# How big could a space-ship get while still being plausible?

Background: In a society where human beings have lived among the stars for tens of thousands of years, they would have accumulated a lot of junk, so they just decided to throw a lot of it onto uninhabitable rocks near colony planets for easy disposal. Kind of like the galactic version of a landfill.

My characters are salvagers, meaning that they go down to these junk planets and bring anything remotely valuable back with them. I want their base to be very large but still mobile. Most of it would be taken up by containers of junk they collected, things to be sorted to see if they're valuable, and things that are valuable if recycled in large quantities. Essentially most of it is just a series of large compartments that act as warehouses, with some recyclers that are either advanced and run by actual people (metal, antiques, jewelry etc.) or very basic and run automatically (pipes, structural supports, broken concrete, etc.).

I want to know how big such a ship could get, because that will tell me how much junk they can handle at any one time and give me an idea of how profitable they would be. It would also give me an idea about how large to make the crew, since my characters only make up roughly twenty or so of the crew.

Currently the ship is roughly the size of a small moon, just a little bigger than Deimos.

The main constraints I already know about are: mass, acceleration speed, construction, expense, and landing.

Mass and acceleration are sort of tied together. It takes a energy to accelerate and decelerate in space, and the more mass something has the more energy is required to do so. I want to know how much energy such a thing would take. Do I need to create a new energy source for this in order to make it feasible? And is hand-waving the energy source acceptable? Acceleration for this ship doesn't really need to be all that fast, although it does need to be able to travel between solar systems in (at most) a month or so.

The construction of the ship would've been enormously expensive, and it would've had to be constructed in space from the outset. No way is it possible to build something that large on a planet. Possibly a shipyard that manufactures everything and puts it together? I was thinking that perhaps asteroids and uninhabitable planets were pretty much looted of all their ore, and then the ore was refined into metal that the shipyard uses to directly build the ships. Seems reasonable, provided the technology is there. The cost to build and maintain the ship would be enormous, but since my characters are mostly low-level grunt workers, I can probably explain it away as the corporation paying for it.

Finally, since there is no possible way such a large ship could get close to a planet without causing a potential apocalypse, let alone land on it in order to salvage valuables. Destroying the very valuables you are there to collect by your very presence is a major issue. So, the ship would probably need to have a hangar that can send shuttles down to carry the cargo back up. They wouldn't be small due to the large cargo area, but it would at least be possible to land on planets so they could gather the materials.

Did I miss any major problems? Should I make the ship smaller?

• What tech level do you have in mind? Star Wars, Star Trek etc? Commented Jun 29, 2020 at 16:33
• Does your civilization have faster-than-light travel? If so, they have a propulsion technology way beyond anything we know, and you can hand wave acceleration and deceleration costs. Commented Jun 29, 2020 at 16:56
• Such a large ship probably would not be allowed anywhere near Earth. It would effectively constitute a very large thermonuclear weapon in orbit. All it would take would be for someone to press the wrong button or for terrorists to take over the ship and the ship might be decelerated into Earths atmosphere. Such a large object would not burn up and it could easily kill hundreds of millions of people upon impact. Commented Jun 29, 2020 at 17:24
• If the ship is the size of a moon, how do they find and collect enough junk to fill it? They have to find dozens if not hundreds of tons worth of material per minute for months to fill it to an appreciable level. Also think of turning this thing. I think you are better using a type of barge system, think Alien. "Small" ship or several ships push the object, and rather than turning it the ships detach and grab the target on the right angle to push it or slow it down. Commented Jun 29, 2020 at 18:05
• If the ship is large enough, it will attract space junk on its own. Commented Jul 1, 2020 at 15:50

Here’s a list of potential issues:

• The gravitational force that the ship exerts upon itself, possibly leading to collapse
• It takes a lot of energy to accelerate a ship that large
• A ship the size of a small moon poses considerable risk to the planet it’s orbiting

Now let’s consider these problems one by one.

Let’s conservatively suppose this ship of yours is approximately the size of Deimos with a mean radius of $$6.2\space\text{km}$$. However, its mass will probably be much smaller than that of Deimos, since it will presumably contain a lot of empty space. Since Deimos’ mass is about $$1.5\cdot 10^{15}\space\text{kg}$$, we might estimate (again, conservatively) that, after being filled with junk, your ship is about $$1/100$$ as dense on average, giving it a mass of $$1.5\cdot 10^{13}\space\text{kg}$$.

