In article ,
Christopher M. Jones wrote:
No, only if you insist on having 1G at a distance of some 6400km. At
one-millionth the distance, you need one-trillionth (10^-12) the mass.
Still kind of a lot, mind you...

I already thought of that. The problem is one of
density. Realistically, you can't get convenient
chunks of matter that are denser than about 18 g/cm^3
(e.g. Uranium).

Remember, the whole point of the question that started this particular
thread is (essentially) that "mass" and "matter" are not the same thing.
Energy too has mass, and the mass of the hypothetical disk with the
relativistic spin is dominated by its energy content, not its matter
content. That is, its density at rest is basically irrelevant.

What *is* relevant, alas, is the impossibly high structural strength it
requires. With the strongest materials we've got, flywheel energy storage
struggles to be competitive with batteries. This falls many orders of
magnitude short of what's needed to store significant *masses* of energy.
There is room for improvement in the materials, notably with the nanotube
composites that many people are trying to make, but that's about one order
of magnitude, which is nowhere near enough.

Plus, as others have pointed out, transporting such vast amounts of energy
is much harder than transporting equivalent amounts of matter. The entire
energy output of the Sun is only about four million tons per second.
--
"Think outside the box -- the box isn't our friend." | Henry Spencer
-- George Herbert |