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For the purposes of this question, let's say that a "worldship" is a starship (constructed by a civilization in the process of moving from K2 to K3, in my case) that is so large that it doesn't need artificial gravity of any sort, because its own mass produces a grav-field strong enough to make exercise machines, spin-gravity, etc. unnecessary.

This civilization's worldships are generally a bit smaller than Titan and a bit less massive, constructed to haul unimaginable amounts of resources between machine worlds as well as act as palaces for the elite of the civilization. Regular propulsion is virtually impossible for a ship this large, and if you did it would screw things up - it relies on its own mass for gravity, and if you meaningfully accelerate it it would throw people off of one side and crush people on the other. The civilization, however, has expanded across the galaxy using FTL warp drive technology, relying on using large amounts of metastable quark matter and extremely precise calculations to generate wormholes right on top of the ship, allowing any ship equipped with a warp drive to jump anywhere in the galaxy in a matter of seconds in the case of interplanetary travel and a matter of hours for interstellar travel (intergalactic travel is rare, but usually takes a matter of weeks).

The warp drives as well as life support systems for worldships take a massive amount of energy, so they run on black hole reactors at the center of the ship that contribute to much of the mass. A constantly-evaporating black hole is continuously fed interstellar gas and other miscellaneous matter that the ship collects, and its Hawking radiation is collected as an energy source of arbitrarily-many exawatts: to increase the power output, they stop feeding it matter for a moment to increase its temperature, and to decrease the power output, they feed it more to decrease its temperature.

Here's the issue I'm having: me, my readers, and many characters in my book would rather not fly around the cosmos in a big orb, because that's just not good enough. Worldships can take up to a century to build and cost quintillions of dollars, and are always subsidized by the civilization's Empire and are owned and operated by the most wealthy and prestigious individuals in the government. So they look more like this -

Citation: https://www.pinterest.com/pin/765471267900657920/

Citation: https://www.pinterest.com/pin/765471267900657920/

-except way larger. In other words, they don't look like planets, they look like really big starships, even though they retain an atmosphere and surface gravity like a planet and can only move around via warp drive.

The people in the government who the Empress blesses with permission to build and own worldships are reasonable people. They know that setting up the structural components and making sure it doesn't collapse in on itself and building all sorts of counterweights and everything so that gravity is still consistent is an enormous pain and requires additional billions or even trillions of worker-hours. On top of that, building the additional warp drive infrastructure to bring the whole worldship elsewhere instead of just a spherical chunk of it is also a painful process, and all these additional difficulties add quadrillions of dollars to the cost of the worldship as well as millions of worker's lives; accidents might only happen once every million thousand worker-shifts when super-tight security measures are in place, but when you have billions of workers going at any given time, thousands will die every hour.

So: what is an actual plausible reason that the worldships of this civilization are shaped like starships instead of the much more economically-feasible spheres?

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    $\begingroup$ So a starship should be shaped like a star? $\endgroup$
    – avpaderno
    Commented Jun 15 at 10:10
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    $\begingroup$ @apaderno it's maybe fairer to say that anything with enough mass will start to reshape itself into a sphere, due to its own gravity pulling on its different components differently. $\endgroup$
    – Jeremy Friesner
    Commented Jun 16 at 13:48
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    $\begingroup$ @JeremyFriesner That happens to stars and planets, but I hope that a ship or a satellite does not reshape itself into a sphere. Stars are spheroids of plasma and planets are formed from large molecular clouds; in both the cases, it is clear why they are spheres. (Anyway, my previous comment was joking about "since they are called worldships, they must be shaped like worlds.") $\endgroup$
    – avpaderno
    Commented Jun 16 at 15:40
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    $\begingroup$ "to decrease the power output, they feed it more to decrease its temperature" - this sounds like a runaway catastrophe waiting to happen when they can't feed it enough matter $\endgroup$
    – Bergi
    Commented Jun 16 at 17:38
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    $\begingroup$ @apaderno anything with enough mass to supply its own gravity will pull itself into a sphere. Not just fluids: Metal and rock melts under gravity stress. It will take extraordinary engineering to overcome that tendency. $\endgroup$
    – SRM
    Commented Jun 17 at 3:08

9 Answers 9

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The ship is long because nearest the black hole there's lots of gravity, too much really, so only robots go near it. Most people live far away from the black hole so they are in a sweet spot of gravity. This naturally requires a long rather than spherical shape, with the black hole at one end, repulsors that push against the black hole to hold up the rest of the ship, and a network of structural supports between the black hole and the lowest inhabited decks.