Possible ship collapse

Good news - the surface gravity exerted by this ship is tiny: $$g_{\text{ship}} = \frac{GM}{r^2}\approx \frac{(6.674\cdot 10^{-11})(1.5\cdot 10^{13})}{(6200)^2}\space\text{m/s}^2\approx 2.6\cdot 10^{-5}$$ So you probably don’t need to worry about it collapsing. In fact, if you look at a picture of Deimos, you’ll notice that it’s visibly non-spherical because the gravity is so weak. Nothing to worry about here, as long as you make sure your ship is sturdy.

Ship acceleration

Apparently, the closest solar system is about $$10$$ light years away, but the nearest one with more than one planet is over $$15$$ light years away. Sorry, but there’s no way you’re traveling that far in under a month. You’ll need faster-than-light travel, which will certainly require a significant amount of hand-waving.

Supposing you can manage faster-than-light speeds, you’d need to accelerate to a speed of at least $$120$$ times the speed of light in order to make the trip in a month. That’s a kinetic energy of

$$\frac{mv^2}{2}=\frac{(1.5\cdot 10^{13})(3.6\cdot 10^{10})^2}{2}\approx 9.72\cdot 10^{33} \space\text{J}$$

To give you a sense of how large that is, that amount of energy is greater than

• The amount of solar energy that strikes the Earth each year
• $$10^{10}$$ times the energy stored in the Earth’s natural gas reserves, as of 2010
• $$10^{12}$$ times the world energy consumption in 2010

That’s a lot of energy! You’re either going to need to invent a miracle energy source, or slow the heck down. Here are some suggestions for getting out of this bind:

• If there are lots of junk planets all over the place, and you don’t care which planet you end up at, have your characters send their ship into random wormholes and scavenge wherever they end up.
• Use cryonics to freeze your “grunts” for 20-30 years while they travel at near-light-speed. This will still require an astronomical amount of energy, but you might manage it by piggybacking off of the gravity of a nearby star, using it to “sling” the ship in the right direction.

Risk to nearby planets

No inhabited planet will want to have this ship orbiting it. If its orbit decays, it will be difficult to prevent it from crashing into the planet and causing a catastrophe. Even if its orbit does not decay, it could still screw up the planet by interfering with the orbits of preexisting moons.

When a body of mass $$m$$ orbits a larger body of mass $$M$$ with velocity $$v$$, the radius at which the circular orbit is stable equals

$$r=\frac{GM}{v^2}$$

If, due to miscalculation or external interference, the orbit decays by some amount $$\Delta r$$, the ship will either need to speed up or move away from the planet to restabilize its orbit. If the former option is taken, the velocity increase needed is about

$$\Delta v\approx \frac{1}{2}\sqrt{\frac{GM}{r^3}}$$

meaning that the energy needed to correct this is about

$$\frac{mv^2-m(v-\Delta v)^2}{2}\approx \frac{GMm}{2r^2}\Delta r$$

For a planet the size of Earth and a satellite the size of your ship, that could still be on the order of $$10^{19}$$ joules if your orbit deviates by just one meter. If you’d rather correct the orbit by increasing the radius, the energy needed is

$$mg\Delta r = \frac{GMm}{r^2}\Delta r$$

...which is twice as much as you would need to speed up the appropriate amount.

Bottom line: your ship needs to be ready to expend $$10^{19}$$ joules at the drop of a hat in order to correct the most minute decay in its orbit. That’s more than the yearly energy consumption of South Korea as of 2009. You’re really going to need some hand-waving to deal with that.

• you cannot calculate the energy required to travel FTL, since we do not have an inkling of how FTL would work.
– ths
Commented Jun 29, 2020 at 23:08
• @ths Fair enough. I guess this calculation just demonstrates that a considerable amount of hand-waving or modifying the laws of physics is necessary to accomplish what the OP wants. Commented Jun 30, 2020 at 0:33
• Due to time dilation It is possible to reach the next solar system in a month of time as observed by the traveller (but many years as observed by from the origin). This requires vast amounts of energy of course. Commented Jun 30, 2020 at 2:57
• Conventional KE energy calculations go to infinity as you approach the speed of light according to Special Relativity. Surprised you even made a Newtonian estimate. FTL drives - like the Alcubierre drive - need exotic matter (i.e. matter that has not been show to exist), and the energy calculations for it possibly/probably depend on the characteristics of that imaginary matter. Commented Jun 30, 2020 at 17:58
• @I'mwithMonica By virtue of being a storage container, it won’t be completely jam-packed full of matter, especially compared to a moon made of solid rock. Even when filled with garbage, there will be lots of empty spaces between pieces of garbage, inside of rooms not intended for storage, within crawl-spaces between walls, etc. Commented Jun 30, 2020 at 18:09

## The biggest constraint for the size of a star-ship is going to be inertia.

This is not a "hard limit" but when you hand-wave away energy and propulsion, I can pretty much guarantee it will be the next engineering limit you will be faced with long before other issues like gravitational collapse or resource availability.

