When discussing the economy of Interstellar Travel, this does not mean the economy needed to develop it, but rather the economy that will exist aboard an interstellar ship where the trip is long term or generational. In an enclosed system with a monitored and regulated population, how will allocation of resources occur? Money as we know it will have little bearing on ship life, but rather a ship-board currency must be developed. Depending on the size of the ship, there may be creature comforts such as personalized furniture, decorative items, and gourmet food.
Books are another item that may be traded, but with the digital age almost all literature and writings from Earth should easily be encodable and accessible to the population at large - so the only books would be those of sentimental or collectible value - perhaps a copy of a favorite novel that is best to curl up with as opposed to reading off a cold, impersonal screen. Although even with this current generation being raised, there is question of how much interaction with the "real world" will be replaced with computer versions of the same item.
So the only real question will be allocation of food rations, in a general sense and in case of emergency. Should different people be allowed greater shares of food, based on personal consumption habits and size? Taste, religious necessity, political choice, all these things may factor into how much and what type of food is distributed to each person. So other than establishing a base-line calorie intake on a periodic basis, what would be the most egalitarian manner of doling out the food?
Would food credits be the staple of currency, with the ability to trade or give away your own food credits to others? Or would they be absolutely non transferable? If they are transferable, what commodity could replace them? Will there be set schedules and time allotted in the recreation facilities, will movies be on a pay-basis (additional use of electricity/power), will there be training facilities or advanced education available on a pay-basis?
This brings up the question of social stratification - there will be different jobs aboard ship, with some being more important and high profile than others. Would the captain of the ship be afforded more credits than a deck scrubber? How would education be determined? By birth, or by merit? How do you prevent nepotism and other unsavory influence in a meritocracy?
What projects will exist aboard ship? Simply the running of the vessel, or other things as well? Science experiments in deep space would be a logical assumption. Construction of new systems, upgrading old systems, and other ship-type maintenance. Who will determine these projects, and how will the resources needed to carry them out be allocated?
It always comes down to the need for some arbitrary unit of measurement to decide who "Gets" what. Money in a closed system would be a finite resource with multiple purposes, so exchange rates and costs would have to be established ahead of time, and most likely would change over time.
