# What would be an appropriate size for an interplanetary bulk cargo ship?

I've been working with a friend to design a sci-fi setting for a TTRPG campaign and one of the issues I've run into is the scale of food production and transportation within the setting. In the setting in order to supply a population of around 25 billion people, a large majority of bulk food goods such as corn and wheat are grown off-world in large structures at the lagrange points of the planet's orbit in order to get similar light levels as the crop's native environment and once harvested, need to be transported back to the populated regions of the system for processing, sale, and then consumption.

According to this article by the University of Michigan's Center for Sustainable Systems, Large scale farms similar to the ones in this setting produce around 60% of overall farm production. With the average American consuming just shy of one ton of food a year, and the total agricultural output of the United States totaling to almost 750 million tons, I am assuming a total food consumption of 20 billion tons of food per year (accounting for economic differences affecting caloric intake) with 42.8 billion tons of production, of which 25.7 billion tons would be harvested off world.

Assuming that there is a sizable fleet of large cargo ships (>100), and each takes two months to make a round trip including loading and unloading, how many and about how large would you make each these vessels to meet this demand?

• You're on the right track. Start with investigating the size of freighters used today on Earth to move grain. Your ships, like those ships, need space for engines, crew, machinery, etc. Frankly, I think they'd be a reasonable size scaled to your needs. After that it's just basic math with whatever modifications you wish to make (e.g., replacing water canon with laser canon for piracy).
– JBH
Commented Oct 8, 2023 at 22:43
• Questions: What is the cost for the trip? What is the cost for the trip in relation to Mass? (e.g. it costs 10 dollars for 10 tonnes of cargo, but 10,000 dollars for 100 tonnes) What is the cost for increased cargo capacity? (e.g. it costs 100 dollars to build a ship with 10 tonnes of capacity, but only 200 dollars to build a ship with 100 tonnes of capacity). These factors, IMO, will be the biggest determinant as to the size of your cargo vessels. Commented Oct 9, 2023 at 3:27
• You'd better place those structures around the planet. Lagrange points tend to contain asteroids (esp L4+L5) that can bump into your farms and disrupt them, also staying within L1 or L2 requires spending fuel, shooting containers at the planet would also ask for extra momentum, thus high expenses, while a space lift-based station can use inexpensive transport system to transfer resources and products. And yes, space lift should be manned to lessen the need of oxygen transfer while also having a constant presence in orbit. Commented Oct 9, 2023 at 5:52
• Don't forget that for every billion tons of food you export, you'll need to import a billion tons of water and CO2, give or take. You'll also be exporting a lot of $O_2$. (I also agree with other commenters that there's little point in doing this at the Lagrange points, instead of in orbit or on a planet, close to the people who are consuming the food and breathing out the needed $CO_2$.) Commented Oct 9, 2023 at 8:00
• Would a spaceship even need to hold bulk cargo internally? A small ship towing a separate cargo compartment makes more sense, as the latter doesn't need life support, etc.
– vsz
Commented Oct 9, 2023 at 9:41

Why would anyone decide to grow a planets food needs off world? I makes no sense as the economic costs would be massive.

But assuming there was some good reason any ships would have to be a compromise between many factors such as scaling optimisation, capital outlay, lifetime number of possible launch /land cycles etc.

It would probably be different for different commodities, conditions and locations/routes so I would expect a wide variety of different sized vessels not one size fits all.