The big reason you can't make a moon sized ship is that that moons are solid masses of stone that typically experience no more than a few cm/s worth of acceleration from thier orbits. In contrast your ship is a relatively thin scaffolding filled with a lot of non-structural weight from the cargo and various systems.

The thing about FTL technologies like Alcubierre drives and Wormholes is that they still require you to move. And the bigger the ship, the more easily it will start to fall apart the second you try to move it.

Picture this: for a ship to accelerate at a speed that feels okay to any sentient race for any extended period of time, you are looking at matching the acceleration of gravity on thier home world. When you attach an engine to something and start pushing it, it does not all move at once. The molecules binding the engines to the back of your ship have to be able to transfer that acceleration all the way up to the nose. At 1G of acceleration, this would cause the same amount of compression and tension in the materials that make up your ship as you see in the materials that make an object sitting at rest on the surface of a 1G planet. So, to find out the maximum size of a ship, we need to look at the maximum sizes of things we can build under gravity.

What is the maximum size we can build under gravity?

Burj Khalifa is currently the tallest building in the world at ~830m tall, but it uses a steel frame construction technique. Rigid carbon nano-fibers can form a structural frame that could theoretically achieve 5 times that height giving you a ship with a maximum conceivable length of somewhere on the order of 4km. That said, for a cargo ship I would not suggest going that big. We think of cargo ships as being big, but because they are designed to carry so much non-structural weight, you can not stack them up super high. When you look at the world's largest freight ships, the Maersk Tripple-E, they are only about 90m tall from keel to the the top most container; so, if you are trying to be realistic, a space freighter should not really be more than about 5x that length (~450m) for it to maintain integrity while fully loaded given our currently understood limits of material sciences.

How to go bigger:

We Earth dwellers like to see our ships thin and long because gravity and water resistance make us do it, but in space, if you want to make a big ship, you go tall or wide and short. This is is because a freighter (hopefully) never needs to turn in a way that exerts more rotational force than forward acceleration force; so, you can make a ship that is 450m long able to accelerate at 1G, and 4500m tall able to turn at 1/10th that speed.

If you want to take this one step further you can go with a giant sideways flying saucer design. A carbon nano fiber freighter could be several kilometers in diameter, and still only 450m long. By placing thousands of evenly distributed thrusters along the broadside of your saucer to push it forward you could make a freighter roughly resembling the alien ships from Independence Day.

Since you are assuming some level of future tech, I would not call it an unreasonable stretch to scale this design up using something a bit better than our current best to get something as big around as Deimos, just probably not in all 3 dimensions.

How to go all the way...

Acceleration for this ship doesn't really need to be all that fast, although it does need to be able to travel between solar systems in (at most) a month or so.

Since you seam to want to go into FTL mechanics, the Alcubierre Drive introduces some very interesting properties. When a ship uses an Alcubierre Drive, the effects of inertia can be mitigated because your ship is accelerated more or less together at a molecular level. (It's sort of like experiencing a free-fall). That said, this sort of FTL ship also experiences a gravitational gradient where the front and back of the ship are prone to accelerate faster meaning that the mid-section of your ship will still experience a bit of an inertial difference from the rest of the ship, but generally only a fraction of what a reactive propulsion system would put on a ship. This means that you can accelerate at more than 9.8 m/s^2 while experiencing a total structural sheer of less than what you would expect 1G to inflict.

Now this is where things look really bleak for you big ship... Since you want to be able to cover 15LY in 30 days. That means you have 15 days to speed up and 15 days to slow down, making your mid-point a distance of 1.41915e17m. By plugging these values into the displacement formula a=(2s)/(t^2) where s=7.5ly and t=15days you get an acceleration of 84,495.5 m/s^2 also known as about 8622G.

Now a reactive engine would crush just about any ship accelerating this quickly like a tin can, but let's look at this with an Alcubierre Drive. If you have a ship that is the length of Deimos (I'll go with the short dimension and say 11 km long) and you want it to survive 8622G, you will need to mitigate about 99.9995% of the central sheer of your ship for it not to break. To do this you would need to project your gravity nodes ~200,000,000,000 km away from your ship with an equivalent mass of +/- 5.1676333628e42kg.

