It is generally accepted that GURPS Vehicles, the vehicle design
supplement for the third edition of that game, is not something anyone
would use at the table; I'd agree with that. But it did form the
surprising basis of a campaign I ran some years ago.
It was the early days of GURPS 4th edition; I remember that
Ultra-Tech wasn't out yet and I think I may have built some weapon
stats using material from its playtest. I wanted to run a
Traveller-ish space free trader game, but without the weight of
background of actual Traveller.
So one of the things I did while setting it up was make some
technological assumptions and then design some spaceships, using the
GVB software for Windows, to see how they'd come out. Specifically, I
allowed reactionless thrusters, but no contragravity. And something
rather special happened: it immediately became apparent that there was
a gradation of efficiency of freight carriage based on how much they
could rely on in terms of local resources at each end of the trip.
There are three basic demands of a trip in this setting: getting the
cargo from the planet's surface to orbit, getting it from orbit to the
outer system where safe jumps are possible, and getting it through
jump. So if the ship that provides the hyperdrive only goes to systems
where they already have the in-system infrastructure to send in-system
lighters out to the jump point and transfer cargo there, it doesn't
need to carry a big in-system drive itself. If a lighter only operates
in systems with orbital ports, it doesn't need to be able to fly
through atmosphere itself, because cargo can be transferred to
shuttles or dropped in re-entry bodies. If a shuttle can rely on a
runway, it doesn't need to be able to do vertical landing and takeoff.
A vessel that can do two different things costs more than two
specialised vessels, because each of its modes needs to be able to
support the dead weight of the specialised equipment used only in the
other modes as well as the live weight of the paying cargo.
Also, the cost/weight of life support and so on doesn't scale linearly
with cargo (double the cargo space, you don't need to double the crew)
so there's a push towards big ships, especially if they don't have to
fly through atmosphere and land.
Which in turn means that the systems with solid infrastructure can be
served by the big cargo carriers with their systems of specialised
ships, but the lower-tech and poorer systems can't. The classic free
trader that can do all the things, jump and in-system travel and
flight through atmosphere and vertical landing, will cost
significantly more per ton of cargo it takes from A to B, but it can
go to places that the specialised ships can't or won't.
For example: there's no orbital port at one end or the other. There's
no lighter fleet. The local atmosphere doesn't support aerodynamic
flight and they can't afford to build a magnetic catcher, or because
of the civil war it's not operating reliably. The delivery of the
bulky cargo needs to be at this specific isolated lab where there
isn't room for a runway. The cargo is delicate and shouldn't be
trans-shipped too much. We can't wait for there to be enough cargo for
this system that one of the big ships will go there, we need this
moving now.
In other words I had a whole pack of adventure seeds for slightly
unusual cargo, falling directly out of the design system (and some
very light economic analysis). A simpler design system like
Traveller might have given me the jump ship versus in-system ship
split, but Traveller lets almost any ship land anywhere, so the rest
of it would have been lost.