Just for a ballpark, how big do you make your planetary armies in relation to the rest of the population? (understanding that this will vary some according to tech, government and law level; also, that a country's standing army is liable to be a sight smaller during peacetime than when at war)
I don't need to get too too detailed in this, because I don't forsee running a mercenary/military campaign anytime soon, but it makes a difference for my overall background.
The setting I'm developing right now is non-OTU, and worlds won't be able to count on the Imperium to bail them out of anything since IMTU there isn't one - though there are local treaty organizations. The subsector I'm focusing mainly on is pretty undeveloped, with few planets over TL 9.
For TL5+ society, I'd suggest 1-5% of planetary Gross Domestic Product be spent on military forces, depending on the degree of threat present. If the world is fighting a major war, it can probably increase this to 5% of GDP each year for about 4-5 years, at which point it will be facing serious economic decline. (To be clear, this means that a nation might have as much as 25% of GDP devoted to war after 5 years).
Planetary GDP will be population x average salary per citizen. Based on starship salaries, I expect that the typical CT world (say, TL 10) will see an annual per capita average salary of Cr12,000.
I think that wealth will *tend* to increase with tech level (it certainly has over the last 200 years). So, adjust the salary by Cr2,000 per TL above 10 and by Cr1,000 per TL below 10. Standing armed forces of pre-industrial planets would be negligable in a Traveller setting, so I wouldn't bother with them.
Wealth will definitely increase with trade, especially if the planet embraces relatively free trade. So, I'd give a 20% bonus for rich or industrialized planets and a 20% penalty for non-industrialized or poor (40% if poor and non-industrialized).
Anyhow, once you have the military budget, you have to decide how much of it the Army, wet Navy and space Navy split. Then determine the cost of representative units (army divisions, space Navy capital ships, wet Navy capital ships). I'd spend 1/4 of the funds on those units and assume the rest are spent on supporting units.
Alternatively, you can figure out manpower by making assumptions about how much a planet spends per military person. The US -- a rich, arguably TL9 nation, with the requirement to deploy forces far away--spends roughly $350K per combatant.
So I'd assume that a comparable TL10 planet (rich) would spend Cr150,000 per combatant. Adjust by Cr50,000 per TL above TL 10 and by Cr25,000 for each TL below 10. Again, this is good only for TL5+ worlds.
Combat troops will be outnumbered by support troops by 1-10 times.
Anyhow, just my cogitations.