This means that such a drive would have to simulate gravitational forces equivalent about twice the total mass in the Milky Way galaxy. Sufficeless to say, this would be a terrible idea for any kind of ship because the wake of your warp drive would be so strong as to wreck... well the whole galaxy.

In other words... don't plan on making a ship that big or that fast unless you plan on hand-waving in some star trek style inertial dampeners and/or structural integrity fields. But, if you're going to do that, then asking how big a ship can be becomes meaningless since you could always explain away bigger with more of the same handwavieness.

• Good thing to remind inertia. Still in such a distant future, after tens of thousands of years of spacefaring, they will have developed materials with properties way beyond our current limits. Moreover the crew could be protected from excessive acceleration or modified to withstand it. Man will not be as we know him. Commented Jun 30, 2020 at 15:55
• @DuncanDrake This is why I stipulate that it is more or less doable with the assumption of future tech. With computer modeling, we are now able to figure out ideal molecular structures to the degree that we can confidently say we are getting close to finding the limits of molecular structures. There are probably plenty of incrementally better configurations left to find, but things a lot better simply can't even theoretically be done with any observable type of molecular bonds. Since the OP is asking about material limitations, hand-waving molecular science away seems out of frame. Commented Jun 30, 2020 at 16:23
• As for the integrity of the crew, a more robust crew/faster ship means a smaller max size ship; so, if the OP wants to make a crew of cyberneticlly and genetically enhanced to sustain 10Gs of acceleration, he can, but that would force him to scale his max ship size way down which seems contrary to his goal. Commented Jun 30, 2020 at 16:26
• @DuncanDrake There are a limited number of types of molecular bonds each with a known and finite binding strength. We know that lighter elements typically make stronger elements than similar heavy elements so we can generalize that as of yet undiscovered heavy elements won't help us; so, from this we know that Carbon, being the lightest element capable of 4 covalent bonds has more binding potential than any other element (known or unknown) for its mass. Knowing that, we can use computer simulations to experiment with numerous carbon configurations per second to find ideal shapes. Commented Jun 30, 2020 at 17:13
• We already know many ideal shapes for various purposes, but the challenge is creating them quickly and without impurities or manufacturing flaws to achieve those ideals in the real world. Doing better than ideally formed carbon nano-structures will require something other than molecular binding. You are right that in the next 20,000 years we might discover something better than this, but there is not even the foundational science to predict what that may be. Without a science to even speculate what it is, such a discovery would fall in the realm of Clarke Tech / Handwave Tech Commented Jun 30, 2020 at 17:20

It's feasible to turn the whole solar system into a spaceship if you like. The question isn't how big is plausible, it's how big is economical? It turns out pretty small.

Let's figure out the economics of your world(s). To do that we need to resolve a paradox: shooting trash into orbit is viable, but once in orbit it has value again. Why?

Do I need to create a new energy source for this in order to make it feasible?

Yes. Nothing too crazy. Conventional sci-fi energy sources are fine: fusion, anti-matter. Energy has to be cheap and abundant for this to work. Here's why...

# Supply, Demand, and Garbage

In a society where human beings have lived among the stars for tens of thousands of years, they would have accumulated a lot of junk, so they just decided to throw a lot of it onto uninhabitable rocks near colony planets for easy disposal. Kind of like the galactic version of a landfill.

Sci-fi likes its junk planets, but their existence has economic implications. We throw things away when it's economically less valuable to make another one than to repair or recycle the item. When that means putting it on a truck and driving it over to the local landfill, the cost of dumping is low.

Getting mass out of a 1G gravity well is very expensive, this is one of the reasons we don't fire our nuclear waste into space. No matter how good your technology is, getting 1 kg into orbit requires 3e6 Joules or about one US dollar worth of energy. The Earth produces 2e12 kg of waste each year, that would take 6e18 Joules, minimum, to put into orbit. That's well within science-based sci-fi.

We can assume your universe has extremely cheap space flight and abundant energy production, or an extremely unsustainable economy, or these landfills contain extremely toxic stuff that make the effort worthwhile. Or all of the above.

My characters are salvagers, meaning that they go down to these junk planets and bring anything remotely valuable back with them. I want their base to be very large but still mobile.

If you're salvaging these landfill planets, something has gone terribly wrong with your civilization. Getting to the planet's surface is expensive. Lifting off the surface is also expensive. What has happened that makes this worthless garbage suddenly valuable? Something very bad.

The flip side is the value of things. A society which overproduces has a supply glut, so the value of its goods will fall. These undervalued goods will be thrown away long before they have no real value, or because it's cheaper to buy a new one than repair an existing one, or simply because there's a better model.

