The single-shell city can be done now (was A buck a squafoot)

> I think the progression will be something like this.
>
> Right now we're at square zero, where every last single thing
> we use in space is boosted up from the surface of the Earth.
>
> Square one might represent using some lunar soil in a
> radiation shield for some humans orbiting above the Earth's
> magnetic field.

I suspect that an economic case could be made right now for a
large orbital-habitat venture (million+ residents) that uses no
extra-terrestrial materials. It would require dropping many of
the current popular-assumptions, though, as well as recognition
of enabling-technologies such as Omax waterjet-machining...
http://www.waterjets.org

...and teleconferencing-telepresence:
http://www.telepresenceoptions.com

The first stage, as I opined before, would involve the
development of ultra-dense, high-capacity,
high-social-efficiency single-shell Terran cities. At the least,
these would serve as proof-of-concept for orbital cities of the
same nature. However, the profit derived from these new,
outcompeting, Terran cities might also be able to serve as part
of the investment capital needed for orbital cities.

> Then we start making solar panels which use lunar material
> for around 99% of their mass. This is the era when SBSP
> really starts to take off.

I doubt that SPS would ever amount to more than a niche market
serving remote locations on Earth. In the general case, it might
even make more economic sense to beam *nuclear-power up* to a
habitat, than to beam power down. There are no lower bounds for
the cost of terrestrial nuclear power. Add automation and scale,
and the cost modulates toward zero. This implies that the cost
of rocket-fuel *also* modulates toward zero -- neutering
arguments for lunar-mass-derived SPS.

Here are my development stages, again:

1. Dense Terran single-shell cities
2. Simulated-isolation of single-shell cities
3. Mass-launch of burned-in hardware and populations