“There is absolutely nothing preventing you from having ships the size of planets in space.”

To be fair, this was in the context of science-fiction space fleet design where Warhammer 40K came up on multiple occasions. Since we’re very clearly thinking about fiction rather than speculating on an actual future, as long as readers buy into (or don’t notice) the inconsistencies with our perception of reality, anything is fair game. On the other hand, if your fiction tries to take itself seriously on a technical level, it invites readers to take it seriously as well, and those multitudes of nitpickers will find out what doesn’t make sense.

Before I get into it though, let me draw your attention to one of my favorite quotes:

The only thing a proof of impossibility shows is the lack of imagination of its author.

I don’t expect to be exhaustive, and quite frankly can’t be bothered to expend a huge amount of creativity here. Let’s begin…

There is absolutely nothing preventing you from having ships the size of planets in space… except…

Hydrostatic Equilibrium

Whoo boy, this is a big’un! Sci-fi fans are often keenly interested in astronomy, and some (lookin’ at myself here) will be able to recite the “recent” International Astronomers Union’s definition of a planet by heart. Among my favorite words in that definition is the title of this section, what a coincidence! Hydrostatic equilibrium is a symptom of an object being “big enough” to qualify as a planet; it occurs when an object has enough mass for its own gravity to deform it—and it almost doesn’t matter what it is made out of. Long story short, a planet-sized space ship will crush itself into a ball of molten scrap.

You could get around this by manipulating gravity. I’ll say it now, but won’t repeat it every time, but gravity-manipulation is incredibly energy intensive as far as we can tell. If you want gravity plating and warp drive, you’re very likely going to have to look outside of general relativity for solutions. In this case, if the parts of the ship are gravitationally isolated from one another, then it can’t pull itself into a ball under its own mass.

If you go the route of diamond-reinforced unobtainium cortenide, then what good are bullets? If your mass drivers can’t penetrate deep into the Earth’s mantle, what hope do they have of making it past a material that can hold up a ship that otherwise would become a moon in a few minutes? Remember: having magic materials changes everything, not just construction.

Thrust-to-Weight and Attitude Control

Obviously, you can’t just increase the size of a ship without also increasing the size or number of thrusters, not if you want it to maneuver as nimbly. Of course, to add thrusters, you need surface area, but it you want to keep the shape the same, you run into the Cube-Square Law.

If you double the linear dimension of a shape (i.e. double the length, width, and height), then you quadruple the surface area and octuple the volume. For thrust-to-weight, that means that as the ship size increases, the increase in mass (linearly related to volume) will outstrip your ability to add more thrusters.

The obvious solution is to change the shape as well as the size. Stretching out only the width or only the width+height while leaving length alone will allow you to keep pace adding thrusters t ocompensate for the extra mass. You could also experiment with fractal designs (think “shaped like tree branches”) which can give even more surface area than volume as they grow; I’d actually love to see something like that in fiction!

Unfortunately, what happens when you need to turn your ship to face another direction? The longer your ship is in any direction, the harder it will be to turn around a perpendicular axis. What’s worse, this problem actually gets out of hand way faster thrust-to-weight ratio! It’s all thanks to something called “moment of inertia”, which is just like mass, except for spinning motions instead of translating motions. You don’t learn about it until you get to dedicated physics classes, but you can bet your publishing deal that your sci-fi nerd audience has had a few of those.

The larger your ship is, the worse the trade-off becomes between “can’t move” and “can’t turn”. At some point, your ship will be a sitting duck no matter how you slice it.

Again, gravity manipulation could be the way out: as far as we can tell, inertial mass (as we are talking about here) is exactly the same thing as gravitational mass (from the last section). Make the mass magically disappear, and neither mass nor mass distribution will be a problem!

Another possibility is some kind of propulsion system that can be mounted internally. None of the thrusters that we have today, or can even theorize about can be put inside a closed room inside a ship; it’d be like putting a fan in the cockpit of a jet fighter and expecting it to help push the plane. However, a “reactionless drive” mechanism like EM thrusters or warp drive (if it works the right way) actually could take advantage of the larger interior volume to keep up with adding thrust. Be warned, thanks to Noether’s Theorem, you might be breaking more fundamental and wide-reaching laws of physics than you ever considered possible!