# Haves and Have Nots

This all sets up a world of haves on the colony planets living in luxury, and the have-nots living off what they throw away. The people on the colony have such wealth they can afford the expense of firing their junk into orbit. The people in space are so poor they consider the surface-dweller's trash to be of value. Why?

The economy on the surface is clearly overproducing and unsustainable. It is, effectively, using their resources once and then paying the cost to lift them out of their gravity well and into space. Once in space, the spacers can collect them for their own repair, reuse, and recycling. The spacers will then keep the majority for their own use.

The only way it is viable for the spacers to trade the garbage back to the surface-dwellers garbage is if they extract and refine the most valuable materials, and if they can do it cheaper than on the surface. This is only possible if the labor of the spacers is cheap, or if they possess technology and industries the surface does not. One situation is toxicity. Processing the waste is dangerous and toxic, but it's safer to do in space. Or it isn't, and the spacer's lives are simply cheaper than surface dwellers.

# E-Waste

Today, we see this situation with electronic waste driven by planned obsolescence. Rather than continuing to use a working, but obsolescent, device, we throw it out. We typically don't even recycle the material, not even its precious metals, because it's cheaper to dig it out of the ground, process and refine it, and ship it around the world, often by using exploited, cheap labor and poor health and environmental standards.

The waste is sent to poorer parts of the world where it is processed. Some is reused in-situ. Some is recycled and sold back. But the process is toxic and dangerous.

# How Big Does Their Ship Have To Be?

Most of it would be taken up by containers of junk they collected, things to be sorted to see if they're valuable, and things that are valuable if recycled in large quantities.

Since it's expensive to bring things up into orbit, the sorting would happen on the surface. Similarly, its cheaper to bring the recycling equipment down than to bring bulky, massive material up. Only the valuable material after processing is brought up.

This also simplifies waste disposal: leave it at the landfill.

The ship only has to be big enough to support the people, fit their equipment, plus its engines. It moves from landfill to landfill, sending out mining parties to extract and refine material and bring it back. Once they gather together enough valuables, they may attempt to trade with one of the trash-producing rich planets.

• I love how compelling your answer is in the face of unknown future tech. Perhaps the only way around this is with something like a teleportation device --- which, presumably, is cheaper per kg. Commented Jun 30, 2020 at 22:45
• @jpaugh Thanks! The cost-to-orbit was just a back-of-the-envelope sanity check to make sure it wasn't going to cost more energy than a planet receives from its star, for example. Commented Jul 1, 2020 at 0:05
• +1 for noting the illogic of "junk planets", could be emphasized even more. There's a YouTube video that goes into why trying to dispose of nuclear waste by shooting it into the sun is more dangerous and more hideously expensive than almost any other option. Commented Jul 1, 2020 at 2:23
• @ScottGartner If you have a sci-fi problem and you think "I'll solve it with wormholes and teleporters" you now have many problems. As anyone who's played Portal knows, wormholes break physics. Put one portal high, one low. Pass a fluid through them so it falls from the high to the low. Use the falling fluid to drive a turbine which powers the portals. Tada! Perpetual motion machine. Star Trek style teleporters are also a problem: what reassembles the matter at the other end? And there are the philosophical issues. Commented Jul 1, 2020 at 19:02
• @jpaugh Like many ethical exercises, The Swampman has no one answer. The Outer Limits episode "Think Like A Dinosaur" presents another view on Swampman. Point is, teleportation carries a lot of baggage; adding it to your world either has implications, or you ignore them and move into science-fantasy. Commented Jul 1, 2020 at 20:27

## Interstellar trips

You will have to resort to a lot of handwaving, using FTL on a Star Wars level to get your scavengers to do interstellar travel in months. Remember that the distance between stars is measured in light years!

Franklin answered this in majestic way!

## Acceleration in sci-fi

The biggest problem with sci-fi ship acceleration is that most of them ignore that this acceleration will throw the crew against the wall, for a long time with an acceleration acceptable to our biology (violating the laws of relativity and with a infinite energy consumption, it takes 11 months to reach the speed of light at 1g) or an acceleration that would transform living beings, other objects loose inside, stuck objects, cargo, fuel, engines and the hull of the ship on a piece of nothing. This, of course, remembering that they make the ships look like planes going forward instead of lift cabins going up and down, which would be more logical.

Asimov develops an elegant proposal for the Trevise's spacecraft that accelerates each atom of the spacecraft at the same time so that the occupants do not notice any acceleration. Clarke uses a similar strategy in Childhood's End and both look like the Alcubierre Warp Drive idea.

