|5.9 LY in 50 years|
54,000 tonnes for 500 tonnes payload
can't even catch a ride
As I and others have frequently noted, space is big. Very big. And while it may be the final frontier its exploration is far from an insignificant enterprise. The technological challenges alone are almost unimaginable, and they are dwarfed by even greater challenges in the form of people. People like to spend mont and time in their own, direct and immediate, interests. Although spreading to the stars is, in my own opinion, the best way for humanity to survive in the long run, most people cannot see the need for starships - those in charge, at any rate. Quite aside from the motivation of the people making decisions, the economics of interstellar travel will prevent it for many years to come. Something like the Daedalus starship of the British Interplanetary Society, pictured above, would cost ~$175 trillion dollars. Much of that is research cost, and thus gives back in the long term, but anything spent on the starship itself can never bee recovered. And as much as scientists may argue the value of good data, few politicians would agree with them.
The solution is to utilised a design that will result in, if not profit, a greatly reduced cost. Any large - scale interstellar exploration will need large orbital construction facilities, probably utilise asteroid mining, and even might harvest fuel from gas giants. All in all there will be a lot of infrastructure that needs to be built, adding to the cost. However, anything geared to mining the asteroids can be put to commercial use once the starship has departed, and represents an investment, not a purchase. The trick is to minimise the amount of material and tech that actually leaves the solar system, while maximising the amount of tech that can be later used to develop the solar system at a possible profit.
here, so I won't go into too much detail about the specific design. Personally I think that one utilising a e-sail/mag-sail and a plasma based beam would work best. The beam can provide more momentum for the same amount of power as a laser, so it gives greater acceleration, countering its short range. Also, the e-sail and magsail are both very effective at decelerating from high speed, so they can be used at the destination. Another advantage is that it would be harder to use the plasma beam as a weapon, due to a range smaller than hat of a laser, and inability to penetrate Earth's atmosphere, which makes it more likely that governments would allow it to be built.
Small scale versions could be perfected and used to explore the asteroids and begin mining operations. These would then be improved as the need for materials increased. By the time the starship is complete, perhaps fifty-seventy years after the project is started, their are enough large beam stations in various solar obits to boost it to interstellar velocity. A good tactic would be to start in a orbit distant from the sun, performing the manoeuvre known as a 'sundive' which combines a gravitational slingshot, Oberth flyby, and can use the sail on the starship as a solar sail close to the sun, where it is most effective.
In a solar system where this has been set up colonisation becomes a reality. The beams can provide fast interplanetary transport, and also form the basis of an economy. Coupled with mining, industries that support the colonists, and a secondary economy based on supplying the stations with the mass for the beams. As more an more people move to the planets and beam stations the need for more mined resources and transport arises, stimulating the economy.
From the perspective of a SF world builder this provides a compelling hard science 'Verse in which to set a variety of stories. The beam stations are the centre of a thriving solar system wide economy. Each could be the centre of a residential space station, income provided by renting the beam and acting as a transport nexus. Not only this it means that any colonised star system has in place the means of interstellar travel, even if it is still uncommon. If each beam station is independent politically, very interesting scenarios could play out, with various factions attempting to gain control of the most vital. Conflict between Earth and the beam stations could provide a refreshing change to colonists on the moon, Mars, or Asteroids.
I'm not an economist, but that seems to be to be a lot less handwaving that if people are just sent out to mine the asteroids. That is likely to lead only to unmanned bases, and robotic ships. The starship project, as an experimental effort, will need people on-sight, and once the infrastructure is in place there is a incentive to use it to regain some of the cost of the starship. In any case, it is but one vision of the future.