Pressuvessels and economics of the first islands in space.

Surely, the most practical shape for a pressure vessel is a sphere? You get
the maximum possible internal volume, for the minimum possible surface
area.

Incidentally, it occurs to me that the first islands to be built in space
will be more expensive than following ones. The capital costs of developing
ISRU techniques and setting up the moon base, mass catcher, mass driver
tugs and manufacturing facilities, will tend to make lunar materials much
more expensive than the few cents per kilo that is generally assumed. For
initial habitat construction, it makes sense to consider using the entire
internal volume of our habitat, rather than just the inner surface.
Island one, for example, weighs in at 3 million tonnes, shielding
included. With an initial materials cost $5/Kg, materials costs alone will
put the price of Island One at $15 billion or about $20,000
dollars/Sq-metre of internal surface area. And that figure does not take
into account the fabrication costs of island one.
What we are really paying for is not internal surface area, but internal
volume. One square metre of internal surface area might cost $20,000, but
one cubic metre would cost only $229. Having gone to so much trouble
fabricating a pressure vessel and importing millions of tonnes of material
as shielding, we waste most of the habitable volume. If we were to use the
habitat as O'Neill suggested, the materials cost of island-one alone, would
be around $2,000,000 per inhabitant.

Given the large initial costs of construction, it would make sense to
divide Island One into internal decks, thereby making use of the full
internal volume. If we were to grant each inhabitant a generous 400m3 of
living volume, the cost of housing him would be a more modest $92,000
(materials costs alone). This estimation is over simplistic, since it
assumes that the internal structure costs nothing. But even if the internal
structure doubles the weight of island one, it still makes much more sense
to consider using the entire internal volume, rather than just its inner
surface.

The larger the initial habitat becomes, the more significant the argument.

Tony