## Cost of a scavenger ship

In an interstellar civilization, the cost of building large cargo ships shouldn't be much. Even if that civilization is ruining and chaos smashed the galaxy into stellar feuds that dispute power among themselves, there will always have old things from the glorious times that can be reused.

## Cargo size

The size of a cargo ship is optimized according to its maintenance cost. If a ship takes X units of cargo, spending Y and another takes 2X spending 3Y, I would prefer to own 2 of the first ship rather than one of the second. The operations of approaching the landfill planets and take the dump, the maintenance of a very large structure and other details need to be calculated to optimize this.

The idea of gaining scale does not have to lead to a single gigantic object. Safety and fuel use make the scale gain pay off, but operations on several smaller units are cheaper and simpler than a large operation. How to solve? Maybe with something like a train?

• Technically, a ship using the Alcibierre drive doesn't accelerate at all - it stays still while the space it occupies moves. Commented Jun 30, 2020 at 9:18
• @occipita, yah, same to ships in Asimov and Clarke stories, its all space around them moving Commented Jun 30, 2020 at 17:32
• @occipita That is a bit of complicate subject... the space around the ship is distorted as though by gravity causing it to "infinitely fall". Acceleration by gravity and acceleration by propulsion are both acceleration. When people say that, they generally mean that Alcibierre drives do not cause whip lash because all adjacent matter in the ship is accelerated relatively in unison, but they do cause potentially destructive sheer because not every point on the ship is pulled/pushed at the same angle and magnitude. Commented Jun 30, 2020 at 18:43
• @jpaugh It is still acceleration, but it accelerates the way falling does, not like a thruster. Alcibierre drives rely on artificial positive and negative gravity. The problem is that gravity has an exponential fall-off; so, when you overlap a gravity source in front of your ship with a negative gravity source behind your ship the sum of pushing and pulling in the middle is not going to be as strong in the center as it is at either extreme. So it's like falling, but where your head and feet fall faster than your stomach. Commented Jul 1, 2020 at 13:21
• @jpaugh actually yes... I have a bad habit or re-evaluating my answers every time someone pokes me. Thanks to this poke I just noticed that I had a unit conversion error in my answer so an Alcibierre drive would not need to simulate the mass of a small star to accelerate an 11km ship to the required speed, but rather twice the mass of the Milky Way... amazing how fast compounding exponential variables can go wrong. Commented Jul 1, 2020 at 14:23

This question is far too concerned about unimportant details; in particular, the mass and size of the ship.

The essential parts of the ship will be:

• The crew's habitat.
• The engines.
• Fuel.

Given that the ship will:

• Accelerate very slowly.
• Remain in space.

The most obvious configuration is simply to attach large cargo nets to it whenever a new load is acquired, and use the ship to tow them along with it. With the low acceleration, there will be very little stress on the lines that secure the nets to the ship, and the cargo can trail behind as far as one wants (given sufficiently long lines and large enough nets).

There are no real restrictions on what the ship has to look like, and in fact, the ship itself can remain relatively small.

The only crucial maneuver would be at the mid-point of any journey, when the direction of acceleration must be reversed. Rather than, as is normally done with a rigid vessel, rotating the ship (which wouldn't work in this case), I'd suggest simply doing a slow and wide U-turn.

The only tricky design feature would be in having the engine exhaust avoid the cargo, which is directly in the line of fire. That can easily be avoided by replacing the single ship with two (or for safety and reliability, several) separate ships that have a common tow-line between them for the cargo lines to attach to.

But given how slowly this thing can accelerate, the goal of "able to travel between solar systems in (at most) a month or so" is completely ridiculous.

I'd suggest that such trips would take thousands or millions of years, making the incredible value of that cargo of scrap even more ridiculous.

This really isn't a premise for a science fiction story. It is pure sci-fi, a story that uses the superficial trappings of science fiction while having almost nothing else in common with the real genre. There is already far too much of that in the world.

Instead, I'd suggest writing this story as a marine salvage operation, here on Earth. All the difficulties that you are wondering how to solve will simply no longer exist.

It might even make a good adventure novel, but it certainly wouldn't be science fiction.

Adding a small idea to all of the rest: You could use "Intertialess technology" like was used in the old EE "Doc" Smith's Lensmen books. It allowed for FTL and huge ships. This may be the answer to some of the "hand wavy" stuff you need to worry about... by not worrying about it.

