Because foam is cheaper? A nice K.I.S.S. solution.
Having seemingly solved this thermal expansion problem "in the same category of problems separating us from a space elevator," I'll be expecting The Fountains of Paradise to come true soon. ;)
> We seem to have solved this thermal expansion problem
I guess I'll have to telegraph my buddies in Hawthorne and at NASA :).
Joking aside, no, foam doesn't solve the problem. You'll still have flexing. This is one of the limits on how long you can have a launch tank on the pad. Imperceptibly small flexing, but of the kind that weakens every metal we know.
>Joking aside, no, foam doesn't solve the problem. You'll still have flexing.
Of course the flexing isn't zero, but by adjusting the thickness of the foam you can drive the diurnal/solar heating thermal gradient arbitrarily low without compromising strength by making the steel walls thinner. That was your material science objection,[1] right?
>but of the kind that weakens every metal we know.
That's true, but misleading. It's true that every metal gets weaker, but in steel and titanium this weakening levels off (all other metals continue to get weaker until failure). Once these two metals are at the fatigue limit they stop weakening and have unlimited flexing cycles.[2] So you can set that as your final material strength and size the tube thickness accordingly.
So repeated flexing is fine, and flexing can be made arbitrarily small with insulation. Foam solves the problem.
At that point why not just bury it? It swaps the thermal problem for the water problem, but we know how to build tunnels.