The long shape also allows for variations in gravity. The poor people mostly live in the lowest >1g decks, which are put there because otherwise there would just be wasted space in the structure, but it is uncomfortable to live that close to the black hole. Many richer folk prefer 0.8g, and the infirm often migrate to the highest decks with low g to ease joint pain. The low g upper decks are also a favored vacation spot for enthusiasts in recreational flying.

There were some comments asking why not build a full sphere around the black hole anyway, since that would produce more living space. True, it would add more living space. However, maybe the long ship already has space for 5 billion people, which would be sufficient, and a sphere would have space for 500 billion people, which would be unnecessary and too expensive to build.

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    $\begingroup$ This doesn't really explain why it wouldn't then be built into a sphere from that longer shape - doing so would just provide more room at any given gravity. $\endgroup$
    – Bobson
    Commented Jun 16 at 12:30
  • $\begingroup$ @Bobson Depends on the budget and the space required, relative to the budget for black holes. If you need even more living space per black hole than a giant long ship would provide, then sure, you could make the living-space end of the ship fatter and fatter until you've built it all the way around the black hole into a sphere. If you don't need quite that much space, then you leave it cigar-shaped. $\endgroup$
    – causative
    Commented Jun 16 at 15:46
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    $\begingroup$ Wouldn't this also result in a more teardrop shape(not bad, just interesting)? Since the space near the black hole is unusable due to high gravity, so you would put just enough support to support the larger section that is farther away. This would lead to a teardrop shape with the pointy bit having the black hole and the bulky bit having all the livable space. $\endgroup$
    – Bubbles
    Commented Jun 16 at 20:10
  • $\begingroup$ But the warp field will be spherical. Seems like the builders would be wasting a huge amount of space within that sphere if you don’t build other stuff on those other sides of the sphere. $\endgroup$
    – SRM
    Commented Jun 17 at 3:16
  • $\begingroup$ @SRM Already answered this in previous comment. Since I'm getting two comments about this I added an addendum to the post. $\endgroup$
    – causative
    Commented Jun 17 at 3:32
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If your ship is large enough in mass to create it's own gravity, then that gravitational force is going to tend to point to the center of the mass of the ship.

Having traditional deck structures would be really weird, feeling like walking up hill if you're moving away from the center of mass, and downhill towards it.

A spherical design makes the most sense in that case.

Just hand-wave artificial gravity and you can make your world ships however you want. If you have FTL, artificial gravity shouldn't break suspension of disbelief.

Most scifi with spaceships do this anyways, since filming actors in actual 0g is not cheap, and simulating 0g is not fun (or cheap).

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  • $\begingroup$ I think your answer starts right, but draws the wrong conclusion. Just because floors/ceilings need be perpendicular to the direction towards the center of mass doesn't mean you end up with a full blown sphere. $\endgroup$
    – Matthieu M.
    Commented Jun 15 at 10:10
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    $\begingroup$ You can only use part of a sphere, aka align decks to be perpendicular to "down" direction, but the decks themselves don't have to compond the whole sphere. Just a couple of brackets O)))))) would form the long "nose" of a ship while fulfilling the constraint of "level" floors. $\endgroup$
    – Vesper
    Commented Jun 15 at 18:22
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Everything that produces or consumes electricity generates waste heat. This heat, unless carefully controlled with a system of coolant loops and exterior radiators, will spread out through the ship and ultimately exit through the hull, warming the entire ship up to an equilibrium temperature in the process — and this temperature may be far higher than the crew and passengers can withstand.

There are, strewn throughout this vehicle, all kinds of heat-generating devices: not just the main reactor, but engines, workshops, scanners, weapons, gravity systems*, air conditioners, freezers, lights, elevators, the crazy-high-speed trams you'd need to move things across a thousand kilometers of spaceship...

On Earth, we have few enough of these that the heat (plus solar heat) typically goes into the air and Earth then loses atmospheric heat to space. But in a closed system that might run much hotter, physically speaking, than a planet's atmosphere, all of these things need to pump their heat, directly or indirectly through additional cycling systems, into the ship's coolant loops, which then need to take it out to the radiators to pump into space so everyone isn't cooked. Coolant loops, however, are most efficient when they are physically small.