Thoughts on perturbing the orbits of existing moons and the "local" planet's gravity: Don't orbit the planet, orbit the star in a matching orbit to the planet (Ahead/Behind/Beside) and shuttle to/from the planet. This will require some energy to follow the orbit because the orbit for a lighter craft than the planet would be different, but given Intertialess as an answer, it's easy to explain.

• "not worrying about it" I think that's the definition of hand-wavy! :-) Commented Jul 1, 2020 at 14:30

I'll try to focus on some of the issues I see in a potentially interesting story.

Size
The size of the ship would be story based.
If you want it to be large you can just make up motives for it to set on a long journey through the stars in its garbage.. ehrm recovery mission. Very much like whaling ships in the XIX century.
Motive could be economical, political, etc. Basically after collecting they can't just sell their load to the nearest civilized planet but have to bring it to a very specific place.

Cost
You are right that a large ship would appear to be too expensive for such an endeavor. I would suggest then that it is a ship / station previously built for a completely different purpose (e.g. defense) then decommissioned and refitted for salvaging operations at a much lower cost. This could also usher in problems in dealing with older, refitted technology.

Energy
Since you need to move to the next destination in a relatively fast time you are going to need to handwave on this. Relativity is not your friend here. But neither your enemy...

Considering mankind has been a space faring race for "tens of thousands of years" it would be quite acceptable for them to have developed physics knowledge and technology way beyond our current limits. I see two options:

Salvaging from the planet
Shuttles would seem the most obvious way, especially if the ship needs to keep a very high orbit. But given the size of the ship I would imagine a large amount of material is brought up on every visited planet. You would need a lot of flights, really a lot. Each shuttle would have loading / unloading times. Does not look good. How instead your civilization is capable of altering the gravitational field? You would need to spend energy for that of course but huge, affordable energy generation (and control) is the basis for a star faring civilization.
Given gravitational field control the ship could go into lower orbits and pull down a space elevator which would act as a conveyor belt, continuously bringing up material from the planet. Shuttles may still be needed though if the salvage sites are not located on the equator.

Conceptual designs place the tower construction at an equatorial site. The extreme height of the lower tower section makes it vulnerable to high winds. An equatorial location is ideal for a tower of such enormous height because the area is practically devoid of hurricanes and tornadoes and it aligns properly with geostationary orbits (which are directly overhead).

Other issues
Other issues that you don't mention may be part or not of your story, it's up to you. Tens of thousands of years far away from now means a lot of change:

• human modifications: technological evolution will change man as we know him. How will they be in your story? I don't expect your crew to look anything like Mal and his motley crew on the Firefly
• social interactions: how is society? do families still exist? has the crew loved ones far away during their journey?
• AI: how does Ai fit in your story? Because there will be AIs. Probably they will be the ones who discover the theory and the technology at the base of making wormholes.
• economy: what makes the enterprise worthwhile for those who finance it? What makes it worthwhile for the crew? possibly mankind is on its path of going from a beyond scarcity economy back to one of scarcity. As you say:

I was thinking that perhaps asteroids and uninhabitable planets were pretty much looted of all their ore,

Two things. Firstly a crewed space ship is in one sense nothing more than a space habitat with an engine attached.

Secondly a lot depends on how 'hard'/believable you want your story to be e.g. are you going to have devices that generate artificial gravity aka Star Trek on the ship or not? If you don't have this kind of device and you want your crew to be able to live and work in gravity then you are going to have to use using centrifugal force. i.e. your ship has to rotate perpendicular to its main axis in order to generate a gravitation field.

The downside of this is that the greater the diameter of your rotating habitat or the higher the gravity you wish to maintain the more mechanical stress you put on the rotating hull i.e. for any given diameter lower/less gravity means slower spin, higher gravity means faster spin. And rotation imposes stress on whatever material it is you use to construct your torus. The bigger the diameter of the ship the stronger the material needed and by strength I mean tensile strength.

People have done the calcs and at the moment the largest diameter structure you could in theory build in space is about 1200 miles wide (i.e. radius of 600). This is based on something called a 'Bishops Ring Habitat" which (in theory) if made from carbon nano rods (Dr Bishop calculated this as having the highest possible tensile strength known to man. So that sets the size limit unless you bring in 'unobtainium' or some other made up material that's stronger. If you don't anything bigger than that simply breaks apart - if you are generating 1 G at the outer circumference.

In reality it would be far easier/safer to go with a slightly smaller radius (say 'merely' 400 miles) and then simply add more 'donuts' to your ship, one behind the other along the access of thrust as needed.

Benefits - Normal G in the living areas - zero g in the centre of the donuts (well close to zero).