• To answer your first question, the in universe explanation is that the farming industry was moved into space to serve as a sort of oxygen refinery in order to terraform planets, (frozen CO2 and Nitrogen go in, frozen O2 comes out ready to be shipped to planets needing different atmospheric compositions) with the food produced initially just being a nice bonus. But once the refineries became large enough, they were also producing enough food to sustain a sizable population by themselves. Out of universe, I think they look really cool and wanted to find a way to include them. Commented Oct 8, 2023 at 22:31
• Asimov imagined Trantor...
– JBH
Commented Oct 8, 2023 at 22:39
• Yes the idea of such massive shipments in vast space going vessels does sound a cool idea for a story. I suggest that you need a cover story / plot device to help some people suspend their disbelief. The big issues would be energy and propellants. Perhaps some form of special drive inertia less / warp / anti-gravity could be used (no need to go into too much detail) then it becomes a lot more believable. As of today there is nothing remotely workable as chemical rockets are hopelessly inefficient and nuclear propulsion in the atmosphere has issues and both would require huge energy inputs. Commented Oct 9, 2023 at 8:03
• Space farming is a way to utilize more energy from the star than is available just on the surface, and one of the more production-ready of the lot. At least mass transfer is a thing we already are capable of performing with decent scale. Eventually (if humanity won't kill itself somehow) we would too build farms over Earth, at least to save on mass transfer to other planets, that is, instead of shipping from surface we could ship from near space. Commented Oct 10, 2023 at 5:16
• Not quite sure what you mean by mass transfer? As things stand today an orbital space launch from Earth requires that 90% of the mass of the rocket is propellant and of the remaining 10% only 1-2% or less is actual payload the rest is engines, tanks and structure. The largest rocket in existence today (Starship) requires almost 5000 metric tonnes of mostly liquid oxygen the production of which consumes vast amounts of electricity to produce and can only lift 150 tonnes of payload. Commented Oct 10, 2023 at 8:22

It doesn't matter

The ships would be automated so no people on board which means no need for life support or living quarters. Think self drive shipping containers.

The answer, I personally believe, would be millions of shipping container sized boxes. They have no engine. The containers are made from metals mined from the asteroid belt. The food/goods are packed into them and they're shot towards their destination using magnetic rail guns. They have a simple control system with basic steering jets to keep it on track. Space tugs grab the containers at the destination and deliver them.

Frame Challenge

I think your approach is wrong. There is more to growing food than just growing food. Wheat and corn require carbon dioxide, water and fertilizer. This means it's easier to grow food beside humans. Plants produce oxygen and food which humans require and humans produce carbon dioxide and fertalizer which plants require. Light can come from the sun or can be artificial grow lamps. Grow lamps are better and plants can be stacked vertically.

Waste from humans is turned into fertilizer and water for plants and directly harvested for people to eat and also providing oxygen. No shipping required.

• Unless you have some tech for the railgun to stay in orbit (in asteroid belt) after repeatedly throwing mass away that also does not consume fuel, and operates on par with mass acceleration of billion tons of food per year towards habitable areas, this won't work. However your points about having humans beside agriculture looks sound. Commented Oct 9, 2023 at 5:55
• @Vesper eh, just point the gun backwards and turn it up to 11, and post stuff back in a retrograde orbit every now and then. Its just rocket science, and therefore nice and simple. Commented Oct 9, 2023 at 10:21
• @StarfishPrime hmm yes, always thought you should not use mass driver as engine if you use it as a mass driver. But this means you'll have to throw away a billion tons of "stuff" per billion tons of food thrown at where it's needed. I really doubt that there would be at least a comparable need to "post stuff" back and forth, where "forth" is towards the planet in question. Commented Oct 9, 2023 at 10:32
• @Vesper not at all... you fire the gun backwards with regular cargo. The retrograde orbit can still get it to the right place, just on a different schedule. Commented Oct 9, 2023 at 11:01
• @StarfishPrime oh, you mean that instead of throwing cargo forward with speed A, you throw it backward with speed A+2*B where B is the speed of your farm relative to the star? (Adjusting to ensure orbit intersection with target in case of unfavorable phase of orbits) Looks doable, as anyway you're to send stuff somewhere; most likely some of your destinations require that much forward speed anyway. But if that launch would exceed the container's onboard delta-V and it'll miss the collector and hit the planet, you're bankrupt and an international terrorist in an instant. Commented Oct 9, 2023 at 11:13

Use a space lift

Lagrange points are too far away from the planet to bother with transfer, especially assuming near-future tech which does not include FTL or handwavium-powered engines. You would lose too much mass of fuel to facilitate building that farm in L1 for example. Instead, you have there a geostationary orbit and already plausible (although not exactly possible) space lift technology, which does not spend fuel in the same amounts as L1 to planet transfer. Remember, fuel spent in space is lost mass for the ecosystem, and on the scale of billions of tons depicted this loss of mass would be noticeable soon enough.

