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In space-trading games like Escape Velocity, Elite: Dangerous & others, cheap FTL exists, but lags in other advancements results in a playable environment that is politically fractured and not a post-scarcity utopia. Pilots can even manage a profit margin trading mundane materials (like water) just a few tons at a time to space stations in star systems with no terraformed planets.

So in a galaxy where stellar neighborhoods can be traversed in hours or minutes, and the demand for moving mundane materials is high, what constraints probably exist that result in tiny, sometimes chintzy & fragile, spaceships being a favored form of transportation for an average licensed pilot? Why are large freighters not seen as generally superior? Or, alternatively, why is the volume of space traffic shown in this type of fiction lower than expected? After all, we just mined x-million tons of asteroidal iron and our water usage is through the roof, as we have many thirsty miners in ships not terrifically larger than an 18-wheeler.

Some common constraints tend to be that A.I. is not a pervasive technology, so robotic swarms are not usually part of these interpretations, and that atmospheric flight is either impossible or prohibited for most interstellar ships. This means ships can usually only dock on space stations or outposts on airless worlds, which are expensive & time consuming to manufacture & operate, and therefore a trade bottleneck. (these could be perfectly acceptable answers to the question, if they are the only limiting factors)

Also worth noting: the FTL drive of Elite: Dangerous' effective range for a single "jump" is inversely proportional to the amount of mass being moved, which presents a hard limit on cargo & theoretical ship size depending on the quality of the drive. Assume this is arbitrary, or that creating increasingly stronger drives isn't insurmountable from a scientific or engineering standpoint.

EDIT: I'm sorry I could only choose one answer, many of the below responses offer a lot in unpacking this common feature in near-future FTL universes. Thanks all!

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    $\begingroup$ / effective range for a single "jump" is inversely proportional to the amount of mass being moved, / isn't this the constraint you are asking for? $\endgroup$ – Willk Feb 13 '17 at 20:46
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    $\begingroup$ Ask yourself: Why do so many people drive to work, or to their relatives' houses for the holidays, rather than take an ocean liner? $\endgroup$ – Spencer Feb 13 '17 at 23:22
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    $\begingroup$ @Spencer The lack of water might be part of the reason. There's not much ocean between where I live and my place of work. While there is plenty of air from here to there, it seems catching an airliner is out too. But basically you're quite right. Bigger isn't necessarily the better choice. $\endgroup$ – a4android Feb 14 '17 at 1:40
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    $\begingroup$ The real answer is that realistic interplanetary or interstellar space-games would be too boring or difficult. So the answer is essentially in-game hand-waving/magic to make the game more fun. $\endgroup$ – RedGrittyBrick Feb 14 '17 at 15:02
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    $\begingroup$ I am not sure they are mostly small. It is just that nothing interesting happens in large craft by design. They have low crew densities and are highly regulated. Think super-tanker or container vessel on the empty ocean. So boring compared to pirate/smuggler landing craft. In Gaunt's Ghosts they and several other platoons + support machines travel on a large cargo vessel that is also carrying a lot of other cargo. Dune also has smaller vessels boarding the larger navigator ones for inter-system travel. $\endgroup$ – TafT Feb 17 '17 at 8:55

15 Answers 15

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The ships would be only big enough to meet demand at the destinations. Hauling a planetary sized mega-freighter of materials to Podunk Space Port-1 which only needs a tractor trailer worth of goods is not very efficient.

You might ask why goods are mostly transported for final delivery in small trucks and not have the mega cargo ship unload directly to all coastal locations.

This type of material management would be just as applicable to FTL transport. If it only takes hours, days, or weeks for a supply ship to show up to deliver goods, you only need to locally stockpile and store enough supplies to last that long. Podunk Space Port can't afford the warehouse space or inventory costs to justify the super-bulk shipments or the infrastructure to support them.

It might be cheaper on a per unit basis, but if you only need 10 units buying them in packs of a billion is uneconomical.

