answers give some interesting insight into problems with the other way around, such as risky pooping (I think that's the biggest and by far most hilarious problem outlined in the thread).
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