World builders in visual mediums will need to consider both internal and external ship structure, but authors may largely ignore this. Absent a schematic or picture, readers typically struggle to picture what we intend. This is true of other elements like the layout of a castle, for example. It’s often better to not be overly specific about where each room lies in relation to another but rather focus on how long or difficult it is for characters to traverse locations within a certain time frame that matters to the scene we’re writing. Still, even if we don’t explain the structure, we should have an idea of it if for no other reason than consistency between scenes. We don’t want characters to reach the engine room from the bridge in two minutes in one scene and in ten minutes during another.
There’s a tendency to invent aerodynamic-looking ships used in space where air obviously doesn’t exist. This is wise for two reasons: we expect such a design and are comfortable with it, and more importantly, it leaves open the option for the ship to enter a planet’s atmosphere even if this is rare. Would ship builders make that nearly impossible with a non-aerodynamic exterior? Only if certain the vessel never leaves space. Since we’ve seen everything from aerodynamic ships to the Borg cube from Star Trek, we can get away with anything regarding aerodynamics.
What other considerations are there for the exterior? One is the weaponry location, which determines which directions the vessel can fire in. Warships typically have this forward facing, with some ability to aim to the sides as well. Rear-facing weapons are for defense while fleeing. Some vessels have top or bottom mounted weaponry that can swivel and fire in all directions—except into itself, of course.
We might also consider where a ship can be boarded. Cargo is typically rear-loaded. People can be, too, or enter from the side, bottom, or even top hatches. Entering from the front is unusual. The decision can affect scenes where a ship has landed and a gunfight breaks out while characters are exiting or entering the vessel. The bottom-entrance seems problematic because if the ship crashes on its bottom, or the landing gear gives out, how does anyone get out? An emergency hatch elsewhere would be the answer. There’s always a solution for these problems, so feel free to do as you please.
The other obvious subject for external structure is the engine location, but this is typically rear-facing, especially for any vessel intended for atmospheric conditions. The engine doesn’t have to be in the actual rear, as propeller planes make clear. Even jet engines can be like this. With laser-guided weapons that can easily target an engine, it makes sense to avoid engine locations that are vulnerable in this way.
From the previous section on propulsion, we should remember that there are fictional drives that don’t operate on the principle of rear thrust; such engines could theoretically be located anywhere, preferably deep within a ship. What this means from a practical standpoint in war is that engines for STL travel might be rear-facing and vulnerable to attack, but FTL engines might be better protected and less vulnerable. Destroying those STL engines has a tactical advantage to inhibit maneuvering in the battle but won’t stop the ship from going to warp to escape, for example. But if something volatile powers those FTL engines, then placing them deep in the ship might be unwise.
A ship that uses rotation to create artificial gravity will have this as the dominant feature of its exterior. The outside doesn’t need to be round, however. It just needs to rotate. It’s the inside that will be curved to some degree mostly because people and items will be pushed against those interior walls (that’s the whole point), which act as the floor. Gravity increases the farther you are from the point of rotation. This is why we often see a configuration that looks like a spoke wheel; almost all of the living areas are far from the center.