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    $\begingroup$ In the real-world example, goods are mostly transported in shipping containers that can go from mega cargo ships to trains of intermodal cars (accommodating two containers stacked between each set of wheels, typically 3 or 5 "wells", for a total of 6 or 10 containers per unit) then on trailers to be hauled behind semi-trucks. So freight-handling in a SF universe could be expected to build upon this, with various sizes of ships capable of handling different numbers of the same shipping containers we're already hauling three different ways now. $\endgroup$ – Monty Harder Feb 13 '17 at 21:51
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    $\begingroup$ Also, having a fleet of small ships allows for higher flexibility than a single behemoth - you can't send 0.3 of a ship to one destination and 0.7 of it to another, but if you have 10 smaller ships, just send 3 to one and 7 to the other (and even better, get 12 and keep 2 standing by for emergencies and opportunities). And you can make more frequent runs, tailor-made to a destination's specific needs. $\endgroup$ – G0BLiN Feb 13 '17 at 23:41
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    $\begingroup$ This basically answers my question, but it would seem that this galaxy would still host temporarily massive volumes of demand. If some system was interested in terraforming a moon or planet, there would be economic interest in creating a supply line for a few trillion (or whatever, could be off by a factor) tons of organic material that would be more substantial than all your neighbor's tractor trailers combined. But given my own rules, Podunk would still be a bottleneck of sorts. $\endgroup$ – Ross Feb 14 '17 at 3:15
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    $\begingroup$ @Ross The thing is, what exactly would you ship in that case? You're talking about a project sized far beyond anything we could logistically support - the kind of thing done often in Star Wars, but certainly not Star Trek or Elite universes. It's unlikely you'd try to terraform a world that doesn't have the raw materials - most of the shipping would be food and amenities for the staff, and all the high tech machinery and various biological agents. That's how we imagine terraforming Mars now - certainly not by shipping Earth's soil there :) $\endgroup$ – Luaan Feb 14 '17 at 11:35
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    $\begingroup$ Ah, the old Costco 3 Pack of Canoes problem! $\endgroup$ – Joshua Drake Feb 15 '17 at 15:30
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Personal cost might be one.

If we're assuming that pilots are owner-operator contractors ferrying materials, odds aren't great that they'd be able to afford massive cargo ships. Even if FTL is "cheap", owning a spaceship of substantial size still requires material and labor costs to build, which people have to pay for. Consider that an 18-wheeler here on Earth costs around $150,000, give or take, and uses a very large version of a relatively basic powertrain (most of them are just huge straight-six turbo diesels). Purchasing something the size of a cruise ship or aircraft carrier is still going to take aircraft-carrier amounts of materials.

Added on is that if your massive ship breaks down you have huge downtime where you're making no money until it's repaired, vs a huge fleet of small, inexpensive vehicles that may be able to maintain 80% uptime or better.

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    $\begingroup$ I like this. Along these same lines: why there are no high rise apartment blocks in the favela. People are doing for themselves as best they can. Maybe the ships are all backyard homebrew Road Warrior type constructions made by the pilot and his immediate family. They halfass it together with whatever they can scrounge up. $\endgroup$ – Willk Feb 13 '17 at 21:24
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    $\begingroup$ They don't even have to be cobbled together. Materials transport is usually "blue-collar" work (without any disrespect to that profession), meaning it may not pay well enough to warrant getting a huge freighter. More to the point, a smaller vehicle will be easier to maintain for longer, decreasing your running costs over time. $\endgroup$ – Chris M. Feb 13 '17 at 21:30
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    $\begingroup$ @Jeutnarg: I think they wanted "personal". It seems to be referring to the cost the operator personally experiences, not the cost of hiring personnel. $\endgroup$ – user2357112 Feb 14 '17 at 0:32
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    $\begingroup$ This is non sensical because it assumes that there is no economy of scale where larger companies are taking over. There are no owner/operator freighters anymore because large cargo ships are more efficient. $\endgroup$ – TomTom Feb 14 '17 at 10:45
  • $\begingroup$ @TomTom Economies of scale don't say that bigger is always better. They say that there is an optimal scale for everything. Large cargo ships are extremely efficient, yes - but the same isn't true for mega-trucks in quite the same way in a well-urbanised country. Elite does have massive freighters, far beyond anything you can buy - those are the large cargo ships (though they don't scale quite as well - bigger ships are less fuel efficient in Elite universe). The small (player-flown) ships are trucks, not "small freighters". The cost of spaceflight isn't dominated by wages. $\endgroup$ – Luaan Feb 14 '17 at 11:30
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Risk reduction? In many fictional universes space is dangerous and filled with pirates, aliens and alien pirates that want your valuable cargo. If all the cargo is stuffed into a single huge spaceship and it is stolen or destroyed then it's a huge financial loss. If the cargo is distributed among relatively inexpensive independent contractors (and you don't have to pay their life insurance premiums) then perhaps it makes economic sense to use smaller ships since there's a better chance that at least some of the cargo gets to its intended destination.