Shaping the vehicle to minimize the distance between any given heat source and the surface (where the radiators are) will result in it having much more surface and much less depth than a sphere (perhaps the worst of all shapes for this purpose). One would imagine a long thin linear vehicle, or a large pancake. To constrain it to a more blocky shape as shown, you could then enforce a minimum width depending on the size of the largest unmovable component (maybe the reactor) or the size of expected cargo objects.

*To be clear, the gravity on an irregular-shaped vehicle will be wonky. A sphere of radially-symmetric density below you is mathematically equivalent to a single point, but if you're somewhere off-center inside a giant rectangular block, the net gravity will probably be somewhere towards the block's center of mass, offset by the relative density of large regions near you in various directions. There will be no uniform "down" across the whole ship — if you want anything vaguely constant, it will need either handwavy antigrav or to be one giant cylinder that rotates and keeps everyone under the same circular acceleration.

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  • $\begingroup$ I think combined with abestrange's answer demonstrating that floors/ceilings need be on a spherical plane centered on the center of mass, we get where we need to. for example, to maximize surface, and minimize distance to surface, you'd get something like an "urchin" design. A small sphere at the center (black hole & equipment) and spires rising radially from there, covered in radiators. Disks could work too, and you could perhaps wrangle a few disks a few degrees apart, etc... there's a lot of freedom even obeying both constraints. $\endgroup$
    – Matthieu M.
    Commented Jun 15 at 10:14
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    $\begingroup$ There is another constraint that can dictate ships being bulky: rigidity. A ship the size of Titan with its own black hole in its mass center tends to fall on that black hole, and in order to prevent it from crummpling certain tension, compression and weight distribution elements would need to exist all around the ship's hull, forcing any protruding shape to actually have a wide foundation to not bend and fall. The segments holding radiators would also require to not fall "down", worse, their length would be enormous, so space ship architects just have to build them bulky. $\endgroup$
    – Vesper
    Commented Jun 15 at 18:17
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Multiple Smaller Black Hole Reactors and Active Support

This is gonna be a bit of a creative answer, but you did put the science-fiction tag, so I'm using gonna try using fairly reasonable science.

So the reason planets are round is because gravity is concentrated in a single place. Your black hole reactors generate similar gravity.

But when we look at Black Hole Energy Capture, the smaller the black hole, the more energy it produces, although it increases instability massively as well as well as shortening its lifespan, as the 'energy' emitted is basically a result of its evaporation. Speed up the evaporation, speed up the energy capture process.

So instead of having one black hole the mass of a planet, you have several, and you put them near key engines and reactors inside the ship for redundancy, running a decentralised power grid. After all, if this thing is the size of a planet, you don't want one big giant reactor that can be blown up (take note Star Wars) or damaged for whatever reason. In fact, counter-intuitively, a big one would give you way less power, although it'd live for far longer.

Each black hole would generate its own gravity field, and you'd probably want to intersperse them around the ship for minimal interference. This also allows you to have Low-G sections of hull if needed where the distance from a certain 'Reactor' is great enough to not matter. By this logic, you would have a somewhat 'bulbous', but otherwise non-spherical ship, and there's no reason why you couldn't extend Zero-G hull sections or not build in High-G areas, you could hand-wave that as launch tubes and docking bays for smaller vessels or similar things, or anything else you fancy.

This is finicky, and relies on you having monstrously good technology, but if we're talking about transitioning from K2 to K3, it's probably a non-issue.

The next option is active support using orbital mechanics. There's a very good video by Isaac Arthur on how this works and what you can do with it, which I'll link below.

Functionally speaking, you create a stream of matter at orbital velocity a given distance from a barycenter, you then sheathe this matter in a vacuum-sealed tube that is then suspended in orbit. No artificial gravity needed to hold it up; it is using the gravitational force of the existing body, and you can even speed it up to create outward pressure, which can then be used as a support strut for key sections of hull. This orbit doesn't have to be spherical either, it can have any eccentricity, as long as it can sustain the orbit. This can allow for 'egg' or 'oval' shaped hull beams, for instance, which the ship could layer or wrap to make smoother but not purely ovoid shapes from.

