In order to illustrate the comprehensive "gravity model" economic system in the GURPS Traveller: Far Trader sourcebook (do yourself a favour and get a pdf copy), let's do a working example calculating trade volume between Tobia (3215 TR) and Tyokh (2226 TR). Please refer to the attached image taken from travellermap.com showing one of the possible routes (in this case eight 2-parsec jumps to cover the journey without need to resort to "wilderness (i.e. gas giant) refuelling"):steve98052 wrote: ↑Tue Sep 26, 2017 6:06 pmThe best way to figure out trade volume is probably the system in GURPS Traveller Far Trader. Some of the on-line maps include a "world trade number" figure (WTN), and from that you can get a "bilateral trade number" (BTN), which is the sum of the WTNs minus a distance modifier. With that, you get the trade between worlds.
That only covers traffic between individual pairs of worlds. A choke point world may be part of the route between several high BTN pairs, and have a lot of through traffic. It may even have a hub traffic, where cargo or passengers change ships in the system but don't end their travel there. (Hub example below.)
For low BTN, the only traffic is the occasional free trader or subsidized merchant. For moderate BTN, there's regular free or subsidized trader traffic. For high BTN, most cargo goes by big freighter, most passengers go by big liners, and the scraps go to free traders. Because the big ships grab most of the cargo and passengers on the busy routes, the amount left for the little guys isn't any better than the amount available on a moderate BTN route.
If you're looking for how much traffic is available as prey for commerce raiders, I'd say that the routes that are busy enough for big freighters and liners are also going to be well served by system defense boats and heavy monitors. In other words, those routes aren't a good spot for small time raiders. The best spots for raiders would be the lower end of moderate BTN routes, where there are enough ships that you can find a victim without such a long wait that the crew starts to get itchy about not getting paid, but low enough that the system isn't guarded by monitors that can turn your puny raider into salvage the moment you threaten a merchant ship.
I haven't mentioned numbers, and I can look them up if no one has the books handy.
The five worlds A, B, c, D, and E map out like this (with dots to force the formatting to work, since I can't get a monospaced font):
A . B
. _ .
_ . _
. c .
_ . _
. _ .
D . E
A, B, D, and E have high WTN, but c has a low WTN.
For travel between A and B, or D and E, traffic goes through the obvious Jump-1 routes. But travel between A and D or E, or between B and D or E, all has to go through Jump-3 routes by way of C (unless it's so insanely urgent that it justifies a TL15 Jump-6 express). There might be dedicated lines for all of A-D, A-E, B-D, and B-E, but it might make more sense to have A-c, B-c, D-c, and E-c lines, and do exchanges at c. That's particularly convenient if the cargo and passengers are carried on modules (or containers, for smaller lines). There might even be some tourist business on c, for passengers who have a few days of layover.
The distance between both worlds is 16 parsecs. Note that only total route length (and not number of jumps or jump lengths) is significant in the below calculations.
First of all, let's get an idea of the relative economic importance of these two terminal worlds and then let's calculate both terminal points' World Trade Number or WTN (a number sadly absent from many world entries at the Traveller Wiki):
* Tobia (UWP A544A55–F) is an industrial (In) sector capital (Cs) high-population world (Hi) with the finest starport class (A) and the most advanced Imperial technology level commonly available (F). It also has an Imperial naval base (N).
- Tobia has a Per Capita Income of 15,000 Cr. This is modified by its Trade Class In (x 1.4) to 21,000 Cr. Gross World Product (GWP) is then 21,000 Cr x 20 billion inhabitants = 420 trillion credits.
- Tobia's Unmodified World Trade Number (UWTN) is the sum of TL Modifier (1.5) + Population Modifier (5.0) = 6.5. This UWTN is modified by the starport class (+0) to give us a final WTN = 6.5.
- Tyokh has a Per Capita Income of 9,375 Cr, no modifiers applied. Gross World Product (GWP) is then 9,375 Cr x 20 billion inhabitants = 187.5 trillion credits.
- Tyokh's UWTN is the sum of TL Modifier (1.0) + Population Modifier (5.0) = 6.0. This UWTN is modified by the starport class (-0.5) to give us a final WTN = 5.5.
* Worlds do not share the same political allegiance: -0.5
BTN is the sum of both worlds' WTN plus the WTCM minus the Distance Modifier (16-parsec trade route, or 2.0): 6.5 + 5.5 - 0.5 - 2.0 = 9.5. [BTN can never exceed the smaller WTN plus 5. So we are well within the permitted limit of Tyokh's WTN + 5 (5.5 + 5 = 10.5).]
A BTN of 9.5 implies the following trade volumes between Tobia and Tyokh:
- 5-10 billion Cr per year (average 7.5 billion Cr per year).
- 0.5-1 million dtons per year (an average of 0.75 million dtons per year), or 10,000-50,000 dtons per week (an average of 30,000 dtons per week). Tramp traders will find 500-1,000 dtons per day available for speculative trade (average of 750 dtons per day).
- 100,000-500,000 passengers per year (an average of 300,000 passengers per year), or 5,000-10,000 passengers per month (an average of 7,500 passengers per month). Tramp traders will find 50-100 passengers per day available (average of 75 passengers per day). (<== A +1 to BTN is applied in this latter case due to the fact that Tobia is a sector capital.)