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    $\begingroup$ I like that. Frequent risks of accidents, piracy, or rogue customs would put a damper on large-scale transportation (all eggs in one basket mode). On the other hand, if we go back to ~16th century, the first "share" system was created to finance ships because at the time there was a significant risk of loss; it seems it could apply here and less the risk somewhat. $\endgroup$ – Matthieu M. Feb 14 '17 at 8:00
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    $\begingroup$ It is easier to defend fewer larger vessels though. People shipping freight past Somalia have not switched to smaller coastal freighters, they still use large fast high-sided ships which can be defended by a handful of armed defenders. $\endgroup$ – RedGrittyBrick Feb 14 '17 at 14:41
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    $\begingroup$ Yes, that's the big weakness of this answer. Perhaps the difference could be explained away because, for the most part, the oceans are safe from piracy with the exception of a few areas (e.g. the coastline of Somalia). The economics of unloading big ships into a fleet of small ships for that single area is not feasible, but if ALL (or most) of space is at risk of pirates then perhaps this answer is valid. $\endgroup$ – GSP Feb 14 '17 at 22:09
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    $\begingroup$ Engineering risk is also a possible concern - the greater the scale, the greater the chance that one component failing brings the entire larger system of the ship down with it, not to mention more avenues for cascading failures. Even with mitigation built in, a smaller ship would be far less of a loss than a massive one, and it's easier to reason about the full set of behaviours of a smaller system, making these smaller ships possibly more reliable to create. Consider a plane vs. a modern rocket as a parallel. $\endgroup$ – Kenogu Labz Feb 15 '17 at 17:51
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1) Lets look at the Honor Harrington universe (David Weber). Normal-space drive performance is inversely related to the size of the ship. In his universe this curve is fairly shallow (it lets small warships run away from bigger ones) lets suppose it's much steeper. Considerable in-space maneuvering is required to use your FTL drive.

Your space-warping normal-space drive can produce 100,000g / the cube of the distance to the farthest point on your ship in meters. (Thus it's linear to the volume of space that must be warped.) To use your FTL drive you must be moving at a Lorentz factor of at least 1 (from special relativity--you must have energy of motion as great as your rest mass--.866c) relative to the hyperspace you are going to enter.

A courier ship that is 1 meter in radius reaches jump speed 402 seconds after clearing the atmosphere.

An explorer in a ship that is 4 meters in radius takes over 7 hours to jump.

A small freighter that is 10 meters in radius takes over 4 1/2 days to jump.

A large freighter 50 meters in radius takes over 19 months to jump and goes over a light year during that time--in an area of dense stars it might not even use it's FTL drive.

2) Now lets add to this pirates. The space warping is quite unfriendly to locating your targets so long range weapons are impractical. You have to get up close and personal to hit and to actually pack enough punch to defeat things like anti-laser ablation layers you need quite a bit of power--and powerful lasers are big.

This makes the best defense be to make your ship smaller than the laser needed to stop you. You will end up with a universe divided between little craft that sprint between stars and some that haul big stuff and have the weaponry to shoot it out with the pirates while plodding between the stars. There will be some intermediate range craft that haul large but urgent cargoes but that's a risky realm to be in.

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Smaller ships are more maneuverable (at least when real-world physics apply at lower than FTL speeds). Lower mass can be moved with less effort/fuel/time.