Geostationary, or a tad farther, is still a good place for space farms

Solar flux is what you actually require from a space farm, and while the planet is still close enough to obscure the farm for some time, the general insolation is just a tad less than what you would get in L1 where the planet won't obscure the sunlight entirely. The time difference of daytime vs nighttime even for ISS is positive, also there should (orbit dependent) be times when a satellite's orbit is completely out of the Earth's shadow, with increasing probability as satellites get farther from the planet, as the complete shadow of a planet gets smaller with increasing distance. Thus, a farm placed at geostationary would be lit by the sun about 96-100% of the time per day (depends on the planet's axial tilt). I say negligible difference with L1. And a space lift is a structure naturally extending to that height (or should I say "distance" here?).

Regarding actual flux value, yes a farm at geostationary would get less flux than in L1 because L1 is closer to the Sun, yet since plants have evolved to grow in the lower sunlight than L1's, it could even be beneficial at least for first stages of establishing space farming to not overheat the farm by placing it too close to the sun. However, the Sun-Earth L1 point is 1% closer to the Sun than Earth, thus solar flux in there exceeds Earth's level by 2% (roughly), this doesn't make enough difference to bother. For your planet the value might be different, but a value of 10% corresponds to the planet having the same mass as Jupiter (for the same mass of Sun), thus you can't get more than 1.5x the flux at L1, and what is 50% for such a variable? Build a bigger farm to compensate, anyway you lose a comparable amount to support structures in a space orangery.

You can even have some artificial gravity in there, should your space farm get eventually expanded into complete ring around the planet, which can be then positioned farther than geostationary and aligned for speed with the space lift, making the planet be locally above the space population, with gravity of several hundredths of G provided by centripetal force allowing them to have an up and a down.

And a space lift-tethered space farm does not require space ships to do mass transfer

Instead, you get way more simple mass flow over the space lift, depending on how it would be built or how many lines would it have, you could effectively build a pipe alongside the lift (attention, high pressure!) to allow food turned liquid to be plain dumped down to the planet. Otherwise, any containers that your lift could effectively raise and lower would do.

• I wonder if a better place for a farm would be in a sun-synchronous orbit. It stays illuminated nicely, and because it sits at a much lower altitude it gets slightly better radiation protection "for free". You might need to run some tethers to discharge the inner van Allen belt a little, but that seems like an easy ask after everything else the OP wants. I guess it does complicate cargo transfer a little, but it doesn't seem insurmountable. Commented Oct 9, 2023 at 11:04
• @StarfishPrime the primary advantage of a geostationary is ground tether, with SSO you always have a delta-V to compensate in order to deliver components to the farm (dropping cargo down to the atmosphere could do with atmospheric brakes for example). This would mean that food grown at SSO would cost a fortune (at least one weight of fuel), while transferring stuff over a space lift is currently estimated at way lower fuel demand per unit of weight. Also rockets, engines, infrastructure, risks of damage at docking (unless they have tractor beams), etc. Commented Oct 9, 2023 at 11:18
• Farms at SSO don't preclude the use of an elevator, of course... drop your freighters (or perhaps "lighters") from the elevator, and inject them into SSO, then inject into a return trajectory. If you're clever you could use aerobraking to reduce the GEO-to-SSO delta-V. It also means that a single elevator can promptly serve the whole of SSO space, whereas there are problematic and slow manoevering issues associated with going from GEO-to-GEO which means there's never a free lunch (literally, in this case). Commented Oct 9, 2023 at 13:01
• @StarfishPrime I am afraid that a thing flying at SSO would eventually precede its orbit into a collision with the space lift (or one of them). Such a collision would definitely destroy the space lift, potentially destroying the farm as well. And you can't steer the farm fast enough to avoid collision, but if you predict one happening from a time distance, you'll find out this should be done regularly. From my perspective, a geostationary tethered farm looks a lot more simple in maintenance than a farm on SSO, lift or not. Commented Oct 9, 2023 at 13:29
• Yeah, the annual precession of a regular SSO is an issue, but I do wonder if it is possible to design the orbit to avoid a specific location on the surface at a particular time... the satellite does make a fixed number of orbits per year, so it won't ever pass over every spot on the Earth's surface, which means it should be possible to avoid the elevator. I'm not sure how much margin for error you get, though. Commented Oct 9, 2023 at 14:22