Neither of these two techniques use artificial gravity (unless you count creating black holes as artificial gravity), and used together they could probably do the trick in breaking up a ship from being a Death Star to something more 'ship' like, although I'd wager it might look smoother or sleeker than the picture, although at the end of the day, you could build it however you like.

https://youtu.be/5QLOAQmZbZs - Space Towers and Supertall Structures - Isaac Arthur

https://youtu.be/oAocMzxPjjo - Black Hole Ships - Isaac Arthur

https://www.vttoth.com/CMS/physics-notes/311-hawking-radiation-calculator - Viktor T. Toth's Hawking Radiation Calculator (also has calculations for gravity and energy output)

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  • $\begingroup$ If the flow of matter into the Black Hole is interrupted for any reason, the Hawking radiation rate of production will grow out of control, similar to a meltdown on a nuke plant. Having multiple Black Hole power sources gives you redundancy to ensure you always have electrical power to push matter around and keep the Black Holes happy. Redundancy is good. $\endgroup$
    – codeMonkey
    Commented Jun 17 at 20:40
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A thinner ship needs a smaller wormhole.

I am imagining your ship generates a (circular) wormhole in front of itself, and that (for some reason) the ship and the wormhole have some relative velocity so that the ship then passes through the wormhole head first. The wider the ship is the wider the wormhole needs to be, the longer the ship is the longer the wormhole needs to last.

You can handwave this trade-off however you like. If making wormholes last longer is super expensive in energy, calculations or whatever, but making them wide is cheap, then efficient ships will be more pancake shaped. Oppositely, if making the wormholes wide is expensive, but making them long-lasting is comparatively cheap the ships will tend towards a needle-like shape.

If you go the "needles are efficient" route you get what you want. The overall shape compromises between long and thin (good for wormholes) and wider to accommodate crew and cargo etc.

More interesting shapes can be further motivated by the wormhole system. For example, I like the idea that the way the ship moves relative to the wormhole is that it projects the wormhole from "arms", and it then pulls the wormhole down over its head with those arms. (Imagine a person pulling a hoola hoop over themselves). This would give you weird water-boatman shaped ships.

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Force is a vector and vectors are always straight lines, duh

Physical laws of universe arent changed in your setting. Your civilization is still bound by them.

Your starship is still a vehicle and need energy to move. The thing that move a mass is, by definition, a force.

Consider an L shaped structure with the lower length side being extremely shorter than the other (larger length) side. Also consider the smaller length side is where the force to move acts on. The ship will move this way:

  <- <- <- L.....

The corner of L has to be extremely rigid. The shorter side of L wants to get away from the longer side. The longer side is just dragging on because of the rigidity of the corner.

At turns especially the corner has to face extreme forces and its prune to be broken.

A sphere also has this problem, though at smaller scale but still an unnecessary one. Where will you put the engine and where will its exhaust go?

The best you can do is place engine right below equator (in a line whose other end is center of the sphere).

The force being straight line still want to take only the equatorial line with it and nothing at north or south of the equatorial line. These areas have to drag along because of the rigidity of the sphere.

If the sphere is made of a gas nothing of it will move except for a brief moment parts directly above the engine. The engine will leave the sphere behind, breaking away from it.

In a solid sphere the whole sphere will move and thats because of the rigidity of the solid. This works but at great cost of inefficiency and at risk of having the engine break away from the sphere.

Now consider this shape:

<- <- <- _ ......

The ship is long, very long as compare to its width. Its engine is at one end, the exhaust going cleanly away from the entirety of ship. All the engine- produced force vector is align with entirety of ship.

Ofcourse with ship being extremely large the tunnel which is the ship has enough breadth to have sufficient gravity (real gravity, not "artificial gravity" which is just centripetal force) for comfortable living of the inhabitants.

