>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

>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. [1] https://news.ycombinator.com/item?id=15460617 https://news.ycombinator.com/item?id=15460617 [2] https://en.wikipedia.org/wiki/Fatigue_limit https://en.wikipedia.org/wiki/Fatigue_limit