Frosted cones and other thoughts

I've been kicking around Bob Hardman's 7-30-02 figures & believe
we MUST lower our sights. Perhaps 500 meters diameter like O'Neill's
Island I (J.D.Bernal sphere) design.

Even at 500 meter diameter with 30 psi design pressure a cylindrical
vessel would need ~23" wall thickness for old aluminum alloy with
15000 psi working stress. Anyone know what allowable working
stress we can use with that new aluminum-lithium alloy?

Also noticed that going from sphere to cylinder shape DOUBLED
wall thickness (for same diameter & internal design pressure).
No wonder G.K.O'Neill mentioned staying with spherical shape for
first colonies in his video tape.

My big hope is to go with cylindrical shape/center light cone design
using the Al-Li alloy with far higher than 15000 psi working stress.
Also consider circumferential cables in tension on outside of shell
every few meters letting us half that required wall thickness.

Hallelujah - Praise Spacesettlers ! ! ! Ed S.