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    $\begingroup$ Slight issue... the ship doesn't have engines. It basically teleports to its destination with wormholes. $\endgroup$
    – Bubbles
    Commented Jun 16 at 20:13
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    $\begingroup$ That's the thing... It doesn't produce a force. It pokes a hole in spacetime and makes a direct path to its destination. The ship never moves, it just goes from being where it is to being at its destination. It never actually applies any force. $\endgroup$
    – Bubbles
    Commented Jun 17 at 4:35
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    $\begingroup$ My body consumes energy while I'm sat in my chair typing this. I am not moving in a direction, unless you count the motion of the Earth itself. $\endgroup$
    – Sod Almighty
    Commented Jun 17 at 11:34
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    $\begingroup$ @Atif the energy being used isn't creating a force, it's warping space-time to make a shorter path to the destination(in this case, one with negligible length). You are assuming that consuming energy always results in a net force, but this is not always the case. Some examples of this include; Our bodies, light bulbs(or really anything that emits in the EM spectrum), things that result in a force in opposite directions, canceling themselves out, most chemical reactions, etc etc. $\endgroup$
    – Bubbles
    Commented Jun 17 at 19:37
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    $\begingroup$ @Atif Even if what you are saying is normally true, in this case, the question explicitly states that this is not the case in this scenario. Since the question has specifically stated that this is not the case, and is not tagged "hard-science", that means that there being no force-vectors applies to the whole thing is an indisputable fact for the purpose of the question, irrespective of whether this is true in general/real life. $\endgroup$
    – Bubbles
    Commented Jun 17 at 19:42
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Liability and Investment Risk

Those quadrillions of dollars are not idle piles of cash that the Empress or their cohorts have lying around. It takes a lot of time and money to build this ship, and you want to put that oversized blackhole generator how close to it? Have you even read what happened to the last ship that had a malfunction? And don't get us started on how much was paid in compensation to the families of the millions which were rapidly spagettified by the drive implosion.

I'm sorry, Brilliant Starship Engineer, but our Department of Not Losing Money has tied your hands on this one. I don't care how beautiful your orb looks, or how practical it is; we aren't releasing your funds until you give us a nice, safe design that clearly puts your omega level reactor at least a few hundred deca-units away from the more expensive pieces of this endeavor. If we can't safely jettison them away at a moments notice, we aren't interested. And we'd rather eat the cost of a 5-10% efficiency loss over a 100% loss of an entire ship and crew in another 'incident'.

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  • $\begingroup$ Interesting take. The Not-Loosing-Money department would certainly only increase the risk of losing money this way, but it is not a rational department. Jettison from any spaceship is usually a fantasy since it would only decrease your chances of returning home in once piece. (Exactly how are you going to create the wormhole home without the massive generator?) But again, rationality is out of the window with the NLM department. $\endgroup$
    – Menno van Lavieren
    Commented Jun 20 at 13:28
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It wouldn't

"is so large that it doesn't need artificial gravity of any sort, because its own mass produces a grav-field strong enough to make exercise machines, spin-gravity, etc. unnecessary."

If it's own mass is large enough to generate 1G worth of gravity then it has the same mass as the Earth (despite the size difference). Now assuming the mass is coming from the black hole, everything will be pulling towards the centre of the hole. Now for a nice even gravity, the ship would have to be a sphere. If the ship was ship shaped, the areas nearest to the black hole would be nice and even but either end would be like climbing a mountain. This would create all sorts of engineering stresses and would be awkward and uncomfortable for the crew.

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Frame Challenge

If they have the ability to move black holes around, they have sufficient mastery of gravity that you don't really need explain how it works to readers.

If you actually want ship shaped ships, they don't need to be so massive. If you get rid of the people, you don't need to worry about life support, gravity, acceleration etc. Store the people digitally in a virtual world. When you arrive at your destination, you grow new bodies for them and transfer their minds back. For anything that needs doing on the ship, people get downloaded into avatars on the ship. These bots don't need air, food or gravity. Once the job is done, the mind is transferred back to the virtual world.

This way, you can have whatever shaped and sized ship you want and it still be a world ship.

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Just Handwave

Sometimes I think people come to this site just to revoke their artistic licence.

You've already built a spaceship bigger than the Moon, capable of making its own wormholes and FTLing itself around the galaxy - a feat that's already unimaginably difficult with countless scientific and technical impossibilities and unsolvable engineering challenges, and the one little thing keeping you awake at night is the shape of it?

Just say it's made of Scifinium - a fictional super-hard material that can hold its shape in spite of its gravity, and move on.

Of all the difficulties that would have faced the designers of this spaceship, how to keep its shape is one of the easier problems to believe they "just solved it".

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  • $\begingroup$ Normally I don't like the -just-handwave- type of answers. But I can't argue with wornholes/FTL >>> structural design, in fantasy grading. $\endgroup$
    – Menno van Lavieren
    Commented Jun 20 at 13:31

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