Brought to you by Budget Security Solutions, the company that replaced all your city walls with apartment buildings.
The heck of it is, things like this actually
work. For example, real world naval battleships had their fuel tanks at the outer edges of the ship, next to (and just below) the waterline. Reason being, that way if a torpedo exploded underwater against the hull, one of the first things it'd exploderize would be one of the gas tanks. Having a fuel tank rupture is inconvenient, but nowhere
near as bad as having hundreds of tons of seawater pour into your engine room.
Basically, the problem with a saucer is that if you try to armor the upper and lower surfaces uniformly, you expend a disproportionate amount of extra weight to protect the outermost edge of the saucer (as in, 50% of the mass of armor goes on the outermost 30% or so of the circular saucer, and 10% of the armor goes to the outermost 5%). Moreover, that part of the saucer is also very
thin. So all that mass of armor plate isn't actually protecting very much of the ship's volume, unless you just happen to be getting shot at from "edge on," with the enemy's ship being in the plane of the saucer*
It'd be much better from a defensive point of view if you could just skip armoring that part entirely, and use the same mass to:
1) Thicken the protective dish of armor covering the central part of the saucer from shots fired 'above' and 'below,' and
2) Put in an internal "tuna can" of armored hull somewhere
inside the ship, encircling the really critical parts of the saucer**.
Sort of like how with city walls (your analogy), you don't put JUST ANY city function inside the walls. Only the ones you foresee actually needing during a siege. Because the more area you enclose with a wall, the more expensive the wall as a whole becomes, and the more you have to reduce its protection in order to be able to afford a complete enclosure.
Although with city walls you have a problem a spaceship's armor belt doesn't have, because the enemy can theoretically use the city walls as cover to approach and scale the walls. There's no equivalent to that IN SPAAAAACE, of course.
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*(Now in that specific case the armor works GREAT because it's so heavily sloped compared to the incoming fire... but big chunks of ablative hull material that don't matter to the ship's combat performance would also work pretty well for that purpose, especially right up against the rim. Plus, the enemy is under no obligation to stay right where you want them to be if they're shooting at you, especially not in three dimensions if there are multiple ships involved)
**Weapon systems, any reactors and propulsion systems the saucer has, any antimatter stored in the saucer (there may be none), key sensors, the main computer, shelters for dependents and noncombatant science personnel to
crowd into during battle, and so on. The less critical stuff includes crew quarters, holodecks, big multipurpose rooms like movie theaters or the ship's gymnasium, deuterium tankage for the fusion reactors, cargo bays, a lot of the science labs, and so on.
This theory would add a sort of ironic note to
@Simon_Jester 's omake about the brittleness of the Constitution -- the designers would have put the nacelles high and clear to help preserve the long-term integrity of the ship. However, in the end the most structurally sound components turn out to be the nacelles due to their interaction with the warp field, and most of the rest is metallic dust held together by SIF and prayers.
I like it.
Leslie: [grunts] "Yeah, it's held together by the structural integrity field and prayer... and most of our officers aren't exactly what you'd call religious."
I'm pretty sure the Redshirt Preservation Society would throw a fit at that.
No, because
during alert conditions, the redshirts are all inside the armored hull manning important systems. They're not off sleeping in their bunks.
In the unlikely event that the ship is attacked while
not at alert and with most of the crew
not manning their battle stations... well, that's a very deadly situation to be in no matter what happens.
Leslie: "Hey, I don't know about you, buddy, but when the
Enterprise got attacked I was always on the bridge manning that one station, or the transporter room, or doing something
important. I wasn't asleep. So I'd rather have the armor protecting the parts of the ship I'm standing in, instead of the parts of the ship I'm NOT in, on account of not lying down on the job."