Any Dust Will Burn

The other thing that surface area affects is weapon placement. Less surface area, fewer places to mount guns. If you want raw dakka, it’s likely more cost-effective to build many ships which could collectively surround an enemy and catch it in a crossfire of your own tactical genius.

Honestly, I don’t have much of a solution to this one. I mean, you can use a fractal design to get more surface, but you still won’t be able to do flanking maneuvers with just one ship.

Internal Component Stresses

So you’ve just turned on the maneuvering thrusters, ready to begin your giant space tree’s maiden voyage and… all of the thrusters tear off the pointy bits and your bulk careens into the side of the shipyard causing even more damage. You see, when a thruster kicks on, it only applies a force to a small part of the ship. The rest of the ship only moves because of all the structural members (e.g. steel beams) that distribute that force around. The only problem is that there’s only so much force that even can be distributed before structural integrity is compromised. It’s actually the same problem that limits the size of aircraft wings: more wing area, more lift, but also more torque on the wings, which eventually causes them to tear away from the fuselage.

If you can come up with a design that distributes the force of maneuvering evenly across an entire planet-sized ship, more power to you. But honestly, I’d reach either for my unobtainium, or for inertial dampeners—my favorite gravity-manipulation technology.

Failure Mode Engineering at Scale

The closest thing we humans have to a planet-sized machine is probably the power grid. The thing is, when the grid fails at one point, that can cause cascading failures. Imagine supplying a planet-sized volume with not only power, but personnel, atmosphere, ammunition, communications, fuel… the list goes on. Even if the cascade stops at some point, can your planet-sized ship operate effectively with part of it “dead”? In a war zone? Where the enemy objective is to cause your equipment to suffer as many failures as possible?

You can fix this by making many smaller ships; that way even the worst failure can only take out one of your bazillion ships… oh wait that’s not a solution! Uh… make your ship invincible with unobtainium armor… but somehow make it so smaller ships and bullets can’t be made of the same material? Honestly, few people will think about this problem. The kind of people who catch this will probably answer that the worst thing that could happen if they go on a vacation is that “I leave the stove on and my house burns down, and also I die in a car wreck.”

Unfortunately, people might pick up on the related…

Strategic Issues

The problem of having one big ship is that, if it isn’t literally invincible, the enemy will spend fewer resources to make a gun that can bring it down. Maybe big static guns will even be several times cheaper than your flying broad side of a barn, so they’ll just deny you the ability to operate in the most key locations. Sure, you could still use your big ship defensively, but then why didn’t you just build the cheaper static installations? It doesn’t matter, if you want to get past the enemy Big-Ass Gun, you’re gonna need to exploit its weakness: only firing on one target at a time. That means—you guessed it—many smaller ships. At the end of the day, one big ship is one big, juicy point of failure to be exploited by hostile forces.

The problem of having one big ship is that it can only be at one place. My guess is that interstellar and even interplanetary war will require attacking and defending multiple strategic objectives. It’s too bad you spent all your spacebucks on one big ship and the enemy is now attacking a hundred critical locations. If only there were some way to defend multiple locations at once… maybe a bunch of ships? But they’d have to be smaller if you wanted to build them all…

The problem of having one big ship is that the enemy destroys it while it’s still under construction. If the enemy focuses on smaller ships, they’ll come off the assembly line and go out into combat long before your investment pays out the fully armed and operational battlestationship you were expecting. By the time the minimal systems are online, enemy ship production has overrun everything in your empire.

Of course, the solution to all the strategic problems is to make planet-sized ships normal rather than unique. Readers might wonder where all the resources are coming from to make these fleets of behemoths, but there’s a worse issue for fiction. The reason to put a big ship in your world is to add a point of interest, but if being big isn’t unique, then you’ve lost the point of interest you were trying to add.