The title of this article is a sentence fragment. And the contents of this blog is a continuation of my first blog, i.e., im going to explain some more misconceptions about spaceships. The intro this second time around is short.
The first two problems boil down to the fact that spaceships travel in space, and space is different from an atmosphere, or the ocean. For example, the only way a spaceship can travel is using a jet engine, or burning small amounts of fuel to shoot large amounts of stuff, or "propellant", in the direction it wants to go. Unfortunately, spaceships can't use the air as propellant like jet fighters do. So they need to carry there propellant with them. A lot of it. In fact for a spaceship to have enough delta-v (delta velocity, or how much a spacecraft can change its velocity before it runs out of propellant) to get around the solar system, two-thirds of its mass should be propellant; the remaining one-third is everything else. On a related note, tanks of propellant would be either spherical, because they have the least surface area for the most volume, or cylindrical, because they also have a decent surface area to volume ratio, but are much easier to build than spheres. In fact the whole spacecraft would probably be one of these two shapes.
The second problem pertaining to spaceships travelling in space, also is directly related to the fact that there isn't much stuff in space. I know that many of you have heard all about the "cold void of space" and junk, but in reality, that's not quite relevant. In fact the temperature problem with space is that things heat up too quickly, because unlike earth-based vehicles, where we can harness the power of conduction to carry our heat away, in space there's nothing to conduct our heat into. In fact the only way to get rid of the heat is through radiation, which is a terribly slow process. This implies huge sheets of metal that we dump the waste heat into, so it can slowly fly away in the form of electromagnetic radiation. Like the kind your computer is now beaming into your eyes. Or you could use coolant, and shoot the coolant away when it gets too hot, but that just adds to the already-difficult-to-make-good propellant mass to everything else mass ratio.
I would do more but this got really big, really fast (that's what she said) so I'm gonna end it here, so seeya later.