This may not matter during standard travels, but it matters when lifting from orbit. OR when trying to outrun those space pirates.

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    $\begingroup$ This x 10. Fuel is expensive - and as mass increases, fuel required increases much, much more. $\endgroup$ – Zxyrra Feb 14 '17 at 0:50
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    $\begingroup$ @Zxyrra True, but the fuel needed to move the cargo is the same if it's in one ship or 500. A large cargo vessels will require less crew than a fleet of small ships moving the same amount of cargo, so you'll have less mass overhead from things like life support and crew quarters when using large ships. $\endgroup$ – Ray Feb 14 '17 at 1:24
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    $\begingroup$ @Ray Small ships can also take the elevator, while larger ships may not - hence the need for more fuel. $\endgroup$ – Zxyrra Feb 14 '17 at 2:03
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    $\begingroup$ @Zxyrra That assumes that much of the fuel is used to get out of the gravity well rather than to accelerate in space, and also that FTL doesn't have mass-dependent fuel requirements, but those seem like reasonable assumptions for the OP to impose. $\endgroup$ – Ray Feb 14 '17 at 2:27
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    $\begingroup$ @Ray This isn't true in the Elite universe - the fuel consumption is worse than linear with respect to mass being transported (or rather, the relationship is non-trivial, with optimal efficiency far below the mass of the rare-ish mega-freighters of Elite). One 100.000t ship needs more fuel for the same jump than ten 10.000t ships, in addition to its much more massive and expensive hyperdrive - and it doesn't even necessarily need less crew (as we've seen in Frontier, where the mid-level "trucks" need more crew-per-ton than the smaller trucks). $\endgroup$ – Luaan Feb 14 '17 at 11:39
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While this question is tagged with "spaceships" and "space-travel", this essentially a science-fictional question about how FTL travel configured in any fictional world.

Since this is about non-relativistic FTL travel, then the author can set whatever worldbuilding rules he or she prefers to institute. For example, the mass limiting rule in Elite where the effective jump range is limited by mass of the vessel.

One of the simplest rules than can be set to keep FTL ships small is to limit the volume of its drive field. If the field's volume cannot exceed a maximum volume, then ships can be no bigger than that volume. if the maximum volume is a cubic kilometre, you can have big vessels like megafreighter. Make it smaller and you might need with only singleships plying the spaceways.

The amount of energy to power a FTL drive might increase exponentially and rapidly too. Especially if it's rate of increase was,say, the fourth power of its mass. The determining factor could the mass of the vessel. Doubling the mass means sixteen times the power to maintain a given FTL speed.

By and large, if you play around with factors associated a FTL spacecraft, like mass, power consumption, size of the vessel, hypothetical drag factors in hyperspace or whatever, and as an author you can design whole ensembles of limiting factors to keep FTL ships small.

This answer has concentrated on devising the general principles for restricting FTL spaceships to being small. Have devising your own rules!

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Don't think space ship, think space truck

Truckers don't own the whole lorry, only the tractor.

A large properly crewed cargo ship is expensive to run, expensive to maintain. A truck is much smaller and cheaper to run.

If cargo pods are standardised, then the haulier doesn't need to turn up with a big cargo ship, he just needs a tractor to haul your trailers. Whether you think truck, or think train with a head and tail engine, there's no reason for big ships when the cargo doesn't need an atmosphere. Just an engine with a cockpit and a tow hitch.

Perhaps there are vast cargo ships that can take enormous numbers of pods on long runs and the little trucks only do shorter, specialist and express runs, but you don't need the big ships turning up for a mere 50 megatons. Just get a couple of truckers on a regular shuttle run.

It's a mature technology

Another factor is availability of the technology. You've said it's cheap and accessible, so treat it like it's cheap and accessible. Early engines were heavy, steam powered, complicated and difficult to maintain. Now everyone has a car. Consider the same cycle, early ships were large, needed vast amounts of bulky fuel and were hard to run and maintain. Now *everyone* has a personal runaround to visit their cousins in the next system at weekends and pop down to Stavromula Beta of an evening. Ships are small, cheap and convenient.

