And so it would be if J1 was cheaper than J2 (over two parsecs, that is).
The context is what happens along a J1 route when higher Jn engines are developed (as otherwise J1 ships would be irrelevant) so the primary argument is whether a J1 ship is more cost-effective than a J2 ship *if constrained to* a J1 route i.e. if the J2 ship is only doing J1 jumps.
If J2+ engines are larger than J1 engines then by definition a J2+ ship of the same size *constrained to* a J1 route must have less cargo space per jump.
This is what creates the possibility of two layer trade along J1 mains: a J1 layer servicing the old J1 truck stops that developed into towns and a top layer between the prime systems using higher Jn ships which bypass a lot of the old truck stops.
In this argument you're saying that J2+ ships are more cost-effective *if constrained to J1 jumps* and so there's no reason for J1 ships - which is clearly wrong.
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The second argument (which distracts from the main one so I want to avoid it) is are J1 ships more cost-effective over longer distances than higher Jn ships (where J1 is physically possible as an option).
My recollection is Aramis' calcs said yes to that but it doesn't matter to me personally as I hand wave that away anyway by treating boonie trade and prime trade as two separate trade systems.
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They also need to make sense in the current Year of the Imperium.
True, like I said it's different for a long-existing setting that has had many layers of civilization laid over the top.
However it's useful for a home-brewed "explorer" type setting like the example I used, take Vland as a home world and roll forward a 100 or so years at J1, then add J2 and imagine which systems would be by-passed, then add J3 etc.
(You could even code it by star port class as a memory aide e.g. A as home world, B as prime colony, C as current truck stop, D as ex truck stop, E as never truck stop.)
However even in the OTU the crucial idea is the same. If the bulk of trade is between prime systems then as Jn increases the systems that get bypassed will make up a second layer of lower Jn trade.
If the through traffic is hundreds of dT per year, a gas/petrol stand analogy makes sense; if the through traffic is millions of dT per year, a huge mega-truckstop is needed. Regardless of the system's history.
Yes, those systems which are *current* truck stops on the route between prime systems in the OTU will be big truck stops - it's more the ex truck stops that have been bypassed that make up the boonie layer.
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Anyway the scenery points I want to make based on the OP are (imo)
1) J1 trader ships servicing the boonie systems along the mains makes perfect sense imo and if you want a campaign where the players *bump into* adventure as they travel then J1 trader ships work fine.
2) You could treat the type S in the same way. If there's a J1 boonie layer then the type S could be the scout version of the Free Trader i.e. it's role is to patrol along the old J1 part of the mains doing stuff like fixing navigation buoys and reporting problems while the separate J3-J6 layer does its own thing.
3) For an away team type game where players are sent on missions far away then a higher Jn ship does make more sense imo. This is easy enough to do - in a scout game for example a 200 dton J3 away team scout ship could be a promotion for a team that has successfully completed some type S J1 runs from say Regina to Lunion or Lunion to Mora.
4) Separately the truck stop idea - current and ex - is a useful way to hand wave away a lot of detail you don't need at the time while allowing you to go back to it later if you have an idea. This may be a useful idea for people who look at the OTU and balk at the number of systems to describe.
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edit: the reason for all this is to create more wilderness *inside* the OTU i.e. as well as systems that were never part of the big ship universe you can have systems that were part of the big ship universe when they were truck stops but slipped back into wilderness when they were by-passed by improved jump tech.