Downsides - either your ship accelerates very slowly up to top speed so the passengers don't notice the two opposing 'gravitational forces' i.e. the centrifugal force and the acceleration or you have to 'spin down' the hubs before you start accelerating so that the crews new 'down' becomes the rear of the ship (towards the engines). Obviously you then have to spin it up again when you stop accelerating or get to your destination. Which takes time - especially when you are talking structures this size. So it would depend on how much 'zipping' around the Universe you intend these things to do. They certainly wont be outmaneuvering any X-wing they encounter.

Just answering a part of the question - the crew.

Given the advanced level or technology, you could expect some massive automation and robotisation. The crew size is not a huge problem, imagine a size similar to the (mining ship, so kinda similar) Red Dwarf. And yeah, I know it's a comedy, but the premise sounds plausible to me. And its size (6 miles long) is probably something like what you're after. Although it's meant to accommodate and be operated by thousands of crew members, it went just fine for millions of years with an active crew of zero, controlled completely by its main computer.

So having a crew of just a few dozen is not a big issue, giving that they'll be mostly decision makers, rather than micromanagers. I don't think your "mostly low-level grunt workers" will work though. You'd have a few of those for the kind of maintenance that can't be done by robots, but most of your crew will be "pilots" and "engineers" - even though there will be a good portion of "machinery operators", they will most likely be white collars!

As Franklin Pezzuti Dyer already scienced out the issue with size/mass constraints, I don't think you'll need an answer on that.

As for what drives the ship - if you want to go with interstellar travel without the use of things like stasis, or making your concept go to science fantasy with the idea of FTL travel (which is impossible according to any theories now existing) - you could go with the concept of folding space (Alcubierre drive style) around the ship. In that case, accelerating would be no factor at all, since you're not technically moving, no G-forces to worry about. You'll need to dive into creating artificial antimatter to get that to work though, as creating folds in space cannot be done in a "safe" and stable manner without it.

The shuttle bay idea isn't bad, since it allows for some playroom with events happening in the space between planet/ship. Makes for an easy tool for character development too if the crew talks on these rides over to the planet or back to the ship.

I'd try to invent some kind of exotic material, maybe a rare metal of sorts, that has started to run out after centuries of mining and failure to recycle - forcing society to dig it up from trash from a time in which it wasn't rare. That would deal with the economics of it quite well I'd think. Also creates opportunities for mystery (who knows what those millenia-old piles of garbage might hold after all).

– L.Dutch
Commented Jul 1, 2020 at 13:00

the largest a ship can get is are 300,000 KM in length, but would require the industry level of a K3 civilization to build. And would only really be practical to a K4 or a K5.

and such ships would be black hole powered as only a small star of a black hole could give you enough power for ships in this size range, and such ship if it was a warship, would be able to conquer an entire large galaxy all by itself.

for context, the yellow dwarf that's our sun, Sol, is 1.392 million km in diameter, so this is a ship that as long as a 5th the sun is wide.

• Why is that the upper limit? Commented Jun 29, 2020 at 18:34
• the strength of materials mostly. another is the cost in the amount of resources to build such a ship. Not to mention the maintenance upkeep will consume entire dozens of planets-worth of resources. Such ships will never be mass produced, even by K4 and K5 civilizations. and the appearance of such a ship in a space battle would be astonishingly rare. you also would never want such a ship entering a star system either as its own tidal forces would in all likelihood knock most of the planets present out of the orbit of their sun! such ships would have the same gravity as a small star! Commented Jun 29, 2020 at 20:38
• how do you conquer an entire galaxy by yourself when you can only be in one place at a time? Commented Jun 30, 2020 at 1:34
• This maximum size seems oddly specific. Why is 300,000 plausible and not 400,000? Does it matter if the ship is spherical or elongated? How can you know with that degree of precision what the limits of material strength and resources are for a K4 or K5 civilization? Surely you cannot be using the laws of physics as currently known to make these calculations, since you also say that such a ship could "conquer a galaxy within a decade", a feat which is not allowed in the laws of physics as we know them, given that galaxies are tens or hundreds of thousands of light years across. Commented Jun 30, 2020 at 17:56
• Without providing any details on speed or shape or capacity, I don't really see much rationale behind the 300,000km figure. Materials strength comes into play when you're talking about how the ship resists forces, like its own gravity (which will depend on shape and density), or its own propulsion (which will depend on shape and power). None of that is described here, so the 300,000km number seems like it was pulled out of a hat. Any rationale behind that number would make this a better answer. Commented Jun 30, 2020 at 18:08