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  • $\begingroup$ +1 simply for the reference to Stravromula Beta. :D ...and because you essentially answered what I came here to say. $\endgroup$ – MichaelK Feb 14 '17 at 15:35
  • $\begingroup$ @MichaelK, I should know round here that no reference is too obscure but I thought that one might just slip through :) $\endgroup$ – Separatrix Feb 14 '17 at 15:53
  • $\begingroup$ Ah, well if you go with Heinlein or Scott Card you might be able to slip something through. But Adams? No. Just... no. (Although I did mention TANSTAAFL in a thread earlier today so do not get your hopes up). $\endgroup$ – MichaelK Feb 14 '17 at 15:58
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I've already seen people voice two excellent reasons - cost and risk. Another big factor which figures into both the cost and the risk of large-scale trade economies in FTL space-faring civilizations and which is in large part synergetic with risk and cost is liquidity - that is to say, a lack of liquidity in the market tends to arise where conditions are costly and risky, and drainage of liquidity from a market tends to engender higher costs and more risk. It's a vicious economic spiral that is mostly resolved historically by regulating and protecting the market (which sometimes means protecting it from undue intervention) until it is self-sustaining, near as I am aware.

In a universe like that of Escape Velocity's, we see in-game that the currency is "universal" because the game wouldn't be "fun" if we had to worry about exchanging currencies at rates which could easily be manipulated by travelers arriving in our own light-cone's past due to FTL (and indeed, the game conveniently ignores the time-traveling effects of FTL travel, breaking physics fundamentally).

In real life, however, a galaxy governed by many small states with many competing forms of currency and very little in the way of liquid capital sufficient to build large ventures on their lonesome would have to start with smaller ventures until they could amass enough capital to gain the economies of scale assumed to exist in the OP. Nobody has the money or the raw materials needed to start out big, so everyone goes small. Since nobody trusts each other and going big is very expensive (which multiplies the risk), nobody is really in a position to solve this problem, either. There might even be larger nations in existence somewhere in the galaxy, but if they aren't interested in what the little guys are buying and selling, then they effectively don't exist in the market. Imagine - what position does the United States take in the barter economies of the Amazon forest tribes? None whatsoever, for the most part.

Furthermore, high risk and volatile markets lead to a lack of trust in currency because you don't really know if Zanzibar IV (where you foolishly decided to do most of your banking because of its excellently-regulated currency system) are going to be conquered by space vikings who don't honor the Zanzibar Simoleon Credit System because they aren't allowed to redeem it. Fiat money is not going to be favored over coin tied to some form of material redemption. This exacerbates the lack of liquidity further because the only kind of currency worth having on the galactic stage is stuff you can physically back - you're not going to be able to use debt to create wealth more efficiently, and thus investment will have to proceed more slowly.

Consider, if you will, a quick instruction on the similar kind of situation which obtained in late middle ages Europe: https://en.wikipedia.org/wiki/Crisis_of_the_Late_Middle_Ages. Introduce enough strain on a market and it all comes crashing down, bringing entire nations and peoples with it:

The European economy entered a vicious circle in which hunger and chronic, low-level debilitating disease reduced the productivity of labourers, and so the grain output was reduced, causing grain prices to increase. Standards of living fell drastically, diets grew more limited, and Europeans as a whole experienced more health problems.

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    $\begingroup$ This is a very good addendum to the cost & demand answers above, thanks! $\endgroup$ – Ross Feb 14 '17 at 14:53
  • $\begingroup$ I already have, if I could accept more than one answer I would. $\endgroup$ – Ross Feb 14 '17 at 15:10
  • $\begingroup$ oh, sorry ::blushes:: $\endgroup$ – Adam Wykes Feb 14 '17 at 15:14
  • $\begingroup$ Folks often assume the space "Empire" is static. But if it is new, and constantly expanding, then massive commercial enterprises can't react fast enough to work. Out on the expanding "frontier" things happen so fast that small owner/operator ships make sense. $\endgroup$ – Jason K Feb 14 '17 at 16:56
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    $\begingroup$ @AdamWykes very true. That is the Dune model. But in a world like in the OP FTL for small ships is trivially inexpensive, so it is easy to imagine the small independent companies could supply the needs of the frontier well below the type of ROI that would interest a large corp, at least for the time period covered by the story. $\endgroup$ – Jason K Feb 15 '17 at 14:16
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Flexibility.

Certainly, big ships are more economical if you are sending cargo in bulk and you are not in a hurry. With slightly more crew than the smaller ships, one of the bigger ships can haul the same mass/volume than a complete fleet of small sized ships.

But they are so big that they cannot land in most of the planets, so once they get to the destination the transport to/from the planet must be done with smaller, transfer ships. This means that loading/unloading operations will take some time, even if everything is perfect1, while a smaller ship can just land on the planet surface and almost do a "door to door" service.

Also, if the regular transfer of goods is small, it does not make sense to keep a full sized cargo ship (with its maintenance and fuel expenses) if it only carries a 10% of its capacity.

Additionally, smaller ships may adjust its fuel consumption to the needs of the service. A small ship carrying an express cargo may use greater accelerations (= energy = fuel expenses) if needed, a big ship carrying one express container and 200 low value containers cannot (because it will spend at least 200x more fuel than the smaller ships)

So, while the bulk of interstellar transport is done in those behemoths, there is still a need for smaller ships for faster delivery and/or secondary trade routes.

1There is an available slot in a parking orbit, transfer ships are not busy serving other heaby ships, etc.

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    $\begingroup$ You must be reading my mind. A comparison in the real world is the difference between Rail shipments, vs Semi-Truck shipments, vs Hotshot van shipments. Flexibility and speed. Rail is by far the most efficient to move large quantities of mass, but trucks move it from places that have rail lines to places that don't. Hot shot trucks do the last mile. Heck, spend 20 minutes to study Fed Ex and the logic stands out in a huge way. A great treatment of this, set in space, is Nathan Lowell's Solar Clipper novels. $\endgroup$ – Paul TIKI Feb 14 '17 at 17:24
  • $\begingroup$ @PaulTIKI Don't forget that poor delivery boy on his bicycle :P The scale goes far on both ends indeed. $\endgroup$ – Luaan Feb 15 '17 at 16:06
  • $\begingroup$ shame on me for forgetting the bicycle delivery guy in an urban environment! should we mention the shoe shine boy on the corner that some Film Noir anti-hero paid a dime to for the delivery of a note :) $\endgroup$ – Paul TIKI Feb 15 '17 at 17:34
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People are where the common resources are

That's pretty much an obvious statement, but there is already quite a lot of metal on earth. If FTL is in humanity's capability, mining the earth's core is not so much out of reach.

The space exploration is most likely to be used for scattered, rare things.

Note that this wouldn't mean big cargos wouldn't exist. They just wouldn't be the main thing.

Asteroids are pretty scattered

According to Wikipedia:

Contrary to popular imagery, the asteroid belt is mostly empty. The asteroids are spread over such a large volume that it would be improbable to reach an asteroid without aiming carefully.

A big ship would have to do a lot of stops, mining one asteroid at a time. It could send a lot of smaller mining ships throughout the belt and then make the trip back but in that case why not send the small ships directly from earth? If the trip is easy to make, it's the stops that are costly.

Small ships can do a lot of mining in parallel.

The government doesn't want a big mining corporation on its back

A big mining corporation can hold a government hostage. That's not something you may want. If in the past, a mining corporation has done that and a great war was provoked from this, then the state may well take extreme measures to avoid that to ever happen again. Of course, it may take care of the mining itself, but it can avoid that job by just ordering mining companies to be really small.

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There's the technical answer, the political answer, and then there's the metagame answer.

This thread has some excellent proposed technical answers. As previously pointed out, your FTL may follow scaling laws which favor smaller ships. Also noted is that asteroid mining in particular is a distributed operation which favors a large number of small operators; that leads in turn to a distributed infrastructure of small space stations and depots, which can themselves be served by small freighters.

The political answers are so complex I won't try to address them here, other than to paraphrase a line from Alien. When asked why the ship was in such bad repair, the reply was that "It's cheaper to bribe the inspectors than to do the repairs." I guess that's my way of saying that in spite of the advantages of the economies of scale and concentration of skilled talent, sometimes large capital-intensive systems simply don't work out well in practice.

Lastly, there's the metagame answer. Since FTL is entirely fictional, any technical or political answer you observe has been crafted to serve a narrative. If you are gaming, then your narrative needs to serve a large number of customers operating alone. Even if you are in an MMORPG, your players are going to favor showing up for a battle in their own ships rather than being assigned a crew position on a larger craft. For purely narrative fiction like books and film, small ships operating independently lend themselves to certain stories and genres; horror, futuristic westerns, and harrowing stories of survival come to mind. I'll bet some of the classic one-man-in-a-spaceship fiction was written in part because the author didn't like writing dialogue. Short stories in particular do not lend themselves to large casts; small spaceships allow the author to introduce a limited cast and a finite setting and then move the narrative along briskly.

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Ok, read all the other answers and didn't see this particular idea, so here goes:

Let's decompose "transportation" into:

  1. Transporting ourselves (or maybe just our consciousnesses, depending on how our world works), and
  2. Transporting everything else - materials/goods/other beings/etc.

As far as the utility of transporting "stuff", most successful races would put value on maximizing efficiency, which could be defined as amount of mass transported per unit of "spacetime" (i.e. travel distance and travel time). This metric would become a driver for lots of constraints on ship size (think economies of scale, etc.), and it seems that things would overall tend towards bigger ships. However, consider for a second what a swarm would mean for this - the big, efficient "ships" are really made up of many smaller units working together as a whole (somehow).

Now, the twist: "personal" transportation - i.e. moving "ourselves", is very different. The human race, and many fictional races, have a strong bias towards personal autonomy and freedom of movement relative to other beings. So we may have a very strong incentive for smaller craft already, if in many cases we explicitly want to transport only ourselves (or a few beings), not to mention the other benefits mentioned for small craft, like maneuverability, lower costs, less liability (only you are at risk when travelling "by yourself"), etc.

Put those two forces together, and it looks an awful lot like a good argument for a swarm-centric model where you have lots of little "personal" craft that consolidate into larger collective units when the efficiencies of bulk transport outweigh the need for personal autonomy.

I know swarms have been mentioned. I'm just trying to further explore how likely it might be for a swarm-based transportation model to emerge naturally, which may determine and/or affect many of the other constraints on ship size.

Just a thought. Great question!

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"Cheap" and "small" are relative terms.

On "small"... Think of current day ocean going vessels. The longest ship was about 450 meters (1500 ft); the largest aircraft carrier is around 340 meters (~1000 ft). While the smallest is/was about half of that. Destroyers go from as large as a "small" aircraft carrier (Zumwalt - 170m) to 1/3 of that. On "cheap"... ship prices for destroyers and aircraft carriers can go from 200 million to around 13-14 billion (Gerald Ford class carrier).

So, a "small" spaceship could still be a very substantial vessel, but would probably still be expensive enough to make it affordable only to the super rich (individuals or corporations). And a large aircraft-carrier sized space-ship would probably only for the well-funded governments, and they do not run trade (at least not in a free market).

You could look at cheaper type ocean-going vessels, like tankers or cargo ships, but for space and FTL, you are probably talking complexity and technology at or beyond the level of a military ship (carrier or destroyer).

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    $\begingroup$ Although military spending numbers are easy to come by, I don't think they are great to use here. The military solve engineering problems by throwing money at them. $\endgroup$ – BobTheAverage Feb 14 '17 at 0:10
  • $\begingroup$ @MarcusYoder - At first I thought you had a really good comment here. I still think you have some good points, but I am not sure your premise that the military solves engineering problems by throwing money at them is entirely accurate. The price of a commercial tanker or cruise ship is not that different from a destroyer, when you consider all the additional technology you need on a destroyer. Since the military put shipbuilding out for bids, from commercial providers, this is not entirely surprising. Even if you reduce the cost by a factor of 10, the point I make in my answer still holds. $\endgroup$ – user11864 Feb 14 '17 at 20:19
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Another possible answer lies in looking at existing shipping on Earth and why they are designed the way they are.

We have 4 main long distance shipping vehicles: Ocean freighters, cargo planes, trains, and cargo trucks.

Of those 4 vehicle types, 3 share a standard shipping container for easily transferring cargo from one vehicle to another.

The important considerations for each vehicle type are:

Ocean Freighter

  • Water produces high drag.
  • Having a lot of mass counters the losses of the high drag.
  • Restricted to water.
  • Can only deliver to docks. The larger the boat, the fewer the ports that have docks that can support the ship.

Airplane

  • High speed.
  • High fuel consumption.
  • High cost.
  • Can only deliver to airports.
  • The heavier the cargo, the fewer the airports that have long enough runways to support the plane.

Train

  • Most fuel efficient.
  • Engineers typically take turns at the controls (can travel for more than 8 hours without law mandated rest periods).
  • Slowest overland travel option (if not considering the option of having multiple engineers to keep the train operating around the clock).
  • Can not share the tracks with other traffic.
  • High up-front cost to setting up the infrastructure.
  • Restricted to land.
  • Can only deliver to stations.

Cargo truck

  • Each vehicle typically carries one cargo container; two at most.
  • Multiple vehicles can share the same road.
  • Drivers required by law to rest (very long trips take more time than the same distance by train) (yes, I know that there are "sleeper" truck that allow multiple drivers; not all drivers can sleep in a moving vehicle; majority of dispatchers don't want to pay for drivers who are sleeping in the cab)
  • Restricted to land.
  • Can deliver to most locations with humans.

Taking all of these considerations in account, and working with the "realities" of typical FTL space travel:

If there were a region of space with high drag on a ship's FTL drive, it would make sense to make larger vessels like ocean freighters. I haven't heard of any such cases in any stories I've read; space seems to be relatively uniform, even in fiction. These ships will need special spaceports for servicing, though thanks to most things in space sharing the same orbit, transferring all of the individual standard cargo containers from the freighter to local delivery ships would be much faster work than the current cranes of today's ocean freighters. If you've ever disturbed a mother wolf spider with hatchlings on its back, I imagine the cargo containers would disperse in a similar way.

If there were a way to inefficiently use energy to produce extraordinarily high speeds, we'll see small, fast, expensive to lease cargo ships.

If there were something about the FTL drives that made them work more efficiently if placed in some sort of conditioned space (subspace slipstreams?), we would have space trains.

Given the most common description of FTL ships; single-family home sized craft that can zip from system to system in minutes or hours, that can land and take off from atmosphere covered planets without needing to spend 80%+ of their mass as fuel or reaction mass, and craft that really don't need to keep to specially conditioned areas of space or avoid certain areas, this is what most individually piloted space freighters are likely to be. They can go anywhere in any reasonable time, and dock at any port, spacedock (space station based ports), spaceport (planet based launch complex), asteroid mining operation, or space homesteader's front door, and be free to offer their services to any dispatcher in the system, provided they're an owner-operator. Serenity and The Millennium Falcon were examples of this type of cargo ship, and both crews were owner-operators.

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People and monopolies, firstly humans don't like being crowded, by-and-large we also don't like throngs of other people around us, frontier types who would actually go out into space where people are generally thin on the ground even more so. So spaceships that require more than say a dozen crew are impractical out on the edges of things because of the dominate personalities out on the edge of civilisation. Secondly where there's a lot of profit to be made corporations will monopolise those profits wherever possible. Andre Norton's Freetraders are a good example of the end result of these two ideas, in the core where inhabited worlds are "close" and populations are large the Combines, big interstellar companies, dominate trade with massive freighters but out on the edge the Freetraders run much smaller multipurpose transport ships between isolated new worlds. Firefly uses a similar model; on the Rim small scale independent traders can make a buck that isn't available on the Core worlds, not because they're unprofitable but because the profits are all sewn up by larger interests.

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