Some thoughts.
When dealing with the number of starships we have to try and deal with the nature of trade between the colonies and Earth, and indeed trade between the colonies.
Now, Earth is interesting. She imports 4 times what she exports (Earth:2300), and indeed, due to this imbalance, there is a tendency to put colonists on ships going out (a colonist will require several tons of material on landing) (SotFA).
However, what Earth is poor of is metals, petrochemicals and food. Bulk metals are mostly found insystem, and are mined at orbital mining stations in other systems where there is no gravity well stopping the launch. Only special metals are actually boosted up a colonial gravity well to be sent back to Earth (see Kafer Dawn etc.). The same may be true of some petrochemicals (ECS notes that demand for petrochemicals has dropped). Food of course is at a premium on Earth, which simply can't feed itself*
Looking simply at Grain, Earth has a population of 5.38 billion, many of whom go hungry. Europe, Indonesia, parts of Asia and parts of Africa simply can't feed themselves, and there are no large grain excesses elsewhere, so they need to import from Offworld.
The economics are such that asteroid mines etc. are possibly more valuable to Earth, and these avoid the need to lift out of orbit.
* A side effect of the loss of grain imports from the French Arm during the 2nd Invasion was a likely series of famines in the parts of the world reliant on such grain.
When dealing with the number of starships we have to try and deal with the nature of trade between the colonies and Earth, and indeed trade between the colonies.
Now, Earth is interesting. She imports 4 times what she exports (Earth:2300), and indeed, due to this imbalance, there is a tendency to put colonists on ships going out (a colonist will require several tons of material on landing) (SotFA).
However, what Earth is poor of is metals, petrochemicals and food. Bulk metals are mostly found insystem, and are mined at orbital mining stations in other systems where there is no gravity well stopping the launch. Only special metals are actually boosted up a colonial gravity well to be sent back to Earth (see Kafer Dawn etc.). The same may be true of some petrochemicals (ECS notes that demand for petrochemicals has dropped). Food of course is at a premium on Earth, which simply can't feed itself*
Looking simply at Grain, Earth has a population of 5.38 billion, many of whom go hungry. Europe, Indonesia, parts of Asia and parts of Africa simply can't feed themselves, and there are no large grain excesses elsewhere, so they need to import from Offworld.
The economics are such that asteroid mines etc. are possibly more valuable to Earth, and these avoid the need to lift out of orbit.
* A side effect of the loss of grain imports from the French Arm during the 2nd Invasion was a likely series of famines in the parts of